26 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION QUANTITATIVE RELATIONS OF THE ACIDITY OF THE RESIDUE TO THE QUANTITY OF ACID USED It is of significance to know the relation of the quantity of acid in the residue after the soil has been treated with the acid t0 quan- tity of acid used. This relation might throw some light on the chemistry of the process. It also aids in judging whether or not the measurement of soil acidity is related to the quantity of acid- forming fertilizers added to the soil, or to the production of nitrates, or to other acidifying influence. q The quantity of acidity of the soil residue was estimated by several methods proposed for ascertaining the lime requirements of the soil, namely, the Veitch, the Jones, and the Hopkins method, and also by barium hydroxide and by barium acetate. LIME REQUIREMENT OF SOIL RESIDUE Eleven soils of low buffer capacity were used. One hundred grams of each soil was treated with sufficient acid to bring the soil _ to a pH of 4.0. as calculated from the pH acid curve. After stand- ing overnight. the soil was transferred to a filter, the supernatant liquid filtered off, and the residue washed twice with water. The soil, aft-er drying, was passed through a 20-mesh sieve. Determina- tions of the lime requirement were then made on the soil residue by the Veitch, the Hopkins, and the Jones methods. In the Veitch method (29) the soil is treated with increasing amounts of lime water until the filtrate upon boiling turns pink with phenolphthalein. In the other two methods the soil is treated with neutral salt solution and the filtrate titrated with sodium hydroxide with phenolphthalein as the indicator. The Hopkins method (5) uses sodium chloride, while the Jones method (7) uses calcium acetate. The results of this work are presented in Table 8. Table 8. Relation of acid added to the soil to the acidity measured by various methods expressed as parts per million of calcium carbonate. Net acid - - - - 1 Laboratory Total Carbonates added Aclqgycgfciflerialdggrfiggzg: ent acid in original 1 equivalent Number added soil to calcium Veitch Hopkins Jones " carbonate method metnod method I 25871 ‘ 312 0 312 410 36 320 25873 812 0 812 1250 48 976 31880 l 1562 450 1112 1607 348 1584 26103 1000 130 870 1250 81 1008 31888 i 781 60 721 821 186 1334 31896 I 1250 180 1070 1607 216 1584 31890 ) 1310 150 1160 1607 78 1680 31804 2615 170 2445 2680 213 2128 31914 I 2190 180 2010 2320 33 2400 31820 1815 490 1325 1960 62 1696 31321 3750 1440 2310 3035 27 2368 Average (11) ______________________________ .. 1286 1686 121 1553 LIBRARY- A 8: M COLLEGE, CAMPUS. E1-332-6M-L180 TEXAS AGRICULTURAL EXPERIMENT STATIUN A. B. CONNER, DIRECTOR COLLEGE STATION, BRAZOS COUNTY, TEXAS BULLETIN No. 442 MARCH, 1932 A DIVISION OF CHEMISTRY RELATIONS OF BUFFER CAPACITY FOR ACIDS A TO BASlClTY AND EXCHANGEABLE BASES I OF THE SOIL AGRICULTURAL AND MECHANICAL COLLEGE OF TEXAS - T. 0. WALTON, President STATION Administration: A. B. Conner, M. S., Director R. E. Karper, M. S., Vice-Director Clarice Mixson, B. A. Secretary M. P. Holleman, Jr., Chief Clerk . J. K. Francklow, Asst. Chief Clerk Chester Higgs, Executive Assistant Howard Berry, B. S., Technical Asst. Chemistry: G. S. Fraps, Ph. D., Chief; State Chemist S. E. Asbury, M. S., Chemist J. F. Fudge, Ph. D., Chemist E. C. Carlyle, M. S., Asst. Chemist. T. L. Ogier, B. S., Asst. Chemist A. J. Sterges, M. S., Asst. Chemist Ray Treichler, M. S., Asst. Chemist W. H. Walker, Asst. Chemist Velma Graham, Asst. Chemist Jeanne F. DeMottier, Asst. Chemist R. L. Schwartz, B. S., Asst. Chemist C. M. Pounders, B. S., Asst. Chemist Horticulture: S. H. Yarnell, Sc. D., Chief *‘“L. R. Hawthorn, M. S., Horticulturist H. M. Reed, M. S., Horticulturist J. F. Wood, B. S., Horticulturist L. E. Brooks, B. S., Horticulturist Range Animal Husbandry: J. M. Jones, A. M., Chief B. L. Warwick, Ph. D., Breeding Investiga. S. P. Davis, Wool Grader Entomology: F. L. Thomas, Ph. D.,.Chief; State Entomologist . H. J. Reinhard, B. S., Entomologist R. K. Fletcher, Ph. D., Entomologist W. L. Owen, Jr., M. S., Entomologist J. N. Roney, M. S., Entomologist J. C. Gaines, Jr., M. S., Entomologist S. E. Jones, M. S., Entomologist F. F. Bibby, B. S., Entomologist S. W. Clark, B. S., Entomologist W. Dunnam, Ph. D., Entomologist **“R. W. Moreland, B. S., Asst. Entomologist C. E. Heard, B. S., Chief Inspector C. Siddall, B. S., Foulbrood Inspector S. E. McGregor, B. S., Foulbrood Inspector Agronomy: STAFF? Veterinary Science: *M. Francis, D. V. M., Chief H. Schmidt, D. V. M., Veterinarian I. B. Boughton, D. V. M., Veterinarial **F. P. Mathews, D.V.M., M.S., Veterinar W. T. Hardy, D. V. M., Veterinarian ———— ————, Veterinarian Plant Pathology and Physiology: J. J. Taubenhaus, Ph. D., Chief W. N. Ezekiel, Ph. D., Plant Pathologis W. J. Bach, M. S., Plant Pathologist C. H. Rogers, Ph. D., Plant Pathologist Farm and Ranch Economics: L. P. Gabbard, M. S., Chief W. E. Paulson, Ph. D., Marketing C. A. Bonnen, M. S., Farm Management **W. R. Nisbet, B. S., Ranch Management **A. C. Magee, M. S., Farm Management Rural Home Research: Jessie Whitacre, Ph. D., Chief Mary Anna Grimes, M. S., Textiles Elizabeth D. Terrill, M. A., Nutrition Soil Survey: T. Carter, B. S., Chief E. H. A. H. Templin, B. S., Soil Surveyor Bean, B. S., Soil Surveyor R. M. Marshall, B. S., Soil Surveyor **M. W. Beck, B. S., Asst. Soil Surveyor Botany: V. L. Cory, M. S., Acting Chief S. E. Wolff, M. S., Botanist Swine Husbandry: Fred Hale, M. S., Chief Dairy Husbandry: O. C. Copeland, M. S., Dairy Husbandman Poultry Husbandry: R. M. Sherwood, M. S., Chief J. R. Couch, B. S., Asst. Poultry Hsbdm: Agricultural Engineering: H. P. Smith, M. S., Chief Main Station Farm: G. T. McNess, Superintendent Apiculture (San Antonio): B. Parks, B. S.,- Chief A. H. Alex, B. S., Queen Breeder Feed Control Service: F. D. Fuller, M. S., Chief James Sullivan, Asst. Chief B. Reynolds, Ph. D., Chief S. D. Pearce, Secretary R. E. Karper, M. S., Agronomist ~J. H. Rogers, Feed Inspector P. C. Mangelsdorf, Sc. D., Agronomist K. L. Kirkland, B. S., Feed Inspector D. T. Killough. M. S., Agronomist S. D. Reynolds, Jr., Feed Inspector H. E. Rea, B. S., Agronomist P. A. Moore, Feed Inspector B. C. Langley, M. S., Agronomist E. J. Wilson, B. S., Feed Inspector Publications: H. G. Wickes, B. S., Feed Inspector A. D. Jackson, Chief SUBSTATIONS No. 1, Beeville, Bee County: No. 9, Balmorhea, Reeves County: R. A. Hall, B. S., Superintendent J. J. Bayles, B. S., Superintendent No. 2, Lindale, Smith County: P. R. Johnson, M. S., Superintendent """B. H. Hendrickson, B. S., Sci. in Soil Erosion ""-‘R. W. Baird, B. S., Assoc. Agr. Engineer . 3, Angleton, Brazoria County: R. H. Stansel, M. S., Superintendent H. M. Reed, M. S., Horticulturist No. 4, Beaumont, Jefferson County: R. H. Wyche, B. S., Superintendent “‘*H. M. Beachell, B. S., Jr. Agronomist No. 5, Temple, Bell County: Henry Dunlavy, M. S., Superintendent C. H. Rogers, Ph. D., Plant Pathologist H. E. Rea, B. S., Agronomist. '_ S. E. Wolff, M. S., Botanist ‘ V. Geib, M. S., Sci. in Soil Erosion "‘*H. O. Hill, B. S., Jr. Civil Engineer N0. 6, Denton, Denton County: P. B. Dunkle, B. S., Superintendent "*1. M. Atkins, B. S., Jr. Agronomist No. 7, Spur, Dickens County: R. E. Dickson, B. S., Superintendent B. C. Langley, M. S., Agronomist No. 8, Lubbock, Lubbock County: D. L. Jones, Superintendent Frank-fiiainefs, Irrig. and Forest Nurs. Teachers in the School of Agriculture Carrying G. W. Adriance, Ph. D., Horticulture S. W. Bilsing, Ph. D., Entomology V. P. Lee, Ph. D., Marketing and Finance D. Scoates, A. E., Agricultural Engineering A. K. Mackey, M. S., Animal Husbandry ‘Dean School of Veterinary Medicine. No. l0, College Station, Brazos County: R. M. Sherwood, M. S., In Charge ' L. J. McCall, Farm Superintendent No. 11, Nacogdoches, Nacogdoches County: H. F. Morris, M. S., Superintendent **No. 12, Chillicothe, Hardeman County: J. R. Quinby, B. S., Superintendent **J. C. Stephens, M. A., Asst. Agronomist No. 14, Sonora, Sutton-Edwards Counties: W. H. Dameron, B. S., Superintendent I. B. Boughton, D. V. M., Veterinarian W. T. Hardy, D. V. M., Veterinarian O. L. Carpenter, Shepherd **O. G. Babcock, B. S., Asst. Entomologist No. 15, Weslaco, Hidalgo County: W. H. Friend, B. S., Superintendent S. W. Clark, B. S., Entomologist W. J. Bach, M. S., Plant Pathologist J. F. Wood, B. S., Horticulturist No. 16, Iowa Park, Wichita County: C. H. McDowell, B. S., Superintendent L. E. Brooks, B. S., Horticulturist No. 19, Winterhaven, Dimmit County: E. Mortensen, B. S., Superintendent **L. R. Hawthorn, M. S., Horticulturist Cooperative Projects on the Station: J. S. Mogford, M. S., Agronomy F. R. Brison, B. S., Horticulture l/V. R. Horlacher, Ph. D., Genetics J. H. Knox, M. S., Animal Husbandry A. L. Darnell, M. A., Dairy Husbandry tAs of March 1, 1932. “In cooperation with U. S. Department of Agriculture. The buffer capacity of a soil for acids is measured by the quantity of acids required to attain a given degree of acidity 0r to change the degree of acidity to a given extent. It is of both practical and scientific importance, in that it measures the re- sistance 0f the soil t0 acidifying agencies. Methods of measur- ing buffer capacity are studied in this Bulletin, together with the relation of the buffer capacity to the carbonates, to ex- changeable bases, and to other properties of the soil. If un- suitable methods are used, the mixtures of soil maybe acid even though undecomposedt carbonates remain. The adequate expression of the buffer capacity of soil requires the construc- tion of a curve, or statement of the total buffer capacity to a definite degree of acidity (pH) and of the specific buffer capa- city between given pairs of pH values. The addition of soluble salts to an acid soil may increase its acidity; washing out soluble salts may decrease its acidity. The lime-requirement methods of Veitch and of Jones recovered approximately the net quantity of acid added to the soil, but the method of Hop- kins recovered! only a small proportion of it. The exchange- able hydrogen corresponds to the net acid on some soils treated with acid, but with other soils is decidedly lower than the net acid, which indicates removal of base from other compounds besides the exchange complex. The total exchange capacity of the soil was not changed by the treatments of acid to estimate buffer capacity or acid consumed. Treatment with acid measures only approximately the ex- changeable bases in the soil, since the net acid consumed by the acid-consumed process represents 86 per cent of the base in the exchange complex, and that by the Kappen method, 81 per cent. The percentage of exchangeable hydrogen varies with different soils at the same degree of acidity (pH) pro- duced by treatment with acid. The base-exchange complex appears to consist of several compounds. The bases dissolved from the soil by dilute acids come from the carbonates, the base-exchange complex, and other compounds. The percentage taken from the base-exchange complex depends upon the de- gree of acidity (pH) and the nature and amount of the ex- change compounds present. The basicity of liming materials containing silicates or phos- phates can be estimated by the method for buffer capacity. Direct solution in acid is likely to give excessively high results on such materials. CONTENTS Page Introduction __________________________________________________________________________________________________ _- 3 Previous work _________________________________________________________________________________________________ ~- 6 Preliminary tests with calcium carbonate .............................................. -_ 7 Method used for buffer capacity ........................................................... _-_ 8 Estimation of carbonates ............................................................................ --11 Effect of time on the action of acid on the soil .................................. --12 Influence of time of contact on amount of acid consumed by soils high in carbonates .............................................................................. __12 Influence of time of contact on acidity of soils high in carbonates_---14 Types of buffer curves of soils ............. ................................................ -_17 Effect of washing out the electrolyte upon the buffer capacity ...... --20 Effect of potassium chloride on the buffer capacity .......................... --22 Effect of potassium chloride with the acid upon the buffer capacity_-23 Quantitative relations of the acidity of the residue to the quantity of acid used .............................................................................................. __26 Lime requirement of soil residue ................................................ __26 Acidity of the soil residue by estimated barium hydroxide---_-27 Acidity of soil residue in terms of exchangeable hydrogen _______________ _.29 Relation of carbonates to the buffer capacity ______________________________________ "33 Relation of the base-exchange complex to buffer capacity __________________ -33 Effect of acid on the exchange complex ________________________________________________ __37 Relation between the acid-consuming power and the base-exchange capacity of the soil ________________________ Q _______________________________________________________ I40 Replacement of exchangeable bases by hydrogen ............................... __44 Nature of the base-exchange complex _____________________________________________________ __47 Relation between certain other soil factors and total exchange capacity ................................................................................................... __47 Influence of soil material other than carbonates and base-exchange complex on buffer capacity_____ _ ______ __48 Method for basicity of liming materials and fertilizers ______________________ __49 Summary ______________________________________________________________________________________________________ __5() BULLETIN N0. 442 MARCH, 1932 RELATIONS OF BUFFER CAPACITY FOR ACIDS TO BASICITY AND EXCHANGEABLE BASES OF THE SOIL G. S. FRAPS AND J. F. FUDGE Soil acidity is -an important problem in the eastern and southern parts of the United States. Large areas of soil are either suffi- ciently acid to require treatment with lime, or liable to become acid under the usual conditions of agriculture. In the southwestern and western parts of the United States, soils are more basic in character than those just mentioned. Texas has some acid soils and some which may become acid, but large areas of Texas are covered by basic soils, some highly so. In a previous bulletin (Bulletin 400) the term basicity was arbitrarily used t0 designate the quantity of acid neutralized by a soil, when applied in excess, under definite conditions, expressed in terms of calcium carbonate. The buffer capacity of a soil for acids is measured by the quantity of acid required to secure a def- inite degree of acidity, or to bring about a definite change in the degree of acidity. The basicity of a soil may also be used to desig- nate its total buffer capacity up to a selected degree of acidity. A number of questions arise in connection with the establishment of this end point to be used in measuring the basicity of the soil. The degree of acidity is expressed in terms of the hydrogen ion concen- tration expressed as pH. A pH of 7 is neutral. Below 7, it is acid; above 7, it is basic. The pH is a logarithmic expression, so that a medium having a pH of 5 has ten times the hydrogen ion concentra- tion of one with a pH of 6. A knowledge of the basicity of soils or the buffer capacity for acids is needed in connection with a number of practical questions. A soil of low basicity may become acid through ordinary natural weathering processes, or by applications of fertilizer such as ammon- ium sulphate, which tends to produce acids. Such a soil would also require small amounts of sulphur or other acidifying materials to render it acid, if it should be desirable to do so for experimental purposes, or to render it suitable for plants (such as certain flowers) which prefer an acid medium, or to aid in controlling plant diseases. The measurement of the degree of basicity of a soil would thus show the extent to which it would be susceptible to the influences mentioned above, and would indicate the quantity of acidifying ma‘- terial which could safely be used without danger of injurious acidity, or which should be used to secure a desired degree of acidity. The buffer capacity measures the ease with which the reaction of the soil may be changed by treatment. The treatment may be arti- ficial as in the case of the decrease in acidity following the applica- tion of one of the various forms of lime or of basic slag to an acid 6 BULLETIN N(). 442, TEXAS AGRICULTURAL EXPERIMENT STATION soil, or in the acidification of the soil with acid-forming fertilizers, such as ammonium sulfate or the addition of sulphur. The change in acidity may be natural, as in the removal of basic material by weathering and the leaching action of water. The change may be either beneficial or detrimental to plant growth, either directly or indirectly. Legumes require a neutral or alkaline soil for best growth. Certain plant diseases, such as potato scab and cotton root rot, apparently are most easily controlled in acid soils. The buffer capacity of the soil and its determination have consequently received considerable attention by soil chemists. A large amount of work has been done on methods for the deter- mination of the amount of lime required to neutralize acid soils, Other work has been done on the buffer capacity of soils for acids. _ The present study on the buffer capacity of soils originated in con- nection with the study of the possibility of making certain soils suffi- ciently acid by treatment with sulfur to inhibit the growth of the ' organism causing the cotton root-rot disease. PREVIOUS WORK Several methods have been proposed for the determination of the buffer capacity of soils for acids. Pierre (19-21) proposed that to 20 grams of soil in collodion bags in extraction flasks, various amounts of acid or base be added, the volume made up to 100 cc., and after three days, the hydrogen ion concentration (pH) of the clear solution outside of the bag be determined colorimetrically. This method is satisfactory for soils of low buffer capacity but is subject to a number of objections when the soil has a high buffer capacity. No allowance is made for a stirring of the soil sufficiently to insure complete reaction between carbonates and acid. The collodion is attacked and the sack destroyed by acid strong enough to neutralize carbonates or to acidify soils of high buffer capacity. The method is, therefore, not applicable to a large group of soils of great agricul- tural importance. Lemmermann and Fresenius (15) treat 100 grams of soil in 100 cc. of water with 10 cc. of 0.1N nitric or hydrochloric acid or 0.1N sodium hydroxide and determine the hydrogen ion concentration (pH) of the mixture. The amount of hydrogen or hydroxyl ions which have disappeared is calculated in the percentage of the amount added. This method merely gives the acid or base consumed by the soil under the conditions of the treatment. Hissink (26, p. 26) calls the buffer value the amount of lime (CaO) in grams which must be taken up by_ or removed from 100 kg. of soil in order that the pH may rise or fall by 0.1. He regards the total buffer capacity of the soil for acids a_s the total exchangeable bases in the soil. The method previously followed in this laboratory (3) has been RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 7 to add acid and water to separate portions of the soil and test the supernatant liquid for hydrogen ion concentration. The tests are continued on separate portions of the soil until a pH between pH 4.7 and 4.3 is reached. The method is satisfactory, but slow. It was also desired to secure a method which would give the buffer capacity at any desired degree of acidity (pH). PRELIMINARY TESTS “IITH CALCIUM CARBONATE Any method for basicity or buffer capacity for acids must in- volve the complete solution of calcium or magnesium carbonates present in the sample of the soil. It is true there are acid soils which contain nodules of hard limestone rock, but these are excep- tional cases and require special treatment. A method for buffer capacity which gives an acid end-point, and yet permits undecom- posed carbonates to remain in the sample, is clearly faulty. Dis- solved carbon dioxide and calcium bicarbonate in solution affect the degree of acidity. A study was made of the acidity of mixtures of calcium carbonate and 0.1 N nitric acid treated in different ways. Varying amounts of acid were added to 0.5 gram of precipitated calcium carbonate, the mixtures well stirred in different ways, and the hydrogen ion con- centration determined in the supernatant liquid. The stirring varied from light stirring or shaking by hand to a prolonged, vigorous stirring with a stirring apparatus operated by a motor. The 0.5- gram sample of calcium carbonate was found to neutralize 96.2 cc. of 0.1N acid. The methods tested were as follows:- (1) The acid was added with hand shaking in such a way that a good suspension was secured during the addition, and the sus- pension was then allowed to stand 24 hours in a stoppered flask. The pH was then determined colorimetrically. (2) The mixture was stirred with a motor-operated stirring machine provided with a stirring rod supplied with the apparatus, in which a simple bend of about 30 degrees was made. This stirrer left a quiet space in the center of the beaker. (3) The mixture was stirred with the machine mentioned above, provided with a special stirring rod so designed that no quiet space existed at the center of the beaker. The results of these tests with calcium carbonate are presented in Table 1. The table shows that in the presence of calcium carbonate the buffer capacity would depend upon the details of the procedure used in the treatment of the soil with acid. With moderate hand stirring, the mixture had a reaction of pH 4.95 when over 20 per cent of the calcium carbonate was still undissolved, even after the mixture had stood overnight. With vigorous stirring by the machine, the mixture has practically the same reaction (pH 5.00) when the 8 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION acid was just about sufficient to neutralize all the calcium car- bonate, while with vigorous stirring with the modified stirring rod on the machine, the same mixture had a pH of 6.20. Under the most favorable conditions, the calcium carbonate was completely neutralized at pH 6.20; under less favorable conditions, the amount of acid to neutralize the calcium carbonate gave a pH of 5.0; and under still less favorable conditions the pH was 3.4. Table 1—Acidity of mixtures of calcium carbonate and 0.1N nitric acid. Nitric acid 1 1 added to 0.5 ‘ Excess acid l With shaking Purchased Modified gram galcium c and standing stirrer ; stirrer carbonate ' 0.1 N DH DH y pH (Equals 96.2 cc) r 95.0 I + 1.3 cc 2.35 3.90 3.50 97.5 f -|- 1.3 cc 3.60 4.40 3.50 97.0 y -|- 0.5 cc I 3.00 4.70 3.90 96,0 = _ 0.2 cc 3.40 5.00 5.20 95.0 - 1.2 cc l 3.35 | 5.05 7.05 94.0 - 2.2 cc 3.30 5.25 7.25 93.0 - 3.2 cc 4.55 5.25 7.05 91.0 - 5.2 cc 4.90 5.35 7.25 35.0 l -11.2 cc 4.90 5.75 7.45 30.0 -15.2 cc 5.25 5.75 7.70 75.0 f -21.2 cc 4.95 5.75 7.70 \ Some of this effect is, no doubt, due to dissolved carbon dioxide and to the presence of calcium bicarbonate in solution. The vigor- ous stirring with exposure to aireliminates the carbon dioxide and decomposes the bicarbonate. The vigorous stirring also enables the calcium carbonate to go into solution, as shown by the pH of 7 or over, when an excess of calcium carbonate was present. _ According to this work, a mixture of- soil and acid with a pH of 6.2 may retain undecomposed calcium carbonate, under some con- ditions, while under less favorable conditions for an equilibrium the mixture may have a pH of 5.0 or even 3.4 and still retain undecom- posed calcium carbonate. This would, of course, enter into reaction later on and decrease the acidity. In measuring the basicity of a soil, it would be necessary to adopt an end point of pH 6, if the procedure is to be effective. It would be well for one to make preliminary tests of procedure and apparatus with the use of carbonate of lime and acid, to see what acidity (pH) was secured with the apparatus and procedure to be used. METHOD USED FOR THE BUFFER CAPACITY In the work previously reported (3), 8 grams of soil was treated with varying amounts of sulfuric acid, the volume made up to 100 cc., and allowed to stand overnight. The pH was determined color- RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 9 imetrically on the supernatant liquid the following morning. The process was repeated with other portions of soil, and quantities of acid added based upon the results previously secured until the de- sired degree of acidity was secured. The buffer capacity of the soil was expressed as the parts of sulfur per million of soil necessary to give a reaction between pH 4.7 and pH 4.3, 1 cc. of 0.01N acid being equivalent to 20 p.p.m. of sulfur. A number of modifications of this method were tested in the present study for the purpose of shorten- ing the procedure and making it more accurate. The amount of soil to be used depends upon the form in which the results are expressed; 8 grams for sulfur because of the ease of calculation of results to sulfur equivalent used in connection with the work on the acidification of soil for the control of cotton root rot. If the results are desired in terms of calcium carbonate or hydrogen, 10 grams of soil should be used. The acid used was hydrochloric acid of three strengths, 0.01 N, 0.1 N, and 1.0 N. The use of sul- furic acid was discontinued because it was thought possible that sulfuric acid on soils high in calcium carbonate might produce an insoluble layer of calcium sulfate around the carbonate particles, thus preventing their complete neutralization, and giving low results for buffer capacity. Table 2. Estimation of buffer capacity, for acid, with acid-consuming power as the basis, on 10 grams soil. Calcium Calcium If acid Acid added carbonate Acid added carbonate consumed correspond- correspond- 15 Min. "-—""——-- ‘ng t° {m8 "~—*————— mg t° a?“ used, 1n I used, in is Normal cc parts per Normal l cc parts per million i million % I 0- 3 0.01 5 100 0.01 10 200 3- 6 0.01 10 200 0.01 20 400 6- 9 0.01 15 300 0.01 30 600 9-12 0.01 20 400 0.01 40 800 12-15 0.01 25 500 0.01 50 1000 15-18 0.01 30 600 0.01 60 1200 18-21 ‘0.01 35 700 0.01 7O 1400 21-25 0.01 40 800 0.01 80 1600 25-30 0.01 60 1200 0.1 12.0 2400 30-40 0.1 12.5 2500 0.1 17.5 3500 40-50 0.1 17.5 3500 0.1 20.0 4000 50-60 0.1 35.0 7000 0.1 40.0 8000 60-70 0.1 50.0 10000 0.1 60.0 12000 70-80 0.1 75.0 15000 0.1 85.0 17000 Tests were made of the effect of various periods of stirring, from 5 minutes to 3 hours, with the determination of the degree of acidity (pH) the following morning. This work showed that a 15-minute stirring of the mixture of soil and acid at a speed of 300-400 r.p.m. was sufficient to give a pH reading identical with that secured 10 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION after prolonged stirring. Consequently the 15-minute period was adopted. The number of mixtures of soil and acid required was reduced by basing the first addition of acid to be made on the quantity of acid con- sumed by the soil. First, 5 grams of soil was treated with 50 cc. of 0.2 N hydrochloric acid, stirred on the stirring apparatus for 15 minutes, and filtered; 20 cc. of the filtrate was then heated to boiling and titrated with 0.2 N sodium hydroxide, phenol phthalein being used as the indicator. If the acid consumed exceeded 8 per cent, the pro- cedure was repeated, except that 10 grams of soil, 100 cc. of 1.0 N acid, and 0.5 N sodium hydroxide were used. The second method was also used if the soil was known to contain more than 8 per cent of calcium carbonate. The first additions of acid in the pro- cedure for buffer capacity were based upon the acid-consuming power, according to Table 2. The amounts "of acid and the buffer values are based upon 10 grams of soil and expressed as parts per million of calcium carbonate. Subsequent additions t0 new portions of the soil depend upon the acidity resulting from the first additions, as shown in Table 2. Individual soils of similar acid-consuming power may vary considerably in the quantity of acid required to reach a definite reaction, but this procedure gives a very convenient starting point and shortens the work required. Determinations of pH were in almost all cases made by means of the potentiometer using quinhydrone and Veibel’s electrode, and with the observance of the various precautions necessary for accurate work with that method, as outlined by the International Society of Soil Science (26). When the colorimetric procedure was followed, Clark and Lubs indicator solutions and LaMotte’s Roulette com- parator were used. The method as finally adopted, based upon the results of the experi- ments mentioned above, was as follows: To 10 grams of soil, add the desired amount of hydrochloric acid, (Table 2) dilute to a total volume of 100 cc., stir for 15 minutes on the motor-operated stirring apparatus, and let stand overnight. Pour off 60 cc. of the supernatant liquid, transfer the remaining soil sus- pension after thorough mixing to a 50-cc. beaker, add quinhydrone and determine the pH by the regular potentiometric procedure. After securing a number of determinations covering the range of acidity between about pH 6.5 and pH 3.5, the data are plotted on regular graph paper, the pH being plotted on the Y axis and the amounts of acid on the X axis. A curve termed the buffer curve is constructed by joining these points as shown in Figure 1. This method of recording the data has a number of advantages over the method in which a definite degree of acidity (pH) is finally reached as the end point. It allows easy estimation of the buffer capacity (d pH/ d acid) over a considerable range of values of either the acid added or the pH secured. By interpolation on the curve, depending RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 11 upon the axis to which the interpolation is referred, the buffer capacity may be expressed as the total amount of acid required to reach a given degree of acidity (pH) considerably more closely than in the method previously used where the end point varies between pH 4.7 and pH 4.3. The buffer capacity may also be expressed as the amount of acid required to give a change of acidity (pH) be- tween two values of pH, or as the change in acidity (pH) caused by a definite amount of acid between any two pH values. In this laboratory, the data and curves secured by this procedure are called the “pH acid” data and curves. This is a convenient ex- pression for either form of buffer capacity. For example, “pH 400 acid” would call for the pH which was reached when the amount of acid equivalent to 400 p.p.m. of calcium carbonate was added to the soil. On the other hand “pH acid 6.0” would call for the amount of acid required to change the soil to a pH of 6.0, “pH acid 6.0-5.0” would call for the amount of acid required to cause the indicated increase in acidity. The data secured from the buffer curves are cal- culated to the parts of calcium carbonate per million parts of soil which are equivalent to the various amounts of acid used. _ ESTIMATION OF CARBONATES The carbonates were determined by decomposing them with acetic acid as recommended by Lunt (16), collecting the carbon dioxide in barium hydroxide, and titrating the residual barium hydroxide. The reaction chamber is a 150-cc. extraction flask, closed by a No. 8 rubber stopper supporting an 8-inch condenser and a dropping funnel. A source of air free of carbon dioxide is connected to the top of the dropping funnel. The top of the condenser is provided with a l-hole rubber stopper from which a rubber tube leads to a Fisher absorption tower containing 200 cc. of standard barium hydroxide solution. The tower is in turn connected to suction. The system, without the tower, is first swept out with air free of carbon dioxide. Then, proper precautions being taken against entry of ordinary air into the system, the absorption tower is connected in place and 50 cc. of a mixture of one part of. acetic acid to 2 parts of water is placed in the dropping funnel, which has the stop cock closed. Lunt (16) has shown that this strength of acetic acid will not decompose the organic matter of the soil, with consequent evolu- tion of carbon dioxide. The dropping funnel is stoppered with a rubber stopper through which purified air is led. In decomposing the sample, slight suction is applied, and the stop cock cautiously opened. After the vigorous evolution of carbon dioxide has ceased, the acid is slowly brought to a boil, boiled 2 minutes, and allowed to cool. Care must be taken that there is no back pressure at any time. An aliquot of the barium hydroxide solution is titrated with stan- dard hydrochloric acid. By varying the weight of the sample and 12 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION the normality of the barium hydroxide, accurate results are obtained over a wide range of carbonate content. The results secured, cal- . culated to parts per million of calcium carbonate, are used in various f tables in this Bulletin. EFFECT OF TIME ON THE ACTION OF ACID ON THE SOIL The data secured in the procedure for buffer capacity are of little practical value unless they can be utilized for experimental purposes with a reasonable degree of accuracy. In such work, the element of time plays an important part. The questions thus arise as to whether or not there is with time a recovery from the acid treatment, that is, a decrease in acidity, and if so, how great it is and how long a time is required for the attainment of final equilibrium. A large number of data which bear directly upon this point are presented by Fraps and Carlyle (3, Tables 4 and 5), who used the original method from which the present method for buffer capacity here described has been developed. They used soils low in carbonates which required acid in amounts up to an equivalent of 2800 p.p.m. of calcium carbonate to produce, with the regular method then used, a pH value of approximately 4.3. The soil may be expected to de_ crease slightly in acidity. The average pH of 24 soils after standing over night, 3O days, 60 days, 5.5 months, and one year were 4.4, 4.9, 4.9, 4.7, and 4.7 respectively. These data show that the soils decrease slightly in acidity but that the results secured with the method for buffer capacity indicate the pH value of the soils after they have been treated with the required amount of acid. ,The procedure may therefore be used with assurance that the results secured may be used where acidification of the soil is for any reason desired. More- over, the data secured represent, .with a satisfactory degree of accuracy, the reaction of the soil after treatment. INFLUENCE OF TIME OF CONTACT ON AMOUNT OF ACID CONSUMED BY SOILS HIGH IN CARBONATES A comparison of the data secured in the regular acid-consumed procedure with the data from the determination of carbonates indi- cated that even with vigorous stirring and an excess of normal hydrochloric acid, there was a possibility that in a few highly cal- careous soils all of the carbonates present might not be neutralized. It was thought that possibly with an increase in the amount of time allowed for the operation a more nearly complete neutralization of the carbonates might be assured. In order to determine whether or not the period of contact of acid on the soil had any influence on the results secured, the following experiment was conducted. Two portions of each of 12 soils high in carbonates were weighed out, after a very careful mixing of the samples. One portion was sub- jected to the regular treatment for estimating acid consumed, that is, RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 13 stirring for 15 minutes, then filtering and titrating an aliquot of the filtrate with standard sodium hydroxide. The other portion was stirred for 15 minutes and allowed to stand over night before filter- ing. A comparison of the results obtained by the two methods is presented in Table 3. Table 3. Effect of period of contact on acid consumed. Acid consumed expressed as % Laboratory ‘ Calcium _ w“ pialcium" carbonate _ _ __A _ carbonate Number content, % 15 Minutes l Overnight 25869 4.56 I 4.00 4.00 31329 5.46 5.78 5.74 26089 5.90 6.88 7.02 29331 8.59 8.82 8.68 31800 9.03 9.42 9.34 28011 9.91 “ 10.40 10.18 31884 12.74 13.14 12.96 30963 14.30 14.48 _ 13.92 25905 15.88 15.38 15.18 26817 17.77 18.00 17.92 31330 25.43 26.50 26.20 31882 40.40 40.08 40.04 Average (12) ........ 14.16 14.41 14.27 l An examination of Table 3 shows that the increase in the length of the period of contact of the soil and acid did not increase the amount of acid consumed, and consequently is not to be recommended, since the procedure with the shorter time is the simpler and equally accurate. ' In only two of the soils, Nos. 25869 and 25905, was the amount of acid consumed significantly lower than the total carbonates present, and in these two soils, the difference was quite small. The differ- ence can be explained as due to the presence of difficultly soluble carbonates which were decomposed by the boiling strong acetic acid but were resistant to the normal hydrochloric acid. In all other soils, the amount of acid consumed slightly exceeded the amount of carbonates present. The conclusions are that the 15-minute period of contact gives as good results as standing overnight and this shorter procedure is therefore to be recommended and that the acid-consumed procedure neutralizes practically all of the carbonates in the soil, even when the carbonate content exceeds forty per cent. 14 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION INFLUENCE OF TIME OF CONTACT ON ACIDITY OF SOILS HIGH IN CARBONATES In the determination of both the acid-consuming power and the total carbonate content of the soil, a considerable excess of acid is added to the soil. In estimating the buffer capacity, excess of acid is carefully avoided. Carbonates which would be neutralized quickly where an excess of acid is present, might g'o into solution only very slowly where the system has a pH value of, say, 5.0. In the latter case, the period of standing after the addition of the acid would have an important bearing upon the pH value secured. The question thus arises as to how long a period should be allowed between the addi- tion of the acid to the soil and the determination of the pH value of the system. The following work was done in order to secure data which might answer that question. Three portions of each of 12 calcareous soils were treated with the same amounts of acid. The regular pH acid determination was run on one portion. After standing over night, the other two por- tions were stirred for 15 minutes, and set aside for 24 hours. The pH of the second portion was then determined in the usual way. The third portion was stirred for 15 minutes and set aside for a second 24-hour period. The pH of the third portion was then deter- mined. The experiment thus gave pH acid data after the soil had stood in contact with the acid for 16, 40, and 64 hours. The results are presented in Table 4. Table 4. Influence of period of contact on acidity of soils. ‘i’ pH after standing Difference in pH due to change Laboratory ; _“—_—t~_i_” ; ‘ " " ” in time of standing Number I 16 f 4° i 64 ‘ 1s to s4 y 1s m 40 40 to 64 hours . hours l hours . hours l hours hours 25869 i 3.69 4.17 4.59 .48 .90 .42 31329 I 3.48 4.01 4.26 .53 .78 .25 26089 | 3.95 3.97 4.10 .02 .15 .13 31800 I 2.87 2.97 2.97 .10 .10 .00 29331 4.36 5.67 5.73 1.31 1.36 .05 28011 ( 4.28 4.53 4.65 .25 .37 .12 31884 I 5.18 5.55 5.68 .37 .50 .13 30963 4.72 4.87 5.01 .15 .29 .14 25905 4.43 4.35 4.47 .08 .03 .11 26817 3.69 4.29 4.33 .60 .64 .04 31330 3.76 5.07 5.03 1.31 1.27 .04 31882 f 4.25 I 4.95 4.93 l .70 .68 ‘ .02 v . L g | g g Average (12)_...l 4.06 ‘ 4.53 4.65 .49 59 I 12 l l 15 RELATIONS OF BUFFER CAPACITY FQR ACIDS OF THE sou. v -.. 2.3 .23.. 82.3 _ 2.3. . .238 8.3.... .......................... .._. .... =55... v2.8 2... 5.0.21 .8... 8.8.5 8...... ocmm ooowm ocmmm 83v ooomm . .. wocmm v A. 8mm 8...... 2.3. 8mm. .88. 2.8. ......................... :58. v5.8 v.58 2... 5.8m .83. ....-v 8v». cowm 83. 88. comm 8mm. 2.20 QZQEH. 83v b no 85 8m. 88. 88 2.3 88. Ede. .320 Oafimfiofl mwwmm v -.. .3... .3... .88. 2.8 .3... 3v... ....................................... =58. v.58 2... 55w 8...... v ... 2... 2.... 2.5 28v 2.3 2.3 ............................................. <55... v... @588... 38.. v ... 8...... 2.... 3v. 8...... 2.3 ..v3 ............................................... .5... 8...... 8...... 88v v -.. 3.. 3.. 8v. 3... 2.8 2.8 .................................. .558. v.58 8.... 2.2:? .223. v ... 3.. 3N. 8...... 38 8v. 3.. .................................. ‘v58. v... S... .......5.< .3... v -.. 3v 8. 8... 3... 8v... 2.v ............................... =58. v.28 2... 8.285.. .88 b -.. 88 3... 2.3. comm 3N. cmo .430 wflfiri 383 v ... 3.. 28 8S 3B 3v. .3... ............................... =58. v.58 2... ........5< 3.2.. v -.. 3.. 8.. 88 3.. 8.. ..v. ...................................... .....8. v.58 2... as... .2... ...-v 8v 8.. 8v. 8... 8v 8.. ............................... .455... v.58 2... o....:.5< 8v... v -.. 8.. 8.. 88 8.. 8v 8.. ............................... =58. v.58 B... 5.5.554 88.... v -.. o3 88 8m. 3v 23 8. ................................. :55... v5.8 9... 85...... 8.... v A. 3... 3v 38 8v. 8.. 3. .................................. =55... v.58 B... 8:26 .8... v ... 8.. 8v. 8v. 8v 8... 3.. ................................... <55... v.58 9... .88. 8...... v -.. c3 8.... 8.. 8.. 3N 8. ............................. ., 58. v.58 2... .8830... v8.8 v -.. 8v... 2... 3.... 3v. 3v. .. 3 58. v.58 B... 025...... 8.3.... v -.. 8v ...... .. 8... 8v 3. 3. .................................. :55... v.55. 2... .059... 233 v -.. 83 , 8.. 8.. 3. 8 .................................... .....8. v8.5. 2... v.83 www... v-.. 3. ...... .. 8v 3. ........ .. .. ................................................ ., .58 2.8. .88 od~ 8 c... o3 8 o6 e... c3 8... A2595“ sunufi HQ m. m. .8. in h... m... 20k wumconvwO on... Sm vwnfizz fiwmcm...m...wa....mmovwwww.....mfi v3.5.3“. 5&3... 150B m..w.%v..w.....w%.v.mnm .. %HOP.QHOBG~H Qvwconumo 2.5.3.3 we 203...... u»... 3...... mm wmmmoumxw fiflom mo v3.2.2.8 vowufim .m w??? BULLETIN NO. 442, TEXAS AGRICULTURAL EXPER-‘IMENT STATION 16 259.3 2. -8 .2228 2828 228$ 222.8 .2283. W .......................................................... .1288 or: _ 288 2A. 222. 222: 288w 288w 222224.. 888......» 2 ........................................... .582 .58 2.22.91 82m 2. -8 228 22:: 22:2: 28E: 28$: 222.5: \\\\\\\\\\\\\\\\\\\\\\\\\ .4 E22 $8 >28 ..a=¢§§_u 28m 2. -8 228 22:: 28$: 222.3 2228: .288: \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ -582 28 28:3 828w @385 228 82. 88$: 228: 2222: 8E2 _ ............................................................................ ., .288 2. -8 88 22: 2283 2283 22282 22282 _ ......................................... .. E8: 22» OM82: 388 ... -8 88 228 228w 2228 88E. 228w _ E22 28 2.5 :88 8 -8 222: 288 22228 25E. 28S .2828 2 E22 $8 .6282 222:... p -8 288 228 .288 2228 2222. .228... ...................................................... 1.38 @235 .828 8 -8 228 288 2228 .288 28$. 2222.... ............ .. =52 .38 .628: 82m 2. 4O 25m 25b 28$ 228.3 OQOFM 28$. 1 E50! 2122mm QC?“ EQNwH fizom 288m QQ é» 8.8 O6 Ow Q6 H: nOnm 8.8 o4» .Q.~.Q.Q HQ in |F|@.v in .6: in pom wwwnonnwO awninz 58m in c: wwcmsu fin: H a a 552280 mm v2.3 28m apounponwq 20w 2.22.88 nwwwsfl to.“ so 293:5 830m. mmpwnonnaO Awozflmuiouvlwuwconuwo E3223 COZZE .22: 3.58m ma 8322586 diow mo 281235 3.35M .m v1.18? RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 1'1 There is a significant decrease in the acidity of some soils with an increase in the time of contact. Eight of the twelve soils decreased in acidity when the period was extended from 16 hours to 40 hours, and two made still further decreases when the time was extended to 64 hours. In the consideration of these data, however, two points must be emphasized. First, the amount of acid added was suffi- cient to bring the soil to a relatively low pH value. There is un- doubtedly a greater shift if the reaction is 4.0 after the 16-hour period than if it had been 6.0. Second, with a soil as calcareous as these, and using an acid as strong as must be used for them, there is considerable opportunity for variations in pH acid values secured, even on identical treatments. Since there is in some cases such a large decrease in acidity due to increasing the period from 16 hours to 40 hours and the difference in cost of time and labor is small, the 40-hour period should be used with soils of high carbonate con- tent. TYPES OF BUFFER CURVES OF SOILS Texas soils studied in this work varied from a light dune sand to heavy calcareous soils. The acid consumed expressed as parts per million of calcium carbonate varied from 0 to 400,800. As will be shown later, these soils varied widely in content of total exchange- able bases. This selection of soils provided a wide range with respect to almost all of those soil characteristics ordinarily associated with buffer capacity. Only the section of the curve on the acid side of neutrality was studied. It was found that as long as there was any considerable amount of calcium carbonate left in the soil there was practically no change in reaction with increasing amounts of acid. Buffer curves for 60 soils were constructed. The data for 36 soils as interpolated on the curves are presented in Table 5. In this table all results are calculated to parts of calcium carbonate per million of soil. Table 5 contains the laboratory number of the soil, which is pre- sented here in order to facilitate comparison with other tables to be presented later. The soil type and the total carbonates, expressed as calcium carbonate, are given. The next three columns give the number of parts of calcium carbonate per million of soil which are equivalent to the acid required to bring the soil to a reaction of pH 6.0, pH 5.0, and pH 4.0, respectively. The next two columns give the amount of calcium carbonate equivalent to the acid required to cause a change of one unit in the pH value of the soil. Three types of buffer curves occur, as plotted in Fig. 1. The differences between the types are almost entirely due to the pres- ence or absence of carbonates. The three types may be briefly de- scribed as follows: Type I. These curves are concave toward the X, or acid, axis. In the soils having this type of buffer curve, a greater amount of 18 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION acid is required to cause the change from pH 5.0 to pH 4.0 than the change from pH 6.0 to pH 5.0. This group includes all of the soils relatively 10w in buffer capacity. The soils belong to Type 1 which are listed above soil No. 31331, in Table 3. This soil requires acid equivalent to 1470 p.p.m. of calcium carbonate to reach a pH of 6.0. Some of the soils more basic than \, Soil No. 31331 also have buffer curves of Type 1. TYPES or BUFFER CURVE-SQ pH ~70 -6.0 -50 -40 Tree I ———— Type 2 —-—-- N30 Type 3 ------- -- I ‘ T/pel» 3/0 940 I560 T/Pez-r 52500 6|26O T/pe5—-> 4375 6250 8|25 ACID A0050 As REM. CACO; Figure 1—Three types of buffer curves. Note also the wide differences in the quantities of acid added. RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 19 Since there are ‘practically no carbonates present, the buffer action in these soils is due to the difficultly soluble silicates for the most part, with perhaps the organic matter ‘playing a small part in some of the soils. Type II. The buffer curves of this type (Fig. 1) are convex toward the X, or acid, axis. In the soils having this type of buffer curve a much smaller amount of acid is required to change the reaction from pH 5.0 to pH 4.0 than to change from pH 6.0 to pH 5.0. The soils in this group are comparatively high in carbonates and have a high basicity. This shape of the curve is probably due to the fact that all of the carbonate has not been destroyed at pH 6.0, and most of this is destroyed in the change from pH 6.0 to pH 5.0. Small quantities of carbonate remain to be decomposed from pH 5.0 to pH 4.0 while practically all of the carbonate is destroyed before pH 4.0 is reached. In these soils, then, the carbonates are the I determining factor in the development of buffer action, to a greater extent than the silicates, which determined the buffer curve in the first group of soils. Attention should be called to the fact that not all of the soils high in carbonates have buffer curves of this type. Soils 23094 and 31800 have buffer curves falling very~ definitely with Type 1, altho the carbonate content is appreciable, as is shown by the amount of acid required to bring the soil to pH 6.0. This difference is probably due to the difference in the nature of the carbonates. For example, a much higher percentage of the total carbonates would be destroyed by mild acid treatment of a soil in which the carbonates were present as finely divided calcium carbonate than would be the case if the carbonates were in coarse granules. In general, however, the curves conform well to the type. Type III. Curves of this type approach a straight line. The soils having curves of this type are comparatively few, and are slightly calcareous in nature. In these few cases, the factors which determine a buffer curve of Type I are apparently approximately at equilibrium with those determining the curve of Type II. In no case, regardless of the nature of the soil or the buffer curve, was there anything like a definite end point or break in the curve. The data presented show conclitalvoly the futility of adopt- ing any procedure in which the buffer apacity is expressed by the pH value resulting from adding an arbitrary amount of acid to a given quantity of soil, as is suggested in the method of Lemmer- mann and Fresenius. They also show that a method similar to that proposed by Pierre or Hissink in which the buffer capacity is ex- pressed as the amount of acid required to cause a change between any two given pH values, while giving valuable information, by no means presents all the data pertinent to the buffer capacity of a particular soil. For example, for soils Nos. 25873 and 31331 the “specific buffer capacities” between pH 6.0 and pH 5.0 are 300 and 180 p.p.m of calcium carbonate, while the buffer capacities 2O BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION to pH 6.0 are 75 and 1470 p.p.m., and to pH 5.0 are 375 and 1650. The quantities of acid necessary to give soils Nos. 29315, an Amarillo fine sandy loam, and 26075, and Irving clay, a reaction of pH 6.0 are identical, equivalent to 1250 p.p.m. of calcium carbonate, but for pH 5.0 the quantities are 1550 and 2300, and for pH 4.0, 2100 and 4400. Thus, if pH 6.0 had been accepted as the single value desired, the two soils would have been practically identical. If, however, pH 4.0 had been the desired end point, soil No. 26075 would have had over twice the buffer capacity of soil N0. 29315. Soil N0. 29315 has an acid-comsuming power of 4000, while for soil No. 26075, the value is 15,500, or nearly four times as great. Obviously in order to express adequately the buffer action of these soils, two sets of data are required: (1) the “total buffer capacity” to a definite pH value; (2) the “specific buffer capacity” between a given pair of pH values, usually, for convenience, differing by one pH unit. EFFECT OF WASHING OUT THE ELECTROLYTE UPON THE BUFFER CAPACITY In connection with the study of the effect of acid upon the soil, it is desirable to know whether the degree of acidity secured by adding acid would be affected by washing out the products of the reaction, chiefly calcium chloride. It is known that the reaction is more acid when a soil is treated with a solution of an electrolyte such as potassium chloride than if it is treated with water. Some of the potassium chloride reacts with the insoluble exchange com- plex to produce potash salt of the exchange complex and hydro- cloric acid. The reaction is reversible and is indicated. as follows: HX+KClzKX+HCl If the acidity of a soil is less when the electrolyte is washed out in the laboratory, the acidity will likewise decrease if the electrolytes are washed out of the soil by rain. This matter was tested on a number of soils. Acid was added as previously described, and the degree of acidity (pH) estimated as usual. The soil was transferred quantitatively to Gooch crucibles provided with filter paper discs and washed three times with distilled water. The acidity (pH) of the residue was then determined as usual. The data, together with others to be dis- cussed later, are presented in Table 6. The figures in the column are to be compared with the original pH in the heading, namely, 6.0, 5.0, and 4.0. It is evident from these results that the washed soil has a lower degree of acidity than is secured by the regular procedure, or the pH is higher in every case. The average difference in pH is 0.7 at 6.0, 0.9 at 5.0, and 1.0 at 4.0. With some soils the difference is large, with others it is small; but in all cases the differ- ence is in the same direction—-a shift toward the alkaline side. The 21 RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 6 6.6. 6.6 6.6 6.6 6.6 6.6 A6: 666625. 6.6 6.6 6.6 6.6 6.6 6.6 ................................................................... =E62 .626 666166666 .6 6.6666 6.6. 6.6 6.6 66 6.6 . 6.6 E666 >666 .6561 66666 6.6 N6 6.6 66 >6 6.6 ......................... 1.66:6 626166.60 6 66666 6.6 6.6 6.6 6.6 6.6 6.6 ..................... .6 .......................... :53: >26 36:66 ~56 66:55 u 366m 6.6 6.6 6.6 6.6 6.6 6.6. ...... .. 6:662 36:66 ~56 55m _ 666$ 6.6. 6.... 6.6 6.6 6.6 6.6 866. :26 8.66861 66666 N... 6.6 6.6 6.6 6.6 6.6. . :56. E6 6618621 666:6 6.6 3. 6.6 6.6 6.6 6.6 S62 66:66 ~56 68.661 6.665 6.6. S. 6.6. 6.6 E62 66:66 6:66 Eswia _ 66666 6.6 6.6 6.6 6.6 6.6 N6 E6606 3:666 066G 6.6002 é 666$ 6.6 6.6. 6.6. 6.6 ........................................................... .5662 66:66 6E6 6:66:54 66666 66 6.6. 6.6 6.6 6.6 N6 ....................................................... .4652 66:66 6:66 666x630 6 66666 6.6 6.6. 6.6. 6.6 6.6 N6 .... .. = :82 66:66 6:66 1625 .6 666$ 6.6 6.6 6.6 6.6 6.6 66 ....................................................... 1:662 66:66 6:66 6666:6666 _ 66666 6.6 6.6 6.6 6.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16:66 2:5 __ C666 6m 66662 666 .666? 6M 6 .662» - 666:5: Si; . £63 at? Si? Si? . 3.33 10w 80am 05666603 _ wflgwwfl onvmmofi onwmmmww W 0665mm W 053.33 065m? Wfimvwmwmw 6.6 w: .86 E64 6.6 m: .86 26¢ 6.6 w: .86 E64 J _ 66.6663 _ .3066 via 50m W0 0.66555 MO 63636.6 5am; UONQQEOU Q5260: 10646656663 MO .w Q~QQP 22 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION curves drawn from the data parallel each other. If a soil is treated with the amount of acid to give a certain degree of acidity by the method previously described, and afterwards washed with dis- tilled water or exposed to rain, it will become less acid. This was found to take place in boxes of soil prepared with different degrees of acidity for experimental work, and was especially noticeable in the surface layer of the soil. The total buffer capacity is greater when the electrolyte is washed out. This must be allowed for in experimental or field work, the conditions under which the work is conducted being kept in mind. EFFECT OF POTASSIUM CHLORIDE ON THE BUFFER CAPACITY In both of the preceeding methods, there is some question as to whether or not the system is sufficiently buffered to prevent large differences in acidity (pH) resulting from small amounts of im- purities or special materials in the system. This subject was also studied in another way. After the elec- trolyte had been washed out, as described above, the soil residue was suspended in a solution of normal potassium chloride, and the acidity estimated. The results also are given in Table 6. As is seen by comparing the parallel columns, the acidity of the residue is usually higher when the electrolyte is added, the max- imum difference from the residue washed with water being 1.4 pH and the average 0.6 to 0.9 pH. It is noted, however, that the average acidity of the soil residue with the potassium chloride is practically the same as that of the original soil with acid. That is to say, the addition of the potassium chloride brings back the acidity to where it was before the electrolyte was washed out with water. The acidity of a soil will vary to some extent according to the quantity of soluble neutral salts added to it. The acidity would decrease when the salts are washed down by rain or irrigation water, unless ‘the irrigation water contained the same quantity of salt as the original soil or a larger quantity. When the salts come to the sur- face in dry weather, the acidity would increase. However, the soils of dry sections are usually slightly alkaline or neutral, so that this effect on acidity would seldom occur. The addition of soluble fertilizer salts, such as potassium salts, nitrates, or sulphate of ammonia, would increase the acidity of the soil, at least temporarily. The equation is:- 2HX+(NH4)2SO4i2NH4X+H2SO4 Gypsum or sulphate of lime would have the same effect, increasing- the degree of acidity of the soil. This was observed in box tests, in which the acidity of the soil to which gypsum was added was higher than at first expected; it was also observed in laboratory tests, in which sulphate of lime was added to the soil. RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 23 EFFECT OF THE POTASSIUM CHLORIDE WITH THE ACID UPON THE BUFFER CAPACITY It 1S well known that the acidity of samples of soil taken from the same field is found t0 be somewhat variable if the measurement is made with the soil suspended in water, but the variation is less if the measurement of pH is made with the soil suspended in a so- lution of potassium chloride. However, the acidity is usually greater in the presence of the potassium chloride than it is really in the field. _ Lemmermann and Fresenius (26) and others have recommended that, in order to avoid the possible variations due to absence of buffer material, the pH of the soil be taken with the soil in sus- pension in a normal solution of the potassium chloride. They state that although this procedure does not give the true acidity of the soil, it gives a valuable characteristic constant of the soil. In order to determine whether this procedure would be of value in the determina- tion of buffer capacity, pH determinations were made similar to those just described, but with the soil suspended in a normal potas- sium chloride solution instead of in water. By using the potassium chloride and water in different co1nbina- Lions, data were secured for the following three treatments of the soil, the amounts of acid added remaining the same. These were: (1) The acidity of the soil (pH) in the presence of potassium chloride. (2) The residue from (1), suspended in a solution of potassium chloride. (3) The residue from (1), suspended in water. The data thus secured were plotted and the curves constructed. Points on this curve are given in Table 7. The amounts of acid added were sufficient to produce the pH of 6.0-5.0-4.0 in the regular procedure, shown in the head of the column. In soils of low buffer capacity, the degree of acidity is greater, or the pH lower, when potassium chloride is present than when water only is used. -The average difference was 0.3 pH, although the maximum difference was 1.0. In soils of higher buffer capacity, the results differ very little. The explanation is simple. In soils of low buffer capacity there is very little salt formed as the result of the addition of the small amount of acid added. In all of the soils of low buffer capacity exchangeable hydrogen is present. The exchange reaction between the soil and acid is a reversible reaction. With a low concentration of salt in the solution, there is little ex- change of the base of the salt for the hydrogen of the soil complex. When, however, the salt content of the solution is comparatively greatly increased by the use of the potassium chloride solution, the solution becomes more acid. In soils of high buffer capacity, salt is produced by the action of the acid on the bases of the soil to such m m T A T S T N E M I R E P X E L A R U T L U m R G A S A X E T 2. 4 A. 0 N m T E L L U B 24 3 w. . . . . . __ . _ . _ v w m H v w m v w w v __ m w %_ v m ........................... QmC mmnnw>< fi-w m6 Qlv mf.‘ w.w H-m . . . ..................................... f . 3. 9m Q Z i Q x TM AHIM ................................. .552 >20 owiwui 3&5 E 3 i, Z 3 E M; 2 Z Eu ma: _T ca; 3 i 3 2w ME we as 3. Z ................. = .83 “Bfiwo $23 Z 3 we m.» 3. T». ww M}. i. .................... \.w...w...,_.-=~.mwfio >32 2E 92.5 Ewmw g we 3 5. m...“ 9m 3w 3 ma. ................................. zwfio. .252 3m haw 3M5 ma 3 Wm S. Tm M3 5 3 i. .................................... =E§ >20 ozmsfi m8? i. 3 Z Tv Z 3 may 2 Tm .......................... T. 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RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 25 an extent that the addition of the potassium chloride has no effect on the acidity of the system. The soil treated with acid was filtered off and washed with water. The acidities (pH) of the residue treated with potassium chloride, and with water, are given in Table 7. The residue treated with potassium chloride differs in acidity (pH) only slightly from the original acidity of the soil and acid mixture, averaging 0.2 to 0.4 pH higher. The residue treated with water is much more alkaline, averaging 1.6-2.1-2.3 pH higher. These results are somewhat similar to those secured by J offe and McLean (6), and the explanation of the results secured in the two investigations is similar. The potash compound of the base-exchange complex dissociates, thus: KX+H-_-O:HX+KOH Working with soils from plats at the New Jersey Experiment Sta- tion they found a considerable difference in acidity, depending upon whether the extractant was water or normal barium chloride. In every case, the barium chloride gave lower pH values than did the water. The pH by water was mostly between 5.0 and 5.5. They found also that even after 10 days’ treatment of the soil with barium chloride, only about 40 per cent of the acid in the soil had been extracted. These results show that a portion of the soil hydrogen was replaced by barium, but that the reaction by no means goes to completion, even when the extractant is finally removed and the soil residue subjected to long washing with the original extractant. In a majority of cases, the acidity of the residue treated with potassium chloride was, within a very small range, identical with the acidity of the mixture of the soil and acid. This means simply that the tendency toward alkalinity due to the residue was counteracted by the tendency due to acidification resulting from the use of potassium chloride in the final determination of reaction. It should be noted, however, that when there is any considerable difference between the two, the residue treated with potassium chloride is always the more alkaline. Irregular results were secured when the residue from the treat- ment with acid and potassium chloride was suspended in water for the determination of pH. The erratic results may be easily ex- plained. After the treatment with acid and potassium chloride, all of the soluble buffering materials are washed out. The exchange complex of the soil contains potassium in place of most of the cal- cium. A very slight hydrolysis of this complex results in the forma- tion of a small amount of potassium hydroxide. Due to the fact that all of the buffering materials of the solution have been removed, this small amount of potassium hydroxide has an enormous effect upon the reaction (pH) of the remaining material. Any slight trace of impurity would also have a marked influence on the re- action. The consequence is that this residue is less acid than the other residues and the results are erratic. 26 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION QUANTITATIVE RELATIONS OF THE ACIDITY OF THE RESIDUE TO THE QUANTITY OF ACID USED It is of significance to know the relation of the quantity of acid in the residue after the soil has been treated with the acid t0 quan- tity of acid used. This relation might throw some light on the chemistry of the process. It also aids in judging whether or not the measurement of soil acidity is related to the quantity of acid- forming fertilizers added to the soil, or to the production of nitrates, or to other acidifying influence. q The quantity of acidity of the soil residue was estimated by several methods proposed for ascertaining the lime requirements of the soil, namely, the Veitch, the Jones, and the Hopkins method, and also by barium hydroxide and by barium acetate. LIME REQUIREMENT OF SOIL RESIDUE Eleven soils of low buffer capacity were used. One hundred grams of each soil was treated with sufficient acid to bring the soil _ to a pH of 4.0. as calculated from the pH acid curve. After stand- ing overnight. the soil was transferred to a filter, the supernatant liquid filtered off, and the residue washed twice with water. The soil, aft-er drying, was passed through a 20-mesh sieve. Determina- tions of the lime requirement were then made on the soil residue by the Veitch, the Hopkins, and the Jones methods. In the Veitch method (29) the soil is treated with increasing amounts of lime water until the filtrate upon boiling turns pink with phenolphthalein. In the other two methods the soil is treated with neutral salt solution and the filtrate titrated with sodium hydroxide with phenolphthalein as the indicator. The Hopkins method (5) uses sodium chloride, while the Jones method (7) uses calcium acetate. The results of this work are presented in Table 8. Table 8. Relation of acid added to the soil to the acidity measured by various methods expressed as parts per million of calcium carbonate. Net acid - - - - 1 Laboratory Total Carbonates added Aclqgycgfciflerialdggrfiggzg: ent acid in original 1 equivalent Number added soil to calcium Veitch Hopkins Jones " carbonate method metnod method I 25871 ‘ 312 0 312 410 36 320 25873 812 0 812 1250 48 976 31880 l 1562 450 1112 1607 348 1584 26103 1000 130 870 1250 81 1008 31888 i 781 60 721 821 186 1334 31896 I 1250 180 1070 1607 216 1584 31890 ) 1310 150 1160 1607 78 1680 31804 2615 170 2445 2680 213 2128 31914 I 2190 180 2010 2320 33 2400 31820 1815 490 1325 1960 62 1696 31321 3750 1440 2310 3035 27 2368 Average (11) ______________________________ .. 1286 1686 121 1553 RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 27 The first column of Table 8 gives the laboratory number of the soil. The next three columns present data relative to the amount of acid added. The third column of the table shows that a small part of acid added would be used in the neutralization of carbonates; the total amount of acid added has been corrected to allow for the amount of carbonate present, the results being termed the net acid added, and presented in the fourth column. The last three columns give the lime requirements of the various soils, as determined by the Veitch, the Hopkins, and the Jones method. The Veitch method indicated a recovery of acid greater in every case than the net acid added. The average excess was about 30 per cent. It should be remembered, however, that the method is not adapted to measuring small differences, because the lime water is added in cubic centimeters and 1 cc. of lime water is equivalent to 410 parts per million of calcium carbonate. However, the order of the soils is the same, and there is no outstanding difference between acid added and acid recovered. The results by the Hopkins method are erratic and low. The average is only 10 per cent of the acid added. This and similar methods are evidently not suitable for work of this kind. With the Jones method, the agreement between the amounts of acid added and the amounts recovered is fairly close, although the lime requirements are uniformly greater than the amounts 0f acid added. The average excess is 20 per cent. Results by the Jones method average a little lower than by the Veitch method, although _ the results in some cases are higher, and in other cases lower. Both the Jones method and the Veitch method give an approximate meas- ure of the quantity of acid added to the soil. ACIDITY OF THE SOIL RESIDUE BY BARIUM HYDROXIDE The effect of the treatment with acid upon acidity of the soil was studied by means of barium hydroxide. The soils used in this ex- periment were low in calcium carbonate and high in total exchange capacity. Portions of the soil were treated in the regular way with amounts of acid sufficient to cause a pH of 4.0, filtered, washed twice with distilled water, placed in Erlenmeyer flasks, and treated with amounts of 0.01 N barium hydroxide equivalent to the differ- ence in amounts of acid required to develop a pH value of 4.0 and those of 4.5, 5.0, 5.5, and 6.0 by the method previously described. For example, the soil No. 29435 required 43 cc. of 0.01 N hydro- chloric acid to produce a pH of 4.0, 29 cc. for pH 4.5, 18 cc. for pH 5.0, 13 cc. for pH 5.5, and 9 cc. for pH 6.0. Five 8-gram portions were treated with 43 cc. of 0.01 N acid. The resulting pH was determined in one portion, and varying amounts of 0.01 N barium hydroxide added to the other four portions. These amounts were 14 cc. (43-29) to each pH of 4.5, 25 cc. (43-18) for pH 5.0, 30 cc. 28 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION (43-13) for pH 5.5, and 34 cc. (43-9) for pH 6.0. After the addi- tion of the barium hydroxide, the flasks were tightly stoppered and allowed to stand with occasional shaking, for 48 hours. At the end of that time, the pH of the system was determined. The results secured are presented in Table 9. Table 9. Recovery with barium hydroxide of acid added, by buffer curve of soil residue. Acid added i pH of Ba(OH)2 treated soil Laboratory (131103 to Original pH i residue when net acid would content soil as i give a pH value on untreated Number p.p.m. p.p.m. i secured ii__ 5°11 of _‘ CaCO3 l i 4.5 i 5.0 | 5.5 I 6.0 29365 i 55 i 1125 4.16 5.49 6.10 6.57 i 7.02 29425 348 I 1720 i 4.22 5.46 5.87 6.16 i 6.64 29434 i 672 i 2438 i 4.16 i 5.53 5.85 6.45 7.02 29427 i 492 2438 i 4.15 I 4.99 i 5.41 6.07 i 6.49 25972 80 2500 i 4.02 5.37 i 5.76 6.23 i 6.64 29435 440 2685 i 4.00 i 5.35 i 5.88 6.37 i 6.45 29441 200 i 2750 i 4.06 i 5.20 i 5.84 6.24 i 6.86 25959 1000 i 3950 4.18 5.75 i 6.85 i 7.50 i 8.10 31327 256 i 4063 i 4.11 5.51 _ i 5.91 i 6.30 i 6.60 26075 i 610 i 4375 4.11 5.66 i 6.67 7.25 i 7.74 31321 i 3132 i 4688 i 4.84 7.01 7.54 i 8.05 i 8.51 25967 1052 i 5155 i 3.97 i 5.33 i 6 15 i 6.50 i 7.34 r l I a i . 1 i Average (12) ............................................. .. i 5 55 i 6 15 i 6 64 i 7.11 I . I The pH values secured by the various additions of barium hydroxide are considerably higher than those which would have been secured by adding to the untreated soil the net amount of acid, that is, the total acid added minus the amount of barium hydroxide added. The average excess is 1.05 to 1.14 pH. The acidity as measured by this method is less than that measured by the methods discussed in the preceding sections. This is explained by the fact that almost all of the soluble salts which would act as buffering materials were removed in the preliminary treatment; consequently, when the barium ‘soil-complex hydrolyzes slightly, the resulting pH is con- siderably increased. This same tendency toward increases in pH after removal of the soluble salts was noted in connection with the work discussed above. A fairly strong base has been added to the system, and the increase in pH is still more pronounced. When an allowance is made for this increase, the curves secured by plotting the results of the experiment closely parallel the curves secured by plotting the pH acid data. That is, if 10 cc. of 0.01 N acid causes a difference in pH of 1 unit, the same amount of 0.01 N barium hydroxide will cause a difference of 1 unit in pH toward the alka- line side. In other words, the slopes of the two curves are nearly identical, but the intercepts are different. RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL Z9 It must be remembered that thepH of the residue of the soil washed with water (Table 6) is on an average 0.7 to 1.0 higher than the residue in the presence of potassium chloride (Table 7) or the reaction products (Table 6). These differences alone are almost sufficient to account for the excess pH secured by means of the back titration with barium hydroxide. ACIDITY OF THE SOIL RESIDUE IN TERMS OF EXCHANGEABLE HYDROGEN In addition to the quantitative estimations of acidity just discussed, determinations were made of the amount of exchangeable hydrogen in soils after treatment with various amounts of acid. The treat- ments included the additions of the quantity of acid necessary to bring the soil to pH values of 6.0, 5.0, and 4.0, in the presence of the reaction products on the regular pH acid buffer curve and after the regular treatment for acid consumed. A determination was sub- sequently made of the total exchange capacity, in order to determine whether or not the acid additions had caused any alterations of the exchange complex, other than the substitution of hydrogen for the various bases, which will be discussed later. The procedure used was as follows: Eight grams of soil was treated with the required amount of acid, the volume made up to 100 cc., and the suspension set aside overnight. The following morning the suspension was transferred to 35-cc. Gooch crucibles, and the residue washed twice with water. The suction flask was washed out and replaced. The soil was then leached with 250 cc. of neutral normal barium acetate solution contained in the leaching apparatus described elsewhere (4). After the 250 cc. had leached through the soil, the filtrate was titrated electrometrically, using quinhydrone and 0.1 N barium hydroxide. Instead of using the regular Veibel electrolyte, a portion of the original barium acetate solution was used and changed after each determination. Two circuits were provided for the titration, one leading through a poten- tiometer, and a second circuit leading directly to the galvanometer through a tap key. The major part of the titration was done with the use of the resistance provided in the potentiometer. The final end point was reached by using the second circuit. By means of this arrangement, the end point Was easily determined, one drop of the 0.1 N barium hydroxide used in the titration causing a very distinct deflection of the galvanometer. The results were calculated as parts per million of calcium carbonate. The procedure for the determination of exchangeable hydrogen after acid consumed was similar with the exception that the regular acid-consumed procedure was carried through prior to leaching with barium acetate. The re- sults secured in this study are presented in Table 10. After the determination of exchangeable hydrogen, the soil was 30 BULLETIN NO. 442. TEXAS AGRICULTURAL EXPERIMENT STATION ‘ll:-IFPD-IFPPFD-E-b-“b-Plf-Ir-E-D-b-b-b-D-II-PL" I C 30222.22 423D 33.. 2.32 Q Q _ Q QQQ333 QQQQ33 QQQQ33 QQQ3... QQQ..Q2. ............................................. .4232. 22.2 h 33323 3333 3332 Q32 Q _ Q QQQ32 QQQ32 QQQ32.2 QQQ32 QQ3332 ............................. =E3Q2 .223 2.2.3232 _. 32.33 Q33 33 Q Q . Q QQQ332 QQQ232 _ QQQ322 QQQ3Q2 Q2332 ........................... -8332 .2323 2.323222 _ . 2.3323 Q33 Q22. Q23 Q . Q _ QQQ2. QQQr. J QQQE QQ23 Q32. ............................. .. E32 .2323 2322.22 QQ323 32.3 Q2... 332 Q ._ Q _QQ233 Q32. QQQ33 QQQ2. QQQ3 .................................... .. .2323 3.223330 2.32.3 3333 QQQ3 2.2 Q Q QQ..32. J QQQ32. I Q32... QQQ3 Q32. ........ -582 32.5.3 3.2.2 23.2332 3322.32 2.3333 QQ23 Q33 . 333 Q32. _ Q QQ333 QQQ3 QQQ33 QQ333 333 ................. .452 32.5.3 3E3 322.2223 32.23 333 Q53 Q332, 2.2.2. Q 3S3 _ QQ332 Q32 Q2.Q2 QQ2.2 ......... -832 .2323 32.533 3.23 355m 22.333 Q333 2.32 Q Q Q 3 _ Q32 _ QQQ32 Q33 QQQ3 Q3QQ2 ......................... ..E3Q2 .2323 Q2385 _ 33333 332. 2.32 Q33 333 Q QQQS QQ332 QQQ3 333 3S2 .................... ..E3Q2 32.5.3 323 322w 33323 Q33 2.33 Q Q Q QQQ32 Q33 QQ33 Q33 QQ33 ......................... .2582 .2323 32382.3 32.23 Q32. QQQ3 ...... .. 3Q22 Q QQ22. QQ33 .......... .. QQQ3 32.3 ...... .... ........ .2582 32.5.3 32.23 333222? 33333 Q33 Q23 ...... .. 2.2 Q 333 Q2... .......... .. Q32 2.2.2 .............. ..E3Q2 .2323 .2223 2.22.3384 23323 252. 333 ...... .. 32.2 33 Q32 Q2... .......... .. Q332 Q33 ......................................... 42320 32322 332.3 332.2. 3333 ...... .. Q3Q2 Q33 253. Q33 .......... .. 333 3.2 ................. =E3Q2 32.5.3 2.23 223.232 2323 Q33 2.2 ...... .. Q332 323 * Q2.3 QQQ3 .......... .. QQ3 Q2. ............ -8332 32.5.3 3.23 2.222238... Q3323 Q33 Q32 ...... .. 3332 333 _ 333 QQ2 .......... .. Q3 Q32 .................. -8332 32.2.33 2.22 22.2.2.2 2.323 Q3Q3 Q32 ...... .. 3332 333 2.33 333 ........... .. Q32. Q32 ................. ..E3Q2 32.5.3 323 322320 3.223 QQ32 Q32 ...... .. 322 333 Q32 QQ2 .......... .. 33 3.. .................. -5332 32.5.3 3:23 23.522 Q3323 QQHN 2.2.32 ...... l Q33 332. 2.2.32 ooHH .......... 1 OwN Q32 .............. sswcfl 32.283 02E" 333322.20 2.22.3 mfimfi 332.2 ...... i ONN. Q33 w 00w; 32.3 ........... .. 33.2 O ............. 58E“: 32.22.33 222.2 03262201.." 3.3333 2.2.3.3 2.332 ...... .. 2.332 22.32 _ 2.2.2.3 QQ2.2 .......... .. Q32 Q32 ................. i853; fiifim 05W“ 23.2222 Q3323 Q23 3332 ...... .. Q232 Q22 . Q32 Q2. .......... .. Q32 Q3 ................... .2382 32.5.3 3E3 232$ 33323 Q33] ...... .. _ .............. .. x Q QQ3 ......... .. Q Q ........................................... .6233 3.2225 23333 ©0535 _ OJ» w O6 2 0d 30m 25222223 . 22am w Eon _. Eon i 2.22.23 . 2.3233323. -252. o2. 9m _. o6 2.232 2222 _ 22.2 22.2 -322 2...... _ 3 .522... 23.2.5.2 QQNCOQ-“NU 22.232222. W Wmrmw %HOHQ . we m2 w 22023.92 o2 22.3.2.2» .333. mOOwO _ Inonwq OH QCQ~N>MUUQ CUMOEU2Q~ Minmwucmsuxfl .2...oO23O .5222 323 33:25am. 3222332252995 E023 3502.222, 2233.223 32203 222 33.32.22.222 32233323322322 .02 322mm. RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 31 leached with 250 cc. of neutral normal ammonium chloride, washed free of excess ammonia, and the determination of total exchange capacity completed in the usual way. This work is discussed on a subsequent page. l The net acid and the net exchangeable hydrogen, expressed in terms of calcium carbonate in parts per million of soil, are given in Table 11. The net acid is secured by subtracting from the total acid used for each treatment, as shown in Table 10, the carbonate of lime in the soil. The net exchangeable hydrogen is secured by subtracting from the total exchangeable hydrogen, shown in Table 10, the exchangeable hydrogen in the untreated soil. It is to be noted, in Table 10, that a soil may contain both exchangeable hydrogen and calcium carbonate. The net acid and the net exchangeable hydrogen is, in some cases, the same within the limits of error, at pH 6, pH 4, and with the treat- ment for acid consumed. With soils 31888 and 26103 the two agree for all three treatments. With some other soils, however, the ex- changeable hydrogen is appreciably lower than the net acid used. With soil 31890, the exchangeable hydrogen is only two-thirds of the net hydrogen added; with soil 26075, it is less than 50 per cent at 4.0 acid and less than 25 per cent on the residue from the acid consumed estimation. Other differences are evident in the table. The differences are usually greater with the residues from the treatment for pH 4 and. for acid consumed. This would seem to indicate that either the acid destroys the base-exchange complex, or that base is removed from other compounds than the exchange complex. As it is shown elsewhere in this Bulletin that acid has little effect on the exchange complex, it follows that the acid has acted upon bases in other compounds. It appears probable that while in some soils the only compounds which give up bases to acids are carbonates and base-exchange complex, other soils contain, in addition, compounds which give up bases to acids without the base being replaced by hydrogen. This is in accordance with the work of Pierre (19), who found that in some soils part of the acid resulting from the application of acid-forming fertilizers was neutralized by soil materials other than those of the base exchange complex. 32 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION . _ QQQQ E QQQQ- owmfi § QQ¢Q1 Q _ QQQQQI Q E QQQ>Q@ QQNQ g QQQQ- QQQH QQQQ| QQQH QQQ2H1 Q QQQNN» QQQQ QQ¢Q| QQQN QQQQ1 Q QQQNH1 Q oowmfix QQQQ _ QQQQ ¢2ow. 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JEN”; 225m o2.“ 25m _ mmwHm ............................................ :52: >20 ~€QQ2> _ £52m .................................. Jnw2 avian our“ @223 _ mwmmm .................................... ..Emo~ x2e 5mm o=EwE< _ 52m ............................................................. <5»? mctrfi _ mgww ....................................... ..Ewo_ 3E3 m5.“ 23mm _ 252m ................................. 15x2 225m 25 QESE< _ omwfim ...................................... 1592 25am 02w mzoofi fi wmwfim ..................................... 15x2 >253 vim 2230 _ v33 ..................................... 15x2 zwcwm mcC 255G owwfiw ................................. 1:522 35mm o2.“ fifixooaO 2 mofiwm ................................ .4532 zwcwm MEG ofiwaofl Q53 .................................. ..E@3 2E2 95 _Q=m§_ QQQS .................................... =E~2 3E2 25 as»? _ wwwS ............................................................ ..Q=@w Qcsm l HQQQN Em 5.2552 ~53 mom k znouwaonwfl 33 mo #55095 o5 via 23w ow. wmwwm Eva Mo 3555a was cmmtiwn cowCQQEoO .3 oEwP RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 33 RELATION OF CARBONATES TO THE BUFFER CAPACITY The carbonate content and buffer capacity are given in Table 5. The soils are arranged according t0 the amount of acid required t0 reach a pH of 6.0. Since the carbonate is practically in the same order, it is apparent that the buffer capacity increases approximately as the carbonates increase. After the carbonate content of the soil reaches about 4000 parts per million, or 0.4 per cent of calcium carbonate, there is some car- bonate still present in the soil when the pH of the soil as determined by the method is at 6.0. This is more clearly brought out in Table 11. The amount still present varies, undoubtedly due to the fact that in some soils the carbonates are more easily dissolved by acids than in others. The last six soils in the list still contain small amounts of carbonate at pH 5.0 (see Table 11), altho in three of these the amount is nearly within the range of experimental error. At a pH of 4.0, some calcium carbonate is still present in two soils (Table 11), but in these the difference is within the range of experimental error (Table 5). It should be remembered that there is a great difference in the procedures for buffer capacity and total carbonates. At pH 6.0, there is only a very small amount of acid present, while in the car- bonate determination there is a large excess of very strong acid. It is to be expected that there would be considerable differences in the amount of carbonates soluble in the two procedures. The degree to which the carbonates are neutralized at pH 6.0 is, in fact, remark- able, and affords a strong indication of the accuracy of the proce- dure for buffer capacity. The data on the total carbonate content of the soil corroborate the conclusions already stated in connection with the discussion of the types of buffer curves. RELATION OF THE BASE-EXCHANGE COMPLEX TO THE BUFFER CAPACITY Various workers have shown that the base-exchange complex plays an important role in the buffer capacity of the soil. The pH of an acid soil is closely associated with the degree of saturation of the exchange complex with bases, altho Pierre and Scarseth (22) have recently shown that the relationship is not as close as had been, assumed by some of the earlier workers. Consequently, the greater the exchange capacity, the more bases must be exchanged for hydrogen before the soil reaction changes materially. Parker (17, 18) has shown that the same proportion of bases exchanged for hydrogen at a low degree of acidity will cause a much larger change in pH than if the exchange were made in a soil already acid. For these and similar reasons,_a study was made of the exchange capacity of the soils for which buffer data had been secured, and the degree of relationship between the exchange capacity and the buffer capacity was determined. 34 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION The total exchange capacity was determined essentially as outlined by Kelley and others (2, 9, 11, 23, 24). Ten grams of soil was placed in a 35-00. Gooch crucible provided with a disk filter paper. The soil was then leached with neutral, normal ammonium acetate until the leachate gave no further test for calcium. The ammonium acetate was then washed out with 95 per cent ethyl alcohol until the leachate gave no test with Nessler’s solution. The apparatus described by Fudge (4) was of great assistance in this work. The soil was immed- iate‘y placed in an 800-cc. Kjeldahl flask, light magnesium oxide added, and the ammonia distilled into 0.2 N hydrochoric acid. The excess acid was titrated with 0.1 N ammonium hydroxide. Results in this study were expressed as parts of cacium carbonate per million of soil. This was done in order to facilitate comparison with the data in other tab'es. One milligram equivalent, the usual unit used in ex- change-capacity studies, is equal to 500 p.p.m. of calcium carbonate, and the conversion can be readily made if desired. The total base-exchange capacity was compared with the buffer capacity of a number of soils. The quantities of acid necessary to, increase the acidity from pH 6.0 to pH 5.0 and from pH 5.0 to pH 4.0 were taken as measurements of the buffer capacity. As previously noted, there are in many soils significant differences between the buffer capacities at these two points, especially in the case of soils low in carbonates, which were used for this comparison. The total quantity of acid required to reach a pH of 6.0 and also the net quantity, are given in the table. The net quantity of acid used for pH 6 was the total less the carbonates. The results of the work are given in Table 12. ' The relationship between the specific buffer capacity, that is, the amount of acid required to cause a change of 1 unit pH, and the total exchange capacity is given in the last two columns of the table. The specific buffer capacity was divided by the total exchange capacity and multiplied by 100. The figures given may be considered as the per- centage of the total bases in the exchange complex neutralized by the acids in changing the pH to the extent given. There is fair uniformity in the results, when the wide variation in the soils is taken into consideration. However, the degree of relation between the two pro- perties of the soils seems to depend upon two factors: the size of the total exchange capacity, and the pH values between which the specific buffer capacity is taken. The percentage of bases taken by the acid tends to decrease as the buffer capacity increases for the specific buffer capacity for pH 6.0 to pH 5.0. The average for the 18 soils with exchange capacities smaller than the equivalent of 14,000 parts per million of calcium carbonate is 9.8 per cent of the bases used, while the average is 6.4 for the 12 soils of greater exchange capacity. With the specific buffer capacity for pH 5.0 to pH 4.0, the average per cent of the sixteen soi‘s with the exchange capacities smaller than 10,000 parts per million is 17.0, while the average is 11.1 for 35 RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL , _ H3 H3 3HH 3H3 33| 33H; 35 .. 8H3 88:3 _ H3333 33H 5 33H 38H 3E 3332 33 .................................................. .. 8H... HH3H88H _ $33 33H 3.3 3S 3H3 33H 3333H 3P ............................ .. 88H 8H3 3mm c5884 _ 3333 3.3 3.3 3H. 333 331 . 33H 33 .................................... 188H 8H» 338380 33333 3.3 3.3 3HH 3HH E33 33HH H3 ............................ <88H388w ~83 8335 33333 H3 ...... .. SHH ...... .. 3 33HH . 33H ........................................ .. 88H 8H0 3333880 33333 3.5 3.3 33H 33H ...... .. 33H: .................................... .. 88H >23 33:3 25384 3333 H.H.H H.3H 3H; 33H 33 3H33H 3H; ........................ 88H 8H8 B3 25384 H338 33H HéH 3H3 38H 33 33H: 33 33H» 882E» 33333 3.5 3.HH 33H 3S 33 3H3 3H .............................. .. 88H 388 383 3:380 :23 33H 3.3 3HH 333 33 33 3H ............................... .. 88H 388 8H3 883 H3H3 3.3 N6 33 com ova mwww owv .......................... 15.33 268.33 mam“ 055384 mmmfim 33H 3.3 E3 333 3H3 333 3H8. .......................... .. 832 3388 8H3 QHHHS8< 333 5H ...... .- 33 ...... .. 3 38 3 .............................. 1 88H .888 ~83 8%? 333H3 3.3 3.3 33 3H 33 3333 3E .......................... 1 833 388 8H3 oHHH88< H333 33H ,,,,,, .. 33 ...... .- 3 3Q 3H ........................ .. 88H >883 38$ H3882 33H3 H}: 33H 33 33 3H, 3333 33 .............................. .. 88H 3388 38$ H335 33H3 W333 H3 83. 3H3 3| 3H3 __ 33H ......................... .. 88H 882 38$ 2HH88< 33333 3H 3.3 33 333 333 3H3 33H ........................... .. 88H 888 8H3 88.58 3333 8M3 33H 33. 33 3H 3H3 __ 3H ........................ .. 88H .682 38H 3381.26 f 3233 3. 3 33H 33 33 33 E33 3.3 .......................... .. 88H >888 ~83 QHH838< £333 3.3 3H E3 83 35 S: _ 3 .......................... .. 88H 88$ 8H3 838803 _ 8333 3.33 ...... .. _ 3H _ . . . . . . . . . . . . . . .. _ 3H3 _. 3 ...................................................... .. H88 356$ $333 _ H. 3.3 3 3.3 8 3H. B 3.3 8 .3 89E 3.3 82m ob 52h :6 Eoarm w in 33.3.3330 . 333.3 333933 o3 E33 3353x388 wfiwwfiHou 39G. mow HQQEUZ wfisfiwnuswi H; wwmHH w Q 3oz H330? 3 803830 hHOuNHOQN-H 33am mo wwwuflmonom a 3H5, 3.30m w A3OOwO c355 awn 33.35 m.» wommwnmxw 333393 3309830 uwwwsn was 3330.393 335286 coozfion HHoflHEwM dH 03mm. 36 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION W: _ 2w 2K2 5w .................................. 1 A32 wuw~w>< 3w § 2. 32 8E 2w 22w 2.3 ......................................... Q20 2222c ~25“ 22w 12 _ i. 2W2 3: 2.? 2:: 2.2 ....................................................... 5% 22>: _ $.22 w: _ E 2.5 32 i. .52: =3 5Q 22>: _ .282 =2 3L 32 .52 8E 25S N2: ..................................................... 22v =8w=om h 552.. Z; _ 3 $2 23 =8 $52 25 ..................................................... $2» 2.82am _ wwmmm 2.2.2 _ 2E. 22 m2 2E 2:2 8N .............................. 1:52 2E2 2E nvasq _ :43 2.” 21.. a? 2m 3m $2.2 2 ........................... 1:82 22:3 25 QESE< _ @222 .3: 3. 22 2a ...... l. 2.22 cw .......................................... 1 =52 >2» zozr.» _ ~52 i: Na £2 2w 2. 22; S; ..................................................... .22“. 522B _ .222 32 Z. $3 2a Q21 2.42 N2 ........................................ =88. 22o £82096 _ 22$ v.3 ME. 33 =2 2:2 22A 3N .................................................... Q2» 222252 __ ~22» .3. B o6 cu oé ow A 9m 3 _ o m 50.2% oé Eoarm o6 Eoah 5d swam w In zfiownwo ucwucoo maoawuonwq Eon zfiuwnwo o» Ho.» wwcasoxw unfi 2am mfinflwnvsw: E wow: . QwZ 130E 82.0250 .6282 wwwwn mo wwwacwuaom uwwwzn urfloonm AwvsimwioOYlAnOOwO c355 .82 2.2.3 ms wwmwwunxw wfismwav 2223.8 amaze was afiuwnmu 022x135 c2533: comfifiofl .2 Baum. RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 37 sixteen soils above that amount. Variations from the average of the percentages of bases used are much smaller at the specific buffer capacity of pH 5.0 to pH 4.0 than at the specific buffer capacity of pH 6.0 to pH 5.0. Results on soils 25871 and 29365 were not in- cluded in the averages mentioned above, because they were much dif- ferent from any of the other soils. Attention should be called to the fact that lighter soils have lower total exchange capacity and lower buffer capacity than heavy soils. In heavy soils a smaller percentage of the exchange complex must be taken up by hydrogen in order to produce a given change in reaction than is the case with lighter soils. Several conclusions with reference to soils containing low amounts of carbonates may be drawn from this work. As the total exchange capacity of the soil increases, the relative amount of acid required to cause a change in reaction of 1 unit pH (the specific buffer capacity) tends to decrease. The specific buffer capacity between pH 5.0 and pH 4.0 represents nearly twice as much of thetotal exchange capacity as does the specific buffer capacity between pH 6.0 and pH 5.0. Assuming that most of the acid is used in the exchange of hydrogen for exchangeable bases, this means that nearly twice as much exchangeable base is removed in changing from pH 5.0 to pH 4.0 as when the change is from pH 6.0 to pH 5.0. However, it requiresten times as many hydrogen ions to change the pH from pH 5.0 to 4.0 as from pH 6.0 to 5.0. The fact that equal amounts of acid changes so much less of the exchange complex in soils of high exchange capacity than in soils of low exchange capacity, probably explains in large measure why, in applying laboratory results to field studies, a larger conversion factor is necessary for the soils of high exchange capacity. When the buffer curve is of Type I, indicating an absence of appreciable quantities of carbonates, a fair indication of the size of the total exchange capacity may be secured from the specific buffer capacity. EFFECT OF ACID ON THE EXCHANGE COMPLEX Treatment of the soil with acid isnecessary in the determination of buffer capacity. Since the amount of acid used depends upon the character of the soil, and a soil containing a considerable quantity of carbonates requires a large amount of acid, it is of some importance to know whether or not there is a breakdown of the exchange complex with the addition of this acid. This question is also of considerable importance in connection with the determination of the exchange capacity of a soil. The method re- quires leaching the soil with a normal neutral solution of ammonium acetate until the leachate gives no test for calcium. Many of the soils used in the present study were highly calcareous, and much time was 38 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION Table 13. Effect of treatment with acid on the exchange capacity of soils (expressed as parts per million calcium carbonate). Acid Exchange capacity efigfitof Labora- con- Soil 7 treat- tOPY Soil tpye lsumed by Original treated ment Number J original soil with with _/___ _/__ 111W N S011 1 __1 1s1cli__1 acid ‘ I 29450 i Amarillo fine sandy loam ....... 12500 i 3400 [ 3540 i ~ 29336 Richland fine sandy loam _______ 22200 5210 5500 —l—— 140 29333 I Midland fine sandy loam ....... 28600 5290 y 4970 —]— 290 31802 Yohola fine sandy loam _____ .1 36600 I 5450 5470 -—— 310 25883 Willacy fine sandy loam __________ 11 9100 5805 5770 -|- 20 25865 Lomalta fine sandy loam ....... -1 15500 6700 K 6610 - 35 31833 Spur fine sandy loam _______________ __§ 17000 7100 7140 __ 90 25871 i Victoria fine sandy loam _________ 14200 7940 7340 _|_ 40 25968 Houston clay loam ................ _-* 15900 3075 J 3000 _ 100 31329 x Potter clay loam _______________________ 62100 9320 9420 _ 75 31325 Amarillo silty clay loam _________ 1.‘ 9700 9375 { 9030 _|_ 100 25891 Donna fine sandy loam ___________ 20750 9500 9430 _ 295 31905 Victoria clay loam ___________________ __ 15000 9010 ' 9710 ___ 30 29436 Wilson clay ......................... 1 12600 10200 10030 _]_ 100 31323 Amarillo silty clay loam __________ _. 9000 10330 11430 __ 120 26824 Lake Charles clay _____________________ _, 22750 11230 11400 _|_1150 29426 Crockett clay loam ___________________ 13100 11030 12320 _|__ 170 29438 Lufkin finesandy loam _____ _._. ____ 12250 11950 11740 _l_ 050 29366 (Unknown ........................... .. ' 9100 12405 12000 __ 210 29425 Crockett clay 10am _________________ 3750 12950 12940 _1__ 255 31326 Amarillo silty clay loam ....... .. 14000 13335 13300 __ 10 31327 Randall clay ________________________________ 11300 13800 14040 __ 25 29434 Wilson clay .................................. ._ 10500 14205 14720 _1_ 240 31828 Randall clay ................................ .. 12100 14315 1 14400 _|_ 515 29427 Crockett clay loam .................... -. 11300 14425 1 14940 _1_ 145 29435 Wilson clay _____________ __ 15100 i 14515 1 14620 _|_ 515 25972 WilSOn clay loam ________________________ _. 11500 14850 15200 _1_ 105 29365 Amarillo fine sandy loam ........ _. 12500 15050 14000 _|_ 410 29441 Lufkin fine sandy loam ............ .. 12000 15725 16120 __ 410 26089 Catalpa clay ________________________________ _. 69100 17540 17170 __I_ 305 25967 301151011 clay 17200 17890 18220 _~ s70 26075 Irving clay ................................... _. 15500 18430 18280 _l__ 330 25959 Irving clay ___________________________________ _» 11900 ' 19110 19520 __ 150 25869 Point Isabel fine sandy loam 48400 19300 18180 _1_ 410 26823 Lake Charles clay ....................... .. 123500 22550 ' 21080 __1120 25966 I Trinity clay .................................. .. 31000 20420 26230 __ 570 ~ | ——— 190 ___ ______ ___ __ __1__ Differences total ____________________________________________________________________ __ 5930 Differences net _ 1770 Average difference .1...I-I,.ITj:IIIIEIIIjIIlI:jiiiiiiilllill _ 50 RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 39 required to leach them free from calcium. If it were possible to add acid to the soil in order to destroy a part of the carbonates without causing a significant alteration in the exchange complex, the determination could be greatly expedited. In a private communication, Doctor Homer Chapman stated that he had added acid to a few calcareous soils without any apparent injury to the exchange complex, but added that the work had been done on only a few soils. In order to test this matter, 36 soils were treated with hydrochloric acid equivalent to 35 cium carbonate and the total exchange capacity from 3400 to 26,420 p.p. city was determined in the residue and in the original soi1s.. The acid- consuming power of these soils varied from 8750 to 69,100 p.p.'m of cal- cium carbonate and the total exchange capacity from 3400 to 26,420 p. p. m. Soils high in carbonates were not used because it was thought that the soils low or medium in carbonates might be more easily affected by the acid than would those high in carbonates. The results of the work are presented in Table 13, with the soils arranged inorder of increasing exchange capacity. In most cases, the exchange capacity is practically the same before and after treatment with the acid. Table 14. Total exchange capacity, in parts per million of calcium carbonate, in original soils compared with soils treated with acid. ' . 1 Laboratory Untreated After i After Soil tvpe pH acid acid Number I soil consumed l 25871 Dune sand _____________________________ _. 350 350 350 31888 Webb fine sandy loam ....... 1, 4240 4250 4190 31890 Miguel fine sandy loam __________ _, 4080 4190 4000 25873 Lomalto fine sandy loam ..... _. 1770 1640 1650 26103 Crockett fine sandy loam ..... 1 2420 2450 2450 31880 Duval fine sandy loam ___________ __ 3710 3550 3510 31914 Orelia fine sandy loam 6160 6125 5890 31896 Moore fine sandy loam __________ _, 3130 3065 3090 31820 Amarillo fine sandy loam ______ .. 4610 4640 4540 31804 Fitch fine sandy 10am ___________ 5750 5775 5560 20075 Irving clay _________________________________ 18430 18810 17100 31321 Amarillo silty clay loam ........ 10500 10100 9790 25883 Willacy fine sandy loam ________ __, 5805 5775 5880 31905 Victoria clay loam __________________ 9610 9850 9680 31833 Spur fine sandy loam 7100 7065 7000 25865 Lomalto clay loam __________________ __ 6700 6765 6580 25891 Donna fine sandy loam .......... .. 9560 8850 9480 31802 Yahola fine sandy loam __________ ._ 5450 5390 5800 25869 Point Isabel fine sandy loam 17300 17175 17420 26089 Catalpa clay .............................. 17540 17375 17720 31800 Miller clay loam ___________________ ._ 11860 11925 10410 31884 Hidalgo clay loam ........... .1 ‘ 8795 8815 7250 25905 Laredo silt 10am 7490 7625 7550 31882 Frio clay . ................................... 13190 13075 12975 l l l’ 157855087511’) .................... ..| 7731 l 700341577107 40 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION In only 5 of the 36 cases is the difference between the results secured by the regular method and that secured after treatment with acid larger than could be accounted for by experimental error. Three of these differences are increases and two decreases, with the use of the acid, so that the differences cannot be due to the acid treatment. These differences are but a small fraction of the total exchange capacity, so that even in these cases they are scarcely significant. The average net differences for the 36 samples is only 50 p.p.m. of calcium carbonate, an amount within the experimental error. The exchange capacity was also determined upon residues left after treatment with acid to secure a pH of 4.0, and after treatment with acid to measure the acid consumed. The results, compared with results on the untreated soil, are given in Table 14. The exchange capacity is practically the same in the residue after treatment with acid to secure pH 4.0, as in the original soil. It is a little less in the residue from the estimation of acid consumed, but the differences are small. The treatment with acid has little or no effect upon, the exchange capacity of the soil. The bases are replaced partly by hydrogen, but the exchange complex is not decomposed. Consequently, the time and work required for making the determination of exchange capacity may be considerably lessened by adding the acid before leaching commences. RELATION BETWEEN THE ACID-CONSUMING POWER AND THE BASE-EXCHANGE CAPACITY OF THE SOIL The determination of the acid-consuming power of a soil is much more simple and rapid than the determination of the exchange capacity. It is a necessary preliminary step in the determination of buffer capacity for acids by the method previously described. The relationship between the acid-consuming power and the exchange capacity is therefore of considerable importance. Kappen (8) states that the determination of the acid-consuming power of a soil which does not contain carbonates will give approxi- mately the amount of exchangeable bases in the soil, and in the case of soils very nearly neutral, the total exchange capacity. If this be true, it is possible to get valuable information concerning a number of factors associated with the exchange capacity without the com- paratively slow and laborious procedure of leaching with ammonium acetate. The following work was undertaken in order to determine the relation between the acid-consuming power and the total exchange capacity of a number of soils. Methods. Two methods for the determination of the acid-consuming power were used. The first is the regular method which has been des- cribed above. The procedure is as follows: Five grams of soil is treated with 50 cc. of 0.2 N hydrochloric acid, stirred with a stirring machine for 15 minutes, and filtered; then 10 cc. of the filtrate is titrated with 0.2 N sodium hydroxide, phenolphthalein being used as the RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 41 indicator. The amount of acid consumed may be calculated to per cent of calcium carbonate, to milligram equivalents of hydrogen icon- sumed per 100 grams of soil, or to parts per million of calcium car- bonate. The latter calculation has been used here in order to give the same units which are used in other experiments reported in this Bulletin. The second method is one which has been proposed by Kap- pen. Fifty grams of soil is stirred or shaken with 250 cc. of 0.1 N hydrochloric acid for one hour. The suspension is then allowed to stand overnight. The clear, supernatant liquid is decanted the next morning, and 125 cc. is titrated with standard sodium hydroxide, phenolphthalein being used as the indicator. The results are calculated to milligram equivalents of hydrogen consumed per 100 grams of soil by Kappen’s method, but for the sake of comparison here, to parts per million of calcium carbonate. In neither of these methods is the amount of acid used in bringing iron and aluminum into solution of any significance, since in the titration with sodium hydroxide and phenolphthalein, their hydroxides are precipitated and the acid neutralized. Obviously, however, the soils must be practically free from carbonates if the acid consumed is to give an accurate indication of the exchange capacity, since the bulk of the carbonates are destroyed before the exchange complex is attack- ed, and the acid consumed by the carbonates is not freed by later titration, as in the case of iron and aluminum. Results. The acid-consuming power of 36 soils was determined by both methods, and the results compared _with the total exchange capacity of the soils. Table 15 gives the results of this study. The total acid-consuming power of the soil, as determined by the two methods, is given in the fourth and fifth columns of Table 15. It has been shown in the preceding work that most of the carbonates are neutralized before the exchange complex begins to exert its full influence upon the added acid. Consequently, the amount of carbonates in the soils has been subtracted from the total acid consumed, giving what is termed the “net acid consumed.” These data are given in the sixth and seventh columns, and are to be compared with the data for total exchange capacity, given in the last column of the table. The net acid-consuming power is practically always lower than the exchange capacity. The difference in some cases is small but sufficiently wide to prevent the acid.-consumed procedure from being used as an exact measure of the base-exchange capacity. It could, however, be used as an approximate measure for total base-exchange capacity. The hy- drogen in the acid does not replace all the bases in the complex. The reaction does not go to completion. It is evident from the compara- tively close agreement of the results that the bases which neutralize acid come chiefly from the bases in carbonates and in the base ex- change complex. In an average of all the soils, the net acid consumed, by the regular method, represents 86.4 per cent of the bases in the exchange BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION _ _ 88H A888 882 882 82H 88 \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ AA E82 8H... 8882895 A 828 8H2; 82H 82H 88H 822 88 \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ A528 88882 h 82$ 88E 8H8 88 88 882 28 A \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ .528 coma? _ $88 882 22H 82H 82H 82H 88 ...................................................... :33 2225M __ 828 88H 28S 88H * 88H 88H 82 .............................. 8E2 8H8 SHE 8:28.25. __ H828 82H 8H2 822 _ 8H2 A 82H 8 ............................. .2882 885$ 22 c225 m 8888. 82H 88H 88H _ 88H 882 888 ............................................. 4H8? 82.2280 828A _ H28 882 88 88H 888 A 88H 82 A. .................................................... .828 EaHHB _ 888 288 8H8 888 88H 88H 888 ......................................... .4282 8H8 82888:’ .388 888 28 82 2H2 888 882 ................................. ..E8H 885$ 2E 258m H88 88 2H8 ~82 88H A 8H3 88 .............................. .282 82:3 25H 228$ H28 88:. 88 82 88H 88H 822 ................................... =82: 885$ 2E. 25m 828 888 82 8H8 82H 882 88H .......................................... £882 8H8 8:885 888 8H8 88 88 88 82 8H .................................. 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J3? :o»w5oHH_ 55$ 33H 53H 33H 2N2 25B $5 ..................................................... law? nafisaw w $53 £5: 2.2 HEwHH 35 Q82 8N .............................. EEmoH 322$ 25H afifisfl # 32a 33H Qafi mHQNH 33H Esfi 2 ............................ .552 $.52 wcG oHHHnwE< _ mafia 38; ammHH QNHQHH =52 253 ow ........................................... JbmoH 52o 52;» Z NEEN 33H £2 QHHNH £2 33H o3» ........................................... :59“; ma? comHH>>% m3? Mfiownwo GQQQQM auHHHmwfl awirwofi flwflnavm Hnwafloo n» QQQESZ o nwsoxo - !l:J w H Sow z 130m. woaswcou Eon uwZ .833 ucmfiflmnouvwmu¢ wfigonHwO uoadaonsfl dofiflfifloOlsAovdHHonawo E3035 £0225 HwQ 253 mu wfifimww: zfiowmwo wwimfiuxw Hwaoa “an 365.3 Cwwguwfi flomfifiofi 6H 3nd? 44 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION complex, while by Kappen’s method, the figure is 81.0 per cent. The total exchange capacity in almost every case is larger than the amount of net acid consumed. This difference is small in many cases, but since it occurs in practically all of the soils, and in some cases is fairly large, it is certainly significant. Moreover, in all of these soils, the titration of the acid solution in the determination of the acid-consuming power of the soil resulted in the participation of some iron and aluminum hydroxides, indicating that a portion of the acid consumed was taken up by solution of minerals. This l would indicate that not all of the acid consumed was taken up by carbonates and the exchange complex, and the difference referred to above would be somewhat increased. The difference in the procedures must be remembered in this connection. In the total exchange capacity, the soil is leached with a large excess of a neutral salt solution. The products of the reac~ tion are constantly being removed, and the reaction can thus go to completion. In the acid-consumed procedure, the soil is treated only once with an excess of acid. The products of the reaction are not re- moved. The final product is the result of a state of equilibrium between soil material, acid, and reaction products. These results indicate that‘ not all of the bases in the exchange complex are replaced by the hydrogen of the acid. Since, however, there is in general a good agreement between the two procedures, a fairly definite proportion of the bases of all the soil exchange complexes must be replaced by hydrogen, regardless of the nature of the soil. This whole question is taken up in detail later in this work. REPLACEMENT OF EXCHANGEABLE BASES BY HYDROGEN 1t has been shown (Tables 13, 14) that the addition of the acid in the estimation of buffer capacity or of acid consumed does not decompose the base-exchange complex in the soil. The exchangeable hydrogen was determined (Table 11) in various residues left after treatment of the soil with acid. From these results, the percentage of base unsaturation of the exchange complex was calculated, and these results are given in Table 15. For purposes of the present discussion, the soils studied may be placed into two groups, based upon their carbonate content. The first 12 soils are relatively low in carbonates, the soil with the highest amount of carbonate being soil No. 31321 with 1440 p.p.m. ‘ of calcium carbonate. The last 12 vary in carbonate contents from soil‘ No. 25883 with 3960 p.p.m. to soil No. 31882 with 404,000 p.p.m. With the soils low in carbonates perhaps the most outstanding point is the comparatively wide variation in the percentage of base unsaturation of the exchange complex at a definite pH value. This value as pH 6.0 runs from 8 per cent for soil 26075 to 41 per cent RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 45 for soil 25873. At pH 4.0, the variation is between 13 per cent for soil 26075 to 64 per cent for soil 26103. A comparison of the data presented in Table 15 will show that this variation is not closely related to the size of the exchange capacity. Pierre and Scarseth (22) obtained similar results, and conclude from their study that, in general, old soils are lower in percentage of base saturation at; a, given pH value than are younger soils. . In the group of soils which contained large amounts of carbonates, there is little relation at pH 6.0 and pH 5.0 between the acid added and the percentage of base unsaturation. In only three of these soils at pH 6.0, is there any exchangeable hydrogen. Indeed, in most of these soils, there is sufficient solubility of bases in the determination of exchangeable hydrogen to make the barium acetate solution definitely alkaline. In seven of these soils at pH 5.0 there is some exchangeable hydrogen, but only in small quantities. In both groups, there is a considerable percentage of the exchange capacity taken by hydrogen at pH acid 4.0, altho there is still a great deal of variation between the individual soils. In general, the base unsaturation in the first group of soils low in carbonates amounts to about 50 per cent, while in the second group high in carbonates this value is about 25 per cent. Table 16. Percentage base unsaturation of exchange complex of soils after various acid treatments. Percentage base unsaturation Labora‘ _ after acid treatment of tory S011 type __..1_..1..__ Number ‘pH Acid‘ pH Acid‘pH Acid Acid _ 6.0 _ 5.0 4.0 -, consumed 25871 l Dune sanH _________________________________________ _, 1 100 »--_ 31888 Webb fine sandy 10am __________________ __ 28 w- 42 58 31890 Miguel fine sandy 10am _____________ ,, 35 ___- 52 89 25873 Lomalto fine sandy loam __________ .. 41 61 59 26103 Crockett fine sandy 10am __________ ,, 40 m- 64 87 31880 Duval fine sandy 10am _________________ ,, 31 -~- 47 49 31914 Orelia fine sandy loam.. 21 42 32 31896 Moore fine sandy 10am _______________ _V 40 m. 53 80 31820 Amarillo fine sandy 10am __________ __ 27 m» 37 80 31804 Fitch fine sandy 10am __________________ H I l9 38 72 26075 Irving clay ________________________________________ _, 8 _... l3 76 31321 Amarillo silty clay loam... 15 _ 24 65 25883 Willacy fine sandy 10am _____________ _, 19 34 82 31905 Victoria clay 10am ______________________ ,_ 0 0 28 78 31883 Spur fine sandy 10am __________________ __ 11 8 24 64 25865 Lomalto clay 10am __________________________ ,, 0 0 25 83 25891 Donna fine sandy clay loam ..... 1 5 13 36 73 31802 Yahola fine sandy loam ______________ __ 9 12 11 37 25869 Point Isabel fine sandy loam ____ .. 0 1 15 17 26089 Catalpa clay _____________________________________ ,, 0 6 25 48 ' 31800 Miller clay loam ............................. .. 0 29 >50 56 31884 Hidalgo clay loam...“ 0 0 31 74 25905 Laredo silt loam.....___. 0 19 18 44 31882 1 Frio clay ____________________________________________ _, 0 0 15 57 46 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION The greatest degree of uniformity with respect to exchangeable hydrogen in the soils is secured after the “acid consumed” treatment (Table 16), in which the soil is treated with an excess of acid. As a rule, the base unsaturation is close to 8O per cent; that is, 80 per cent of the exchange complex is taken up by hydrogen. Soil 25869 - is a decided exception to this statement, the base unsaturation for this soil being only 17 per cent. The reason for this exception is not known. The variation in the percentage of base unsaturation after the various acid treatments and the difference in this respect between different soils offer a partial explanation of the fact that different soils vary widely in the quantity of acid which is necessary ao give a definite pH value. This is particularly true with respect to the first group of soils, in which exchange and acidity relationships are not obscured by the presence of large amounts of carbonates. The ex- planation is found in the fact that while the percentage of the base unsaturation at “acid consumed” tends to be fairly constant, there is a great deal of variation in the percentage at any of the “pH acid” values. The conditions of the experiment must be remembered in studying these data. In the buffer-capacity procedure, a quantity of acid is added which will leave only a very small amount of acid after the reaction with the soil has reached equilibrium. In soils originally containing considerable amounts of carbonates, such as those soils lower than No. 31905 in Table 11, there is at the time of determining pH and exchangeable hydrogen, a considerable quantity of calcium chloride and very little hydrochloric acid. As the result of mass action, the tendency is very strong towards the exchange of calcium for small amounts of hydrogen which might have been taken up by the exchange complex. Such an exchange would leave the exchange complex completely taken up by calcium while there would still be enough hydrochloric acid in the solution to cause a pH of 6.0, or even 5.0. This explains why there is such a small amount of exchangeable hydrogen in the calcareous soils at these pH values. The same reason- ing applies to the other soils and pH values, but not to such a degree. The determination of the total exchange capacity after the acid treatments (Table 10) shows that there is no destruction of the exchange complex in these soils with the addition of the acid. This fact is of considerable importance for two reasons. The first and i. most important point from the standpoint of the present work is that since there is no destruction of the base exchange complex with the ad- dition of acid, the method proposed is satisfactory from this important point of view. The second is that, in the determination of the total exchange capacity with ammonium acetate solution, the carbonates can be destroyed by treatment with acid and the time required for the determination is thereby greatly reduced. RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 47 l NATURE OF THE BASE-EXCHANGE COMPLEX It is generally believed that the inorganic base-exchange complex is made up of one or more alumina-silicates. Truog and his students (13, 14, 28) have separated colloidal material from soils, which they claim is a single compound and alone responsible for base-exchange phenomena. This compound was found in a number of samples of bentonite. It is perhaps significant to note that the material separated by Kerr (13, 14) had an alumina-silica ratio of 1:6, while that separated by Chuka (28) had an alumina-silica ratio of 1:4. Kelley and his coworkers (9, 10, 12), on the other hand, claim that a number of different compounds are involved, and there may be wide differences in the base-exchange activity of the several compounds. Baver and Scarseth (1) state that soil material capable of base exchange may originate in a number of ways, and that the properties of the material will vary with (the factors conditioning their development. According to Britton (1 A, page 474), the soil colloids are simply mixtures of hydrated alumina and silica. The data presented in this paper appear to show that there may be two or more compounds in the inorganic base-exchange complex. The exchangeable hydrogen introduced into the soil by acid in estimating acid consumed is not always a definite percentage of the total exchange capacity. Thus in Table 16, with soils low in carbonates, the percentage of base unsaturation of the exchange complex after acid consumed varied from 49 for soil 31880 to 89 for soil 31890. With soils high in carbonates, there was a wider variation. The percentage of base unsaturation after acid treatment to produce definite pH values was subject to still greater variations. As previously noted, these results are in accord with those of Pierre and Scarseth (22). The light, noncalcareous soils listed in the first part of Table 16 are similar in most of the properties which influence the relation between the pH value and the percentage of base saturation. The only way in which they could vary sufficiently to account for the differences in percentage of base saturation at similar pH values is in the nature of soil acids. The only way in which the same pH can be secured under identical experimental conditions by two different quantities of acid is to have different acids having different dissociation constants. These facts all indicate that the base-exchange complex is made up of several different acid compounds. RELATION BETWEEN CERTAIN OTHER SOIL FACTORS AND TOTAL EXCHANGE CAPACITY There appears to be no regular relation between the carbonate content of the soil and the total exchange capacity, though soils containing carbonates have higher exchange capacity than others. This is to be expected, since the exchange capacity is associated § 48 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION almost entirely with the colloidal siliceous and humate materials of i the soil. There is a very close relationship between the physical character of the soil and the total exchange capacity. As the soil class becomes heavier the total exchange capacity becomes greater. This was to be expected, since in the heavier soils there is generally a greater amount of colloidal material, and a portion of this is the source of‘ _‘ the base-exchange constituents of the soil. INFLUENCE OF SOIL MATERIAL OTHER THAN CARBONATES AND BASE-EXCHANGE COMPLEX ON BUFFER CAPACITY The work reported above has indicated that heavy, noncalcareous soils have a greater buffer capacity than light noncalcareous soils, after due allowance has been made for the influence of the exchange complex. This indicates that the finely divided materials of the soil, other than those associated directly with the exchange complex, play a significant role in the buffering of the soil. Pierre (19) has shown that in fthe application of the buffer-curve data to field work a factor of more than 1.0 must be used and suggests that this is because of the reaction ‘of the acid added with non-exchangeable material in the soil’. Joffe and McLean (6) maintain that when a soil has a reaction of orl below pH 5.6 or 5.4, it contains free acids from inorganic sources, which may react with silicates other than those of the exchange complex and bring iron and‘ aluminum into solution. This material is probably for the most part complex alumino-silicates in which the hydrogen of the acid replaces bases in a manner analogous to the weathering of feldspar with the formation of kaolin. The re- action of the acid with this material is rather slow and indefinite and at present no method is known by means of which a quantitative measurement of the extent to which this material influences the buffer capacity may be made. It is possible and even probable that consider- able acid may be absorbed by this material, without bringing into solution any significant amount of substance. An analysis of the solution would therefore be of no value. Extraction of the soil with 0.2 N nitric acid in the determination of active phosphoric acid and potash removes appreciable amounts of iron, aluminum, and silica. This material is undoubtedlyibrought into solution by the breaking- down of the non-exchange material. It has already been shown that treatment with an excess of 0.2 N acid is not sufficient to remove all of the bases in the exchange complex. From these two facts must follow the conclusion that the non-exchange complex is consuming some acid while there is still an appreciable amount of bases in the exchange complex. The fact that non-calcareous soils of similar acid-consuming power an_d exchange capacity vary widely in the percentage of the acid con- RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 49 sumed required to give a definite pH value in the buffer-curve deter- mination indicates quite strongly that the amount and activity of this portion of the soil material varies over a wide range. While, under the conditions of the determination of buffer capacity in the laboratory, this portion plays a minor part in the results obtained, it is probable that in the field over a longer period of time, it bears a significant relationship to the change in reaction following any treatment. Be- cause of the uncertain composition and reaction of this material, however, no attempt was made in this work to evaluate it. No attempt was made to determine the influence of organic mat- ter on buffer capacity. All of the soils reported in this study are relatively low in organic matter and hence this portion of the soil material played a very minor part. Undoubtedly, in soils high in organic matter, this factor plays an important part in the buffer capacity. Even though a detailed study of the influence of organic matter was not made, the “pH acid” procedure itself is adaptable to this type of soil, as well as to all others. METHOD FOR BASICITY OF LIMING MATERIALS AND FERTILIZERS The estimation of basicity of carbonates of lime or magnesium is merely the estimation of carbonates. There are other materials, such as mixtures of limestone and superphosphate, basic slag, and calcium silicates, in which the estimation of basicity is not so easy. The work here presented gives the basis for a method for such materials. The buffer capacity of such materials depends upon the degree of acidity (pH) secured. In order to estimate basicity, it is necessary to decide on a degree of acidity (pH) at which to stop. The total buffer capacity at this point would be the basicity. The pH decided upon must be low enough to ensure the decomposition of carbonates, but high enough to exclude buffer capacity which is not useful in decreasing soil acidity. If the basicity of liming materials is defined as the power to neutralize acidity which is harmful to plants, then the pH should be between 4.5 and 5.5. A pH of 4.5 is harmful to most plants, While a pH of 5.5 is harmful to only a few plants. The use of 5.0 is suggested. If plants to be grown are very sensitive to acidity, a pH of 6.0 could be adopted were it not for the fact that pH 6 is too high to ensure decomposition of all the carbonates. A pH of 5.0 is suggested as a tentative stopping point for this determination._ The pH acid procedure is applicable not only to soils but to any other solid material possessing buffer capacity. As examples of possible applications, the buffer capacities of two samples of rock phosphate and one of colloidal phosphate were determined. The buffer curves plotted from the data secured were very smooth; in no instance was a pH acid determination as much as 0.1 pH off the regular curve. 50 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION The rock phosphates required acid equivalent to 310 and 220 p.p.m. of calcium carbonate for pH acid 6.0, and 620 and 540 for pH acid 5.0. The corresponding figures for the colloidal phosphate were 11.25 and 1875. On the other hand, the acid consumed by one sam- ple of rock phosphate was 14.5 per cent, and by the colloidal phos- phate, 28.0 per cent, both as carbonate of lime. Thus entirely mis- leading results regarding the basicity of these materials may be obtained from the acid consumed by them. These data are suffi- cient to illustrate the usefulness of the method for materials other than soils. SUMMARY The total buffer capacity of the soil for acids is measured by the quantity of acid required to secure the desired degree of acidity. The specific buffer capacity is measured by the quantity of acid required to cause a definite change in degree of acidity. The buffer capacity for acids measures the reserve bases of the soil and the resistance of the soil to acidifying agencies, such as weathering, acidifying fertilizers, or other additions. A mixture of calcium carbonate and acid may be slightly acid (pH 6.2) when vigorously stirred while the same mixture may have a pH of 5.0, or even 3.4, when only moderately stirred. A mixture of acid with an excess“ of calcium carbonate may be acid (pH 4.25, or even 3.3) unless the conditions are such as to promote the escape of carbon dioxide, the decomposition of calcium bicarbonate, and the completion of the reaction. Manipulation for basicity may be tested by ascertaining the acidity (pH) of mixtures of calcium carbonate and definite amounts of acid. A modified method for buffer capacity of soils for acids is described, the buffer capacity at a desired acidity (pH) being read from buffer curves. The method used for estimating carbonates is described. A mixture of soil and acid may decrease slightly in acidity (pH) during 6O days. Stirring vigorously for 15 minutes gave as good results for acid consumed for soils high in carbonates as stirring‘ and standing over- night. In the estimation of buffer capacity of soils high in carbonates, stirring for 15 minutes followed by standing for 40 hours was found slightly better than stirring and standing overnight. Three types of buffer curves of soil are discussed. The adequate expression of the buffer capacity of soils requires the construction of a curve, or statements of the total buffer capac- ity to a definite degree of acidity (pH) and of the specific buffer capacity between given pairs of pH values. If the salts are washed out after the reaction between the soil RELATIONS OF BUFFER CAPACITY FOR ACIDS OF THE SOIL 51 and acid,- the residue becomes less acid in an amount varying in different soils but averaging about .9 pH. The addition of potassium chloride to the acid decreases the buf- fer capacity; that is, it increases the acidity (pH) of soils with low buffer capacity, but may have little effect upon soils with high buffer capacity. The increase varies, but averages about .6 pH. Addition of potassium chloride to a residue of a soil treated with acid and washed with water increases the acidity (pH) of the. mix- ture to approximately the acidity of the original mixture of soil and acid. The acidity of a natural soil would vary according to the quan- tity of salts present, increasing to a certain extent when the quan- tity of soluble salts is increased and decreasing when the soluble salts are washed out or otherwise removed. Additions of nitrate of soda, sulphate of ammonia, gypsum, and similar salts may tem- porarily increase the acidity of acid field soils. The lime requirement was estimated on washed residues from soils treated with acid to secure a pH of 4. When the amount of acid was corrected for the carbonates in the soil, the Veitch method for lime requirement gave approximately 30 per cent more than the net acid added, and the Jones method 20 per cent more. The Hopkins method gave erratic and low results, only about l0 per cent of the net acid added. Titration of the soil residue with barium hydroxide also gave results in excess of the acid used. Exchangeable hydrogen was determined on some soil residues after treatment with various amounts of acid. A soil may contain both exchangeable hydrogen and calcium carbonate. With some soils, the net exchangeable hydrogen agrees with the net acid used, within the limits of error. The net acid is the total acid added less that used by carbonates in the soil. With other soils, the exchange- able hydrogen is appreciably lower than the net acid used, which seems to indicate that bases are removed from other compounds in addition to the exchange complex. Some calcium carbonate was still present in some soils when acid had been added to secure a pH of 6.0, and some soils still contained small amounts of carbonates at pH 5.0. For soils low in buffer capacity 9.8 per cent of the bases in the exchange complex was removed in changing the pH from 6.0 to 5.0, while with soils of greater exchange capacity only 6.4 per cent was removed. To change the pH from 5.0 to 4.0, 17 per cent of the bases of the complex was removed with soils low in exchange capacity and 11 per cent with those with high base-exchange capacity. This indicates differences in the base-exchange complex in different soils. The total base-exchange capacity of the soil was not changed by treatment with acids to secure various degrees of acidity or in the estimation of acid consumed. 52 BULLETIN NO. 442, TEXAS AGRICULTURAL EXPERIMENT STATION The net acid consumed by the regular method represented 86.4 per l cent of the base in the exchange complex while by the Kappen method i it represented 81.0 per cent. "' _' The percentage of exchangeable hydrogen varied with different soils at the same degree of acidity produced by treatment with acid.‘ i At pH 6.0 it varied from 8 to 41 per cent and at pH 4.0 from 13 to? .; 64 per cent. A H ' i i‘ ._ In the residue left after treatment for acid consumed approxi- ' mately 80 per cent of» the exchange bases wasreplaced by hydrogen; i‘ The base-exchange complex appears to consist of several different compounds. a l The percentage of base extracted by the acid from the exchange complex depends upon the degree of acidity of the mixture (pH) and on the nature and quantity of the base-exchange compounds in the particular soil used. The bases of the soil neutralized by acid came from the carbonates, from the base-exchange complex, and from other compounds. A The basicity of lime materials containing silicates or of fertilizers can be determined by the procedure here described, if a definite degree of basicity is used as the end point. The use of pH 5.0 is sug- gested. Acid consumed is not a correct measure of the basicity of some of these materials, for it exceeds the real capacity of the ma- terial to neutralize soil acidity by a very large amount. Heavy... 1a. §0 10. 11. 12. 13 14. 15. 16. 17. 18. RELATIONS OF BUFFER CAPACITY-FOR ACIDS OF THE SOIL 53 REFERENCES Baver, L. D. and Scarseth, G. D., 1931. The nature of the soil acidity as affected by the S:O2-sequioxide ratio. Soil Sci. 31:159. Britton, H. T. S., 1929. Hydrogen Ions, their determination and importance in pure and industrial chemistry. D. Van Nostrand Co., Inc., New York. Chapman, H. D. and Kelly, W. P., 1930. The determination of the replaceable bases and the base exchange capacity of soils. Soil Sci. 30:391. Fraps, G. S. and Carlyle, E. C., 1929.: The basicity of Texas soils. Texas Agr. Exp. Sta. Bul. 400. Fudge, J. F., 1931. Apparatus for continuous leaching with suction. Ind. Eng. Chem. (Anal. Ed.) 3:114. Hopkins, C. G., 1910. Soil fertility and permanent agriculture, p. 267. Joffe, J. S. and McLean, H. C., 1926. Colloidal behavior of soils and soil fertility: II The soil complex capable of base exchange and soil acidity. Soil Sci. 21:181. Jones, C. H., 1913. 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C. 3:372. \ Hydrogen ion concentration, aluminum j