TEXAS AGRICULTURAL EXPERIMENT STATION A. B. CONNER, Director College Station, Texas BULLETIN NO. 647 APRIL, 1944 SOME FACTORS AFFECTING THE UTILIZATION OF PHOSPHORIC ACID IN SOILS BY PLANTS IN POT EXPERIMENTS G. S. FRAPS and J. F. FUDGE Division of Chemistry i ‘. "T" T]? a1". JG! ' w’: $4.2: A.‘ fialleze fipiian, ‘atexas o;,:-._ AGRICULTURAL AND MECHANICAL COLLEGE OF TEXAS F. C. BOLTON, Acting President F" .. " D-17-444-2500 [Blank Page in Original Bulletin] The relations between the quantities of phosphoric acid re- moved by corn and milo or kafir and the composition and other characteristics of Texas soils was ascertained from the data of 375 pot experiments. The average quantities of phosphoric acid removed per crop increased with the phosphoric acid soluble in 0.2 N nitric acid (active phosphoric acid) when the soils had a basicity of less than 2%. When the basicity was over 2%, the quantities of phosphoric acid removed by the crops increased with the total phosphoric acid of the soil. The quantities of phosphoric acid removed per crop from surface soils which contained similar quantities of active phosphoric acid were greater than those removed from subsoils. With similar quantities of active phosphoric acid, the quantities of phosphoric acid removed by the crops increased with increases in total phosphoric acid, in total nitrogen and in active potash. Phosphoric acid came either directly or indirectly from soil com- pounds in which the phosphoric acid was insoluble in 0.2 N nitric acid, since the quantities of active phosphoric acid in some soils were practically the same before and after cropping. Soils varied widely with respect to the availability of their total phosphoric acid. The availability of the total phosphoric acid in some soils was higher than in rock phosphate, but not as high as in super- phosphate. Availability of total phosphoric acid was higher in calcareous than in noncalcareous soils. A crop of corn recovered from 20% to 30% of the phosphoric acid fixed from solution by different soils, which means that the phosphoric acid fixed in these soils was highly available. This work shows that the quantities of phosphoric acid which a soil will furnish to a crop is related not only to the active phos- phoric acid or total phosphoric acid contained in the soil, but also to its basicity, content of total phosphoric acid, nitrogen, active potash, and whether it is a surface soil or a subsoil. CONTENTS Introduction Different kinds of phosphates in soils Previous work ..................................................................................................... .. Methods and samples Relation of the active phosphoric acid of the soil to the amounts of phosphoric acid removed by crops and to the weights of the crops...‘_;_§ k7. Relations of other factors to the quantities of phosphoric acid x removed bycrops ..................................................................................... 1...‘; Low, medium, and high availability of phosphoric acid in soils containing similar quantities of active phosphoric acid ................... Statistical studies of the factors ................................................................... Correlation coefficients ....................... Q. ........................................... ...... .. Regression equations ........... Q .................................................................... Qflf Correlation and regression within groups based on active phosphoric acid ............................................................................. ............ .. Effect of cropping on active phosphoric acid of some soils ................... Availability of total phosphoric acid of some soils ..................................... Availability of fixed phosphoric acid ............................................................ Acknowledgement .... ..................... It‘ Summary ................................................. .. References ............................................................................................................. .. i SOME FACTORS AFFECTING THE UTILIZATION OF PHOSPHORIC ACID IN SOILS BY PLANTS IN POT EXPERIMENTS By G. S. Fraps, Chief, and J. F. Fudge, Chemist, Division of Chemistry The relation of the chemical composition of soils to the production of crops and possible deficiencies is an important field for research work. The subject is very complex, because the yields of crops are influenced not only by the quantities and forms of combination of various elements in the soil, such as phosphoric acid, potash, nitrogen, calcium, magnesium, iron, manganese, boron, and zinc, but also by the amount and distribution of rain, the capacity of the soil to receive and hold water, the depth of the soil suitable for root growth, length of day, intensity of light, temperature, kind and variety of plants grown, and perhaps by other factors. In such a complexity of factors, it is usually necessary to study one factor at a time, keeping all the others constant so far as possible, except those whose effects upon the factor being studied are varied in order to ascer- tain the effects of such variations. Phosphoric acid (P205), because of its economic and its scientific im- gportance, is one of the factors which has been given considerable atten- tion at the Texas Agricultural Experiment Station. Pot experiments were used in much of the work, because it is possible, in pot experiments, to mix the soil to secure uniformity and to keep constant some of the other ffactors which affect plant growth. The results of pot experiments cannot always be applied directly to farm practice, but need confirmation by field experiments. Previous work has shown definite relations between the quantities of active phosphoric acid in the soils and the amounts of phosphoric acid withdrawn by crops in pot experiments (4, 5, 6, 7, 8, 9, 10, 11, 14, 18, 22, 24).. When the soils are divided into groups according to their content of active phosphoric acid, within each group there are considerable variations from the average amount of phosphoric acid removed. It is important to ow what factors cause differences in the use by plants of the phosphoric acid in different soils. Larger amounts of phosphoric acid were removed by plants from soils containing the same amounts of active phosphoric acid when the soils had a higher content of nitrogen or of total phosphoric acid than the average, and from surface soils as compared with subsoils. The object of this publication is to present the results of additional studies on these relations. Different Kin-ds of Phosphates in Soils Phosphoric acid may be found in soils as inorganic compounds such as jlpatite or calcium fluophosphate, derived from the original rocks, or as rganic compounds derived from plant or animal residues (12, 13). It may be present as tricalcium phosphate, and as various phosphates of aluminum and of iron. If commercial fertilizers containing phosphates are used, 6 BULLETIN NO. 647, TEXAS AGRICULTURAL EXPERIMENT STATION monocalcium phosphate and dicalcium phosphate may be found for periods of time. Calcium phosphate may combine with fluorine to f calcium fluophosphate. Soluble phosphates will form less soluble w, phates of calcium, of aluminum, and of iron (20, 34), or be fixed by _V linite and some other silicates (2, 30, 32). Both pot and field experiments have shown that there are d ~ differences in the quantities of phosphoric acid which plants can re n? from different kinds of natural phosphates (16, 19). Tricalcium phosp y has a high availability to plants, while ground rock phosphate (cal V f: fluophosphate) has a low availability. Phosphates which differ littl,‘ solubility in reagents may be quite different in availability to plants Phosphates may also be physically enclosed within soil particles and be inaccessible to the roots of plants (10). These particles may be d posed or dissolved by chemical solvents, so that phosphates which may physically unavailable to plants are dissolved in the chemical anal ~ _. Even weak acids will dissolve particles of calcium carbonate which ~’ enclose phosphates. Different kinds of plants also differ in their poweg utilize phosphates in the soil. This may be due to differences in depthi penetration by the roots, in the solvent powers of the roots, or in the i tent to which the fine feeding roots occupy the soil. " g-vfil/Nlewiqykwivfl’ f. I ’ It has previously been pointed out (26) that part of the phosph acid dissolved by the 0.2 N nitric acid or other weak solvents may», taken out of solution‘ by the fixing power of the soil. The results ofa_ f analyses represents the equilibrium between the phosphoric acid dissol ‘ and the fixing power of the soil for phosphoric acid. Further, the pf phoric acid dissolved includes not only that upon the surface of the particles and exposed to the roots of the plants, but also that from wi p’ , such soil particles as are soluble or partly soluble in the solvent and l‘ exposed to the roots of the plants. When a soil contains 10 or less p per million of active phosphoric acid and has a fixing power of less 5' 80% and a basicity of less than 1%, the phosphoric acid is prob found as basic phosphates of iron or aluminum, and probably none?) found as apatite or calcium phosphates (10). When 10 or less active phosphoric acid are found and the fixing power for phosphoric r exceeds 75%, calcium phosphates may or may not be present. The calci 1 phosphate may go into solution and then part of the phosphoric acid _[ , _ be removed by fixation. Soils which contain more than 10 p.p.m. of acti, i phosphoric acid probably contain calcium phosphates. Soils which contfi more than 100 p.p.m. of active phosphoric acid and have a basicity of l, than 2% probably contain considerable amounts of calcium phosphafi which is accessible to the roots of plants, although this may not always the case. If the soil has a basicity of more than 2%, part of the phony‘- phates may be protected from the roots of plants by calcium carbonate. It is obvious that the determination of the various kinds of phosphatfii in soils and the relation of these phosphates to crop growth is a task-oi considerable magnitude. Soils of different origin may differ widely in ‘the kind of phosphates which they contain, and results secured with one kind of crop may be different from those secured with other crops. a 5 FACTORS AFFECTING UTILIZATION OF PHOSPHORIC ACID BY PLANTS 7 Previous Work The relation of the phosphoric acid of the soil to the phosphoric acid in crops and the effects of fertilization with carriers of phosphoric acid has been the subject of a very large amount of work by many investigators and covers a period of nearly a hundred years. Work done on Texas soils . alone by the senior author covers a period of over forty years. It is ob- viously impracticable to cite all the literature dealing with the results of this vast amount 0f work; however, certain points dealing particularly with Texas soils are of interest. The work on the relation of the phosphoric acid of the soil to the phosphoric acid removed by plants in pot experiments has already been mentioned. Soils vary widely in their capacity to fix phosphoric acid from fertilizers, and this affects the availability of phosphates. Fraps (20) found that soils having a fixing power of more than 50%, when treated 'with superphosphate and subjected to percolation, held practically all of the added phosphoric acid in the upper layers; with most soils having a fixing power of less than 50%, considerable amounts were frequently washed to lower layers of the soil. The action of phosphate fertilization in causing an increase in active phosphoric acid and a downward movement of phosphoric acid in the soil horizon was studied in two sets of field plats. Active phosphoric acid in a Lufkin fine sandy loam at College Sta- tion (27) was markedly increased by applications of superphosphate and there was a considerable downward movement of phosphoric acid into lower levels of the soil. On the other hand, active phosphoric acid in a Lake Charles clay loam at Beaumont (28) did not increase significantly, except with very heavy applications of superphosphate, and no evidence was found to indicate a downward movement into lower levels. The phos- phates were fixed in the Lake Charles clay loam in compounds which were insoluble in 0.2 N nitric acid. When soils are high in iron which is soluble in dilute acids, they have a considerably higher fixing power than soils which are low in that respect (34). The presence of various compounds other than phosphates may have a considerable effect upon the efficiency with which plants may utilize phosphoric acid already in the soil or introduced in applications of super- phosphate. Calcium carbonate in the fertilizer or in the soil may decrease the availability of superphosphate (17). Basic compounds of calcium ap- plied to the soil to reduce soil acidity may reduce the availability of phos- phate if applied at the same time as the fertilizer or if mixed with the fertilizer (17 ). Nitrate of soda increased the effect of calcium carbonate in reducing availability. The relative effect of liming materials on the availability of phosphoric acid already present in the soil varied widely with different kinds of soils. Working with 6 selected soils of low basicity from the East Texas Timber Country, it was found (15) that limestone increased the size of the crop and the amount" of phosphoric acid removed; the effect of the lime was small at first but increased with succeeding crops. The quantity of phosphoric acid removed by crops had practically no effect upon the quantity of active phosphoric acid remaining in the soil at the end of the experiments (15); evidently the phosphoric acid taken up by the plants was drawn directly or indirectly from the more insoluble phosphates in the soils. 8 BULLETIN NO. 647, TEXAS AGRICULTURAL EXPERIMENT STATION Methods and Samples The methods used in the present work were briefly as follows: Total phosphoric acid, P205: The soil was ignited with magnesium-y nitrate, the mixture was dissolved in hydrochloric acid, filtered, and phos-- phoric acid was then determined by the volumetric method of the Associa- v tion of Official Agricultural Chemists (1). Active phosphoric acid, P205: (10). Basicity: percentage of calcium carbonate. Soils: Nearly all of the samples used had been sent in by members of the Division of Soil Survey, U. S. Bureau of Soils, as being representative 1 of the principal types of soil in the various areas of Texas. Surface soils in most cases were collected to a depth of 7 inches; most of the subsoils I were from the depth of 7 to 19 inches. The soils were dried, passed through a quarter-inch mesh sieve to remove plant material and rocks, mixed, and stored in galvanized iron cans with closely fitting covers. The _ average chemical analyses of Texas soils, together with other information, are given elsewhere (25). Pot experiments: Five kilograms of soil were weighed into galvanized iron pots, the fertilizer mixed with the soil, and water equivalent to 50% of the water-holding capacity of the soil was added. Two or three pots. containing soil to be tested for phosphoric acid received 1 gram of am- monium nitrate and 1 gram of potassium sulfate (NK), while other pots. received the same additions plus 1 gram of dicalcium phosphate (NPK). Weighted quantities of seed were then planted. After the seedlings were well established, they were thinned to 3 plants of corn or about 12 plants of grain sorghum per pot. The pots were kept in a greenhouse. The crops, after harvesting, were dried at about 40°C., weighed, and analyzed for phosphoric acid by the volumetric method. In most cases, corn was grown from about March 15 to May 15. After the corn was harvested, milo or kafir was usually planted in the same pots. The quantity of phosphoric acid removed by the crop was calculated from the weight and the analysis, and is expressed in parts per million of dry soil. Relation of the Active Phosphoric Acid of the Soil to the Amounts of Phosphoric Acid Removed by Crops and to the Weights of the Crops The average relations of the active phosphoric acid of the soils to the phosphoric acid removed from the soil per crop, to the weights of the crops, and to the average basicity and total phosphoric acid in all of the soils are shown in Table 1. The phosphoric acid removed by the crops and the weights ‘of the crops from the soils which did not receive phosphoric Two hundred grams of soil were di- V, gested 30 minutes at 40°C. in 2,000 cc. of 0.2 N nitric acid and filtered After evaporating and drying to remove silica, phosphoric acid in an . aliquot was precipitated with molybdate solution, the precipitate dissolved 1 in caustic potash and titrated with nitric acid as in the volumetric method. An aliquot of 10 cc. of the filtrate from the digestion, above was heated to boiling and, after cooling, titrated with 0.1 N sodium; hydroxide, using phenolphthalein indicator. The percentage of nitric acid ; neutralized by the soil, divided by 10, gives the basicity, expressed as the a hi), Fv.»q;,,;m.£m.,~/_4i' .w¢.~»1»1 » ~ , r,a A - .55.; w). i ‘ win-v 4- ._.___...__.=_-__i.4__._..._,n..4_i FACTORS AFFECTING UTILIZATION OF PHOSPHORIC ACID BY PLANTS 9 -‘l‘able 1. Average relation of active phosphoric acid of the soil to the phosphoric acid removed by crops and to the weights of the crops. to active Active Phosphoric Weight Weight Total Basicity Number oric acid phospho- acid re- per crop per crop phospho- of soil of ric acid moved per NK NPK ric acid soils _ crop in soil .p.m. p.p.m. p.p.m. gm. gm % % 0 to 10 8 2.6 6.9 27.7 .037 1.73 75 110 20 15 4.9 11.6 28.8 .042 1.79 68 t0 30 25 8.3 16.6 30.2 .052 3.85 40 to 50 40 8.0 16.9 28.3 .056 3.98 57 to 100 74 8.8 17.3 25.3 ~ .071 4.71 46 1 t0 200 138 11.0 19.4 29.5 .077 5.31 52 to 400 279 13.1 23.3 34.7 .088 3.05 21 er 400 536 18.2 25.6 28.8 .133 4.46 16 (‘acid increased with the active phosphoric acid content of the soils. This is in accord with work previously published (10, 18‘), such as the data in ,.Table 2 which are taken from Bulletin 267, published in 1920. The total phosphoric acid and basicity in the soils averaged in Table 1 show a ten- Table 2. Relations of active phosphoric acid of soils to phosphoric acid removed by crops and to weight of crops (From Bulletin 267. 1920). ‘ Active phosphoric Number of soils Phosphoric acid Weight per crop acid removed per crop NK NPK p.p.m. p.p.m. of soil gm. gm. 7.3 18 2.7 7.0 28.3 14.8 44 4.5 10.3 21.2 25.1 9 5.6 14.0 30.1 34.8 8 12.8 25.9 39.6 48.3 10 8.2 19.5 28.8 f dency to increase with the active phosphoric acid. The average weights of the crops produced on the soils with complete fertilizer were reasonably ,, uniform, ranging from 25.3 to 34.7 grams. Correlation and regression coefficients are discussed in a later section. Relation of Other Factors to the Quantities of Phosphoric Acid Removed by Crops In order to secure information on the relations of factors other than active phosphoric acid to the quantities of phosphoric acid removed by the crops, the data were arranged in several different ways and averaged as shown in Table 3. The groups used for active phosphoric acid, total phos- a phoric acid, total nitrogen, active potash and basicity are the same as those used in classifying the constituents of the soils of Texas (25) into grades 1, 2, 3, 4, 5, with the exception of one group of active potash. ~ The number of soils which contain less than 50 parts per million of active potash was too small to justify a separate group. Table 3 shows that the quantities of phosphoric acid taken up by the _ crops is related to the quantities of active phosphoric acid in the soil, but ' is influenced to a considerable extent by other factors. With the same quantities of active phosphoric acid, crops grown in surface soils took up considerably more phosphoric acid than those grown in subsoils. There are some exceptions, but this is the rule in most of the comparisons in Table 3. This is in line with the conclusion previously stated (18), that, 10 BULLETIN NO. 647, TEXAS AGRICULTURAL EXPERIMENT STATION 5 s: - .;p'q: . .geF: u. . . -q:gq . . . - . >¢ . .g;g; a .@\_Q . . . . -|_~_gg - .Q1@L- - . . . cq - . - 4N . . - Q] . - . . . g a 3 O . he J; *5 d“: ' ‘m. ‘ ' ° “WW”! ‘W? “t? “Q59”! "'1 ‘m. ‘ __,2'S - we - - <9 wovua ns-m -L- womvco um ~00 .5. N g5 w Ed. g u oi.- ‘i ... c F‘ 4 F‘ “o-u * “i”??? ‘"'.°‘!°'?“2 °°“?"‘.".‘°. ' ‘°?"!“'. "’I°°."1 § ° fig-aé w-now-u: ~r-4LIJIDOO ww-mmvco - wrwco -r.\1coc\1 x» i: “a . ,_. . . . § u "‘ l: g ‘U 2 .25 |.. g q ‘an v E g 8=€ 109w? - <=2¢\1.==.~=:<=. “wwfiwc =°.<".°’%"i°. k q; or! Q W wuovcu - vvmmoo Nvccmc: Hwcomco -m<:<»-1 1,, 1i v-u-q O ~ v-1 _ m H - O p; ,: 3g <1 m u " z -° - Q °.°3°?°‘."! QC}??? "*°‘.°°°. ' NPOFQ“! “YEW”. ‘j Em “g wmmmoo w-uxmwc- NQQNQO - cummwcv .-. '---w. ' -l~.<=2¢. 3 a Q >3 - won-o - wmow . . . .¢¢ - -c': - - w-wn - - -occ> - -m a5 --O\=¢>r- cnoo<-:"=. 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In the groups of soils which contained from {O to 30 p.p.m. of active phosphoric acid, the phosphoric acid removed by icthe crops averaged 4.9 p.p.m. from soils which contained less than .025% Ztotal phosphoric acid, and 14.0 p.p.m. from soils which contained more than .150% total phosphoric acid. The phosphoric acid removed by lcrops averaged 4.9 p. p. m. from soils containing less than .03% total fnitrogen, but was 9.8 p.p.m. from soils containing more than .120% l‘ total nitrogen. The phosphoric acid removed by crops averaged 4.9 p.p.m. {from soils containing less than 100 p.p.m. active potash, but was 14.0 p.p.m. from soils containing more than 400 p.p.m. active potash. Similar increases are evident for the soils which contained from 31 to 100 p.p.m. active phsophoric acid. With soils which contained more than 100 p.p.m. active phosphoric acid, the effects of different amounts of the constitu- ents referred to above, while still evident, are not so regular. However, the number of soils in these groups was quite small. The relations between the active phosphoric acid in the soil and the phosphoric acid removed by the crops were especially close when the soils were low in total nitrogen, total phosphoric acid, and active potash. From soils which contained less than 30 p.p.m. active phosphoric acid, the plants removed larger quantities of phosphoric acid when the basicity was over 2% than when it was below 2%. This relation was reversed with soils containing over 100 parts per million, since the plants removed less phosphoric acid when the basicity was above 2% than when it was below this amount. The capacity of the soils to fix added phosphoric acid had no regular effect upon the quantity of phosphoric acid removed by the plants. The relation of the composition of the soils to the quantity of phos- phoric acid removed by crops, expressed as percentages of the number of soils in the different groups, is shown in Table 4. When the active phos- phoric acid was less than 30 p.p.m., 68% of the soils produced crops which contained less than 4.9 p.p.m. of phosphoric acid; when the soils contained from 31 to 100 p.p.m. of active phosphoric acid, 36% of the soils furnished less than 4.9 p.p.m. of phosphoric acid to the crops. Table 4 also shows that the quantities of phosphoric acid taken up by the crops are related not only to the amount of active phosphoric acid in the soils, but also to the amounts of total phosphoric acid, nitrogen, and active potash. There seems to be little relation between the quantity of phosphoric acid removed by the crops and the quantities of acid-soluble potash, acid-soluble lime, and phosphoric acid absorbed by the soils. From the data available, it may be concluded that, with soils of simi- lar active phosphoric acid content, higher quantities of phosphoric acid were removed from soils in which total phosphoric acid, total nitrogen, and active potash are relatively high than from those in which these con- stituents are relatively low. 12 BULLETIN NO. 647, TEXAS AGRICULTURAL EXPERIMENT STATION Table 4. Percentage of soils in different groups based upon the quantity of phosphoric " acid removed per crop and grades of various constituents. _. Num- Percentage of soils in groups based on Removai. ber quantity of phosphoric acid removed at Constituent and grade of 0 5.0 10.0 over 50% of soils to to to 19.9 soils 4.9 9.9 19.9 ,~ p.p.m. p.p.m. p.p.m. p.p.m. p.p.m. p All soils 363 48 25 18 9 Active phosphoric acid, p.p.m. Grade 5 0 to 3O 174 68 24 7 1 2.9 ” 4 31 to 100 97 36 28 34 '2 5.2 _, ” 3 101 to 200 54 35 17 22 26 8.5 " 2 201 t0 400 22 14 41 27 18 9.2 " 1 Over 400 16 0 25 44 31 13.9 Total phosphoric acid, % Grade 5 0 - .026 83 88 17 0 0 2.5 " 4 .026 - .050 126 44 29 25 2 5.9 3 .051 - .100 85 40 28 19 13 6.3 " 2 .101 - .150 59 27 27 29 17 8.8 " 1 Over .150 10 0 20 60 20 16.9 Nitrogen, % Grade 5 0 - .030 25 72 16 12 O 2.6 " 4 .031 - .060 86 68 23 8 1 3.7 ” 3 .061 - .120 177 49 26 21 4 5.1 " 2 .120 - .780 59 12 32 27 29 10.7 ” 1 Over .180 10 10 20 20 50 19.9 1 Acid-soluble potash, % A Grade 5 0 - .10 23 62 30 8 0 3.8 ” 4 .11 - .20 36 3.6 33 25 6 6.2 " 3 .21 - .40 87 71 18 7 4 2.7 ' " 2 .41 - .80 83 45 27 19 9 6.1 ” 1 Over .80 42 30 23 28 19 8.6 Active potash, p.p.m. Grade 5 0 - 50 20 80 20 0 0 2.5 i ” 4 51 - 100 63 73 22 5 0 3.0 3 101 - 200 110 53 28 18 1 4.4 ” 2 201 - 400 95 39 30 21 10 6.3 " 1 Over 400 65 15 20 37 28 13.2 ‘ Basicity, % Grade 5 0 - .30 65 54 31 14 1 4.1 " 4 .31 - .60 44 61 14 18 7 4.1 " 3 .61 - 2.00 113 41 27 20 12 6.3 " 2 2.01 - 5.00 41 51 22 22 5 4.5 ” 1 Over 5.00 100 47 27 19 7 5.1 Acid-soluble lime, % q Grade 5 0 - .10 21 63 16 16 5 3.9 , ” 4 .11 - .20 25 40 44 16 5 5.0 L " 3 .21 - .40 38 50 29 16 5 5.2 ” 2 .41 - 2.00 98 50 22 18 10 4.9 " 1 Over 2.00 81 50 25 17 8 5.0 Phosphoric acid absorbed, % 0 - 25 17 41 29 23 7 5.2 26 - 50 46 48 35 13 4 5.0 51 - 75 57 54 19 21 6 4.7 Over 75 50 68 18 10 4 2.6 Low, Medium, and High Availability of Phosphoric Acid in Soils Containing Similar Quantities of Active Phosphoric Acid In order to study further the causes for differences in phosphoric acid removed by crops from soils similar in active phosphoric acid content, the soils were divided into groups according to the active phosphoric acid in the soils, and then further divided into subgroups containing soils from which the crops removed quantities of phosphoric acid which could be con- sidered as low, intermediate or high for each main group. The results are shown in Table 5. The soils similar in active phosphoric acid content from which the plants removed relatively low quantities of phosphoric acid contained 13 FACTORS AFFECTING UTILIZATION OF PHOSPHORIC ACID BY PLANTS 3;. vc.v 3v cc.v mg mm.v am.v av. S» v2. mvm W: ma.“ v.8 avc. ca swam 2. ma.» 3.” w”. mam wvc. 3E w... mam v.3 S... S. vusvosufinH 2: 3.» N; av. m2 cvc. 3v v.m a.mm m... mmc. cw B3 N2 mumwnoz. cwuocww $4 S. va. $3 N2. 9:. v3 v.cv 3v 3A. 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O-AQH. 14 BULLETIN NO. 647, ‘TEXAS AGRICULTURAL EXPERIMENT STATION smaller amounts of total phosphoric acid, nitrogen and active potash than the soils from which intermediate or high quantities of phosphoric acid were removed. The differences were greater with soils which were low in active phosphoric acid than with those which were high. The relations be- tween total posphoric acid, nitrogen and active potash and the availability of the phosphoric acid in soils containing similar amounts of active phos- phoric acid are thus similar to those brought out in a different way in >1‘>l the preceding section. Acid-soluble lime, total potash, and the quantity of phosphoric acid absorbed by the soil had little apparent relation to the relative quantities of phosphoric acid removed by the crops. Soils from which high quantities _ of phosphoric acid were removed contained relatively high amounts of. these constituents, with the exception of total potash. The greatest diff ferences between the groups were in the weights of the crops; this was to f be expected, since the higher quantities of phosphoric acid removed, for which these groups were subdivided, would naturally be associated with larger crops. The average phosphoric acid removed by two successive crops from soils which contained similar quantities of active phosphoric acid but dif- fered in other characteristics is shown in Table 6. As in work previously Table 6. Effect of level of active phosphoric acid upon the phosphoric acid removed by two crops from different soil groups (p.p.m. of soil). 0 to 30 31 to 100 101 to 200 201 to 400 Over l“ Active Active Active Active Active All soils 8.7 16.6 23.3 26.0 38.8 Surface soils 13.4 20.3 30.4 34.2 41.2 ‘ Subsoils 6.6 10.8 10.3 11.5 30.6 Basicity below 2% 9.5 14.1 19.8 14.8 29.1 Basicity above 2% 8.4 18.8 32.6 39.4 75.6 Humid section 10.3 19.1 21.2 26.8 31.9 Subhumid section 10.0 13.5 20.2 26.2 41.7 Humid, low basicity, surface 11.8 22.4 38.3 . . . . . Humid, low basicity, subsoil 6.2 12.7 . . . . . .. . Humid, high basicity, surface 22.3 22.4 31.8 28.3 43.7 Humid, high basicity, subsoil 6.0 15.1 10.9 16.9 13.7 Subhumid, low basicity, surface 9.0 21.4 35.9 49.3 74.4 Subhumid, low basicity, subsoil 5.3 9.9 12.9 16.2 77.9 Subhumid, high basicity, surface 22.1 14.0 17.2 15.6 28.9 " Subhumid, high basicity, subsoil 12.3 7 1 9.0 6.7 17.2 discussed, a relation between active phosphoric acid in the soil and phos- phoric acid removed by the crops is evident. The degree and effect of the relation differed with different groups of soils. The differences are larger with surface soils than with subsoils. When the soils contained less than 30 p.p.m. of active phosphoric acid, the phosphoric acid removed by the crops averaged 13.4 p.p.m. from surface soils and only 6.6 p.p.m. from sub- soils. Phosphoric acid removed by the crops from soils which contained less than 30 p.p.m. of active phosphoric acid averaged 9.5 p.p.m. in soils in which the basicity was below 2%, and 8.4 p.p.m., or nearly the same, in the soils in which the basicity was above 2%. In surface soils and sub- soils from the subhumid section of the state, phosphoric acid removed by crops had no relation to active phosphoric acid in soils of high basicity FACTORS AFFECTING UTILIZATION OF PHOSPHORIC ACID BY PLANTS 15 (an irregular range from 6.2 to 17.2 p.p.m.) but showed a very high rela- tion to active phosphoric acid in soils of low basicity (a regular increase from 9.0 to 74.4 p.p.m.). The data in Table 6 clearly show that the phosphoric acid removed by crops from soils of similar active phosphoric acid content depends to a certain extent upon whether the sample is a surface soil or a subsoil, whe- ther the basicity is less than 2% or more than 2%, and whether the soil came from the humid or subhumid region of the state. A further study of the factors which affect the amounts of phosphoric acid removed by crops from soils arranged in groups of similar active phosphoric acid content but of different general characteristics is given in Table 7. In this table, the number of soils which, though similar in active Table 7. Distribution of soils within different groups from which plants removed low, intermediate, and high amounts of phosphoric acid. As percentage of total As percentages within number of soils each group Groups compared Low Inter- High Total Low Inter- High ' mediate mediate (36) (45) (19) (100) Humid 15 24 12 51 29 47 24 subhumid 21 21 7 49 43 43 14 Surface 11 26 17 54 20 48 32 Subsoil 25 19 2 46 54 41 5 Basicity below 2% 18 34 14 66 27 52 21 Basicity above 2% 18 11 5 34 53 32 15 Light texture 18 27 7 52 35 52 13 Heavy texture 18 18 12 48 38 38 24 Humid, low basicity 10 19 8 37 27 51 22 Humid, high basicity 5 5 4 14 37 36 27 Subhumid, low basicity 8 15 6 29 27 52 21 subhumid, high basicity 13 6 1 2O 65 30 5 Light texture, surface 4 16 7 27 15 59 26 Light texture, subsoil 14 10 1 25 56 40 4 Heavy texture, surface 7 10 10 27 S6 37 37 Heavy texture, subsoil 11 9 1 21 52 43 5 Low basicity, surface 4 18 13 35 11 51 38 Low basicity, subsoil 14 16 1 31 45 52 3 High basicity, surface 7 8 4 19 37 42 21 High basicity, subsoil 11 S 1 15 73 2O 7 Humid, surface 4 13 11 28 14 46 40 Humid, subsoil 11 11 1 23 48 48 4 Subhumid, surface 7 13 6 26 27 50 23 Subhumid, subsoil 14 8 1 23 61 35 4 phosphoric acid content, gave up low, medium, or high quantities of phos- phoric acid to crops are compared in groups of factors. The percentage distribution of the soils with respect to both the total number of soils and to the number of soils within each subgroup are given. The term avail- ability will here be applied to the active phosphoric acid for convenience, though it is not correct, since the active phosphoric acid does not seem to be removed by the crops. ' When only one factor at a time is considered, the data given in Table 7 show that the relative availability of active phosphoric acid was higher in surface soils than in subsoils, in soils from the humid section of the state than in those from the subhumid section, in soils of basicity lower than 2% than in soils of basicity higher than 2%, and in soils of heavy 1s BULLETIN N0. e47, TEXAS AGRICULTURAL EXPERIMENT STATION texture than in those of light texture. When more than one factor at; time is considered, the same general relations hold, but information n] cerning the interaction of factors is obtained. In surface soils, availabW‘ was higher in heavy textured soils than in light textured soils, but was little difference in subsoils. In soils of basicity over 2%, availab’ f was higher in soils from the humid section than in those from the = humid section, but there was little difference due to source of soils I which basicity was low. Availability was highest in surface soils of 1d basicity and heavy texture from the humid section and lowest in subso of basicity over 2% and light texture from the subhumid section. ‘ In another arrangement of the data, various groups of soils averaged as shown in Table 8. Averages given in this table show that, all the groups, surface soils contain more active phosphoric acid :.j___ slightly more total phosphoric acid, yielded larger crops, and provi more phosphoric acid to the crops than did the subsoils. Soils withj basicity below 2%, as compared with those having a basicity above 2 contained smaller‘ quantities of active phosphoric acid and total phospho acid; however, the weights of the crops and the phosphoric acid by the crops from the two groups of soils were practically the saine. So‘ of the humid section, as compared with those from the subhumid secti contained smaller quantities of active phosphoric acid and smaller quan ties of total phosphoric acid, but produced practically as large crops W111 contained as much phosphoric acid. The surface soils of the humid secti with low basicity contained smaller amounts of active phosphoric acid .- of total phosphoric acid and yielded smaller crops and amounts of ph phoric acid to the crops than corresponding soils of high basicity. The so of low basicity from the subhumid section contained larger quantities ng‘ active phosphoric acid and total phosphoric acid and yielded larger cro l1 and more phosphoric acid in the crops than corresponding soils of A humid section. The subhumid surface soils of high basicity, as comp with similar soils from the humid section, contained larger amounts s» active phosphoric acid and total phosphoric acid but yielded slightl smaller crops and less phosphoric acid in the crops. Table 8. Average phosphoric acid in soils, phosphoric acid removed by crops, and weight of from different groups of soils. Number Phosphoric acid Phosphoric acid Weight of '1) i Soil group of in soils removed by - " - soils Active Total One crop Two crops One crop p.p.m. % mgm. mgm. gm. All soils 320 88 .060 8.3 16.2 15.1 Surface soils 188 103 .064 11.2 21.7 19.5 Subsoils 132 66 .056 4.5 8.9 9.0 Basicity low (less than 2%) 188 54 .040 7.7 15.5 14.9 3asicity high (above 2%) 132 136 .089 8.9 17.2 18.2 Iumid section 162 _ 40 .055 8.0 15.5 14.5 iubhumid section 158 118 .065 8.6 17.0 15.6 Iumid, 10w basicity, surface 54 29 .035 8.5 16.7 16.7 Iumid, high basicity, surface 38 130 .098 14.7 28.5 22.0 {ubhumid, low basicity, surface 62 101 .051 12.2 24.5 21.5 {ubhumid, high basicity, surface 34 195 .099 10.2 9.1 18.0 Iumid, low basicity, subsoil 41 15 .035 3.6 8.1 7.0 lumid, high basicity, subsoil 29 95 .070 5.4 10.5 10.5 ‘ubhumid, low basicity. subsoil 31 65 .038 5.1 9.9 9.7 ubhumid, high basicity, subsoil 31 117 .086 4.2 8.7 8.6 FACTORS AFFECTING UTILIZATION OF PHOSPHORIC ACID BY PLANTS 1'7 Statistical Studies of the Factors Statistical studies of the data from each individual soil offer certain advantages in addition to the presentation of the data by averages. Each individual set of observations is given its proportionate weight in the va- rious calculations. The significance of differences and relations between the data may be mathematically estimated. Where relations are of high significance, quantitative statements of the relations may be calculated. The methods used are described by Snedecor (33) and by Fisher (3). The individual sets of data, from which the averages given in Table 8 were calculated, were used in connection with the statistical studies to be dis- cussed below. Correlation Coefficients The correlation coefficients for active phosphoric acid and for total phosphoric acid in different groups of soils, as related to the phosphoric acid removed by one crop and by two crops and to the weights of the crops, are given in Table 9. The relations between active phosphoric acid and total phosphoric acid in the soil are also given. The coefficients are significant when they exceed the smallest signifi- cant coefficient as given in the first column of figures. When all of the soils were combined into one group (line 1 of Table 9), the coefficients of correlation for all of the comparisons were highly significant. The correlation coefficients for phosphoric acid removed by the crops were usually higher for two crops than for one crop, or for the weights of either one or two crops. The coefficients for active phosphoric acid averaged slightly higher than for total phosphoric acid. When the soils were divided into two groups, for single comparisons, the coefficients were still highly significant in nearly all cases. The coef- ficients were very similar for surface soils and for subsoils. As already shown, less phosphoric acid was removed from subsoils than from surface soils of similar active phosphoric acid content, but this did not affect the degree of correlation. Coefficients for active phosphoric acid were much larger in soils of low basicity than in those of high basicity. Coefficients for total phosphoric acid were considerably smaller in soils of low basicity than in soils of high basicity. Coefficients for soils from the humid region of the state were usually considerably smaller than those for soils from the subhumid section. When the soils were divided into eight groups, the coefficients for active phosphoric acid were highly significant for soils of low basicity, while those for surface soils of high basicity from the “humid region are barely significant. Coefficients for total phosphoric acid were highly sig- nificant in surface soils of high basicity and in soils of low basicity from the subhumid region. Active phosphoric acid and total phosphoric acid were significantly correlated in only three groups of soils, all of which were from the, subhumid region. Correlation coefficients for the relation between the phosphoric acid removed and the weights of crops were calculated for all the groups listed in Table 9. With the data from one crop (corn), the lowest figure was .87 for humid, high basicity, surface soils and subhumid, low basicity sub- soils, and the highest (.99) for humid, high basicity, subsoils, and subhumid, 18 BULLETIN NO. 647, TEXAS AGRICULTURAL EXPERIMENT STATION oomfiflouooo ocoofimcwmw fiawi... flomeaiouoo. ufloofimflwwm .... .3”. mm. .3 2m. 8. Nu. S. mo. 8. mm. ~63... £xc§fi .32 ifi=i=w .23. i3. i3. fig. i3. 1.5.. $11.. **vm. 5.5. mm. momma... iomomman 32 émfiinsm g. S. om. 2. um. 2. 2. ‘N. 3. w”. sown... .235; fir. 335i mo. 2. mm. ow. S. .28.. 3%. ...&w. .28. 3. c022. 5223... 32 .255 .2": 32.. in». ...&m. .1265. ‘N. ma. *3“. mm. mm. .552... 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I3. :. mfiom =< .598 no.5 maooo n26 mflom naouo mono naouo no.6 uflomo o3? oiO 95H 0:0 E Boa 93H. oiO 9BR. 0G0 Auuooo moooo wo>oEoa Ho“ 22:3 2:5 @255?- Eoo 050 9.8a flow no fi-Mmo? oiosano-Tm éoan no i-umo? omooinmo-Tm Ania? 33s i» miom E Eon oioznmonn 13oF n=ou E Eoo omooifinoaa o>3o< |=oEm doooo wo EH9: on: noooo o5 E Eon oiozomozo via mom o5 E How omuosonono iooiooa flown-Bongo 25E» mo wofiomoflwooO d Baum. ~r FACTORS AFFECTING UTILIZATION OF PHOSPHORIC ACID BY PLANTS 19 high basicity subsoils. For two crops, they were lowest (.80), with humid, 10W basicity, surface soils and highest (.99) with high basicity soils. The correlation was high in all cases, but was not 1.00 in any group of soils. The proportions of the variations in the data which may be attributed to concurrent variations in active phosphoric acid or total phosphoric acid were estimated by squaring the coefficients of correlation-(Table 10). For active phosphoric acid, the groups in which these proportions were appre- ciable were soils of low basicity. Active phosphoric acid is thus a reason- able basis for prediction with soils which are low in basicity; it is not a good basis for soils with a basicity of over 2%. For total phosphoric acid, the relations were appreciable in surface soils of high basicity and in soils of low basicity from the subhumid region; total phosphoric acid is thus a reasonable basis for prediction in these soils but not in other soils. The relations are closer with two crops than with one crop, with phosphoric acid removed by the crops than with the weights of the crops, and with soils from the humid region of the state than with those from the subhumid region. Regression Equations Regression equations show the mathematical relation between two quantities, so that, if one is known, the other can be calculated. If the re- lation between the active phosphoric acid and the phosphoric acid removed by crops can be expressed as a straight line, the phosphoric acid removed can be calculated by multiplying the quantity of active phosphoric acid by a factor and adding a constant. Equations for simple regression, which conform to the equation of a straight line, = mx + b, were calculated for the data secured in this work. For this purpose, y represented the calculated quantity of phos- phoric acid removed by the crops in parts per million of the soil; x, the quantity of active (or of total) phosphoric acid in the soil in parts per million; m, the coefficient of the active (or total) phosphoric acid which Table 10. Percentage of variation in phosphoric acid removed by crops or in weights of crops attributed to concurrent variations in phosphoric acid in soils. . Active phosphoric acid in soils Total phosphoric acid in soils F Soil group Removed Weights Removed Weights One crop Two crops One cropTwo crops One cropTwo crops One cropTwo crops t1 soils 12 14 s 4 19 27 7 s face soils 19 22 13 12 32* 31* 14 7 soils 16 24 10 18 8 16 6 6 ‘w basicity (below 2%) 59* 66* 31* 1s 1s s s h basicity (above 2%) 6 7 6 8 36* 31* 19 20 mid section 17 16 13 12 26 18 11 11 humid section 25 29 16 18 26 29 16 16 id, low basicity, surface 57* 59* 51* 3 i‘ 0 0 2 2 id, high basicity, surface 14 10 14 10 76* 16 24 14 humid, low basicity, surface 84* 80* 44* 32* 67* 71* 40* 31* _ humid, high basicity, surface 5 11 6 l3 35* 56* 21 16 id, low basicity, subsoil 23 74* 24 29 6 8 3 9 id, high basicity, subsoil 1 6 4 11 3 6 3 8 humid, low basicity, subsoil 76* 88* 49* 56* 67* 71* 51* 64* humid, high basicity, subsoil 1 1 1 5 15 15 17 14 ‘High relation active (or total) phosphoric acid in the soil; and b, a constant which“ states the difference between the quantity expected and the quanti _ which would be expected on the basis of changes in active (or total) phosphoric acid alone. The numerical values of these characters for a particular group of soils may differ considerably from those for some,’ other group. In the tables to follow, the quantity b will be designated ash the constant, while the quantity m will be designated as the regressio 7° coefficient of active phosphoric acid or other basis for calculation. The a gression coefficients for total phosphoric acid were based upon parts ~ million rather than per cent, in order to conform to the units used for quantity of phosphoric acid removed. g The regression constants and coefficients of various groups of soifi?“ are given in Table 11. The active phosphoric acid, in parts per millio multiplied by the appropriate coefficient, plus the constant, gives the Table 11. Characteristics of equations for regression of phosphoric acid removed by crops J on active and total phosphoric acid in the soils. A i Average Active phosphoric acid Total phosphoric acid phosphoric Soil group acid removed Constant Coefficient Constant Coefficient‘ D-D-m- Surface soils, one crop g Humid, low basicity“ 8.5 2.6 .206* 8.5 0002 E Humid, high basicity 14.7 10.8 .029 —12.3 0276‘ ; i Subhumid, low basicity 12.2 6.1 .060* 0.1 .0238‘ _. i Subhumid, high basicity 10.3 8.6 .008 1.3 .0090‘ L Surface soils, two crops \ Humid, low basicity 16.1 7.2 .310* 14.6 .0004 '- ; Humid, high basicity 28.5 23.3 .040 6.6 .0246‘ 5 Subhumid, low basicity 24.5 12.0 .1331‘ 0.3 .0488‘ E Subhumid, high basicity 19.1 15.8 .017 2.2 .0171‘ Subsoils, one crop ‘ Humid, low basicity 3.6 1.5 432* 2.9 .0019 Humid, high basicity 5.4 4.9 .005 3.3 .0031 i! Subhumid, low basicity 5.1 1.2 .060* —4.7 .0260‘ Subhumid, high basicity 4.2 3.5 .003 ~0.8 .0059‘ ; Subsoils, two crops t Humid, low basicity 7.0 0.7 .414* 5.6 .0041 Humid, high basicity 10.5 10.0 .016 6.2 .0061 Subhumid, low basicity 9.9 2.8 .109* -7.2 .0454‘ Subhumid, high basicity 8.7 8.0 .006 1.9 .0125‘ * A significant proportion of the phosphoric acid removed is accounted for by this regres- sion coefficient. ‘ "Low basicity, less than 2%; high basicity above 2%. culated number of parts per million of phosphoric acid that would be ex- pected to be removed by the crops. Thus, for 10 p.p.m. of active L phosphoric acid in a surface soil of low basicity from the humid section, - the amount of phosphoric acid to be expected in one crop would A 10x .206 + 2.6 = 4.66 p.p.m. The table shows that there are relations be-i I tween the active phosphoric acid and the phosphoric acid removed by the. crops with soils or subsoils of less than 2% basicity in the humid region. A relation between the total phosphoric acid and the phosphoric acid re- Z moved occurs in soils of more than 2% basicity from both the humid and the subhumid regions. In soils of low basicity in the subhumid region, 1 there is a definite relation between the quantities of total phosphoric acid Hvv \v Il-I I FACTORS AFFECTING UTILIZATION OF PHOSPHORIC ACID BY PLANTS 21 Table 12. Average relations of certain factors at different levels of active phosphoric acid ln surface soils of low basicity. lge of active phosphoric acid, p.p.m. 0 to 15 16 to 30 31 to 100 Above 100 Inber of soils in group 23 30 30 19 hrages Active phosphoric acid, p.p.m. 10.4 21.8 51 0 214.8 Phosphoric acid removed, p.p.m. 7.9 22.6 36 6 41.9 Weight of two crops, grams 19.6 29.7 47 8 66.1 rrelation coefficients ' I _ Active phosphoric acid and phosphoric acid removed .48* .46‘==‘~‘ .45’~“""' .00 Active phosphoric acid and weight of crops .31 .44* .39’-“-_ .21 Phosphoric acid removed and weight of crops .91"*“?‘ .88’5‘* .77*‘-‘ .47* {ression characteristics Active phosphoric acid and phosphoric acid removed Constant 1.47 3.61 10.31 42.81 Coefficient of active phosphoric acid .618 .417 .241 .00 Active phosphoric acid and weight of crops - Constant 9.98 6.60 26.55 65.12 Coefficient of active phosphoric acid .927 1.057 .416 .00 Phosphoric acid removed and weight of crops Constant 0.16 2.89 - 4.67 23.38 Coefficient of weights of crops .394 .330 .376 .279 Significant correlation ‘Highly significant correlation and the phosphoric acid withdrawn by the crops used. The numerical value of the regression constants and coefficients differed widely between different groups of soils. Correlation and Regression Within Groups Based on Active Phosphoric Acid Since the relation between the active phosphoric acid in the soil and the phosphoric acid removed by crops is greatest for surface soils of low basicity, the relations were further studied within groups of these soils. The four groups studied were as follows: 0 to 15, 16 to 30, 31 to 100 and over 100 p.p.m. of active phosphoric acid. The results are given in Table 12. The coefficients of correlation between active phosphoric acid and phos- phoric acid removed were nearly the same within the first three groups, but was zero in the group of soils which contained over 100 p.p.m. of active phosphoric acid. The regression coefficients for active phosphoric acid and phosphoric acid removed were .618, .417, .241, and 0 respectively. The regression constants increased, being 1.47, 3.61, 10.31, and 42.8. This indicates that factors other than the content of active phosphoric acid be- came increasingly important with increases in the amounts of active phosphoric acid in the soils. The active phosphoric acid was more closely correlated to the phosphoric acid removed than to the weights of the crops, except in the last group. With less than 100 p.p.m. of active phosphoric acid, there is thus a high relation between the active phosphoric acid in the soils of low basici- ty and the phosphoric acid removed by the crops and the weight of the crops, but above the 100 p.p.m. levels, the relation becomes much lower; it is perhaps of'minor importance in most soils containing more than 100 p.p.m. of active phosphoric acid. Effect of Cropping on Active Phosphoric Acid of Some Soils When soils were analyzed for active potash before and after cropping in pot experiments, it was found that the loss in active potash accounted 22 BULLETIN NO. 647, TEXAS AGRICULTURAL EXPERIMENT STATION for about 40% of the potash taken up by the crops (21). Previous work indicates that cropping does not decrease the active phosphoric acid of the soil (18). The active phosphoric acid wa§ determined before and after cropping in some pot experiments. The average results for four crops are given in Table 13. Although appreciable quantities of phosphoric acid Table 13. Average effect of cropping on active phosphoric acid of soils (p.p.m.) Number Group based on Average active phosphoric Phosphoric acid removed of active phos- acid in soils by crop soils phoric acid Before crop- After crop- ping ping 8 7 to 10 8.2 17.3 10.5 13 11 to 20 16.3 23.1 19.3 8 21 to 30 24.3 21.5 21.8 18 31 to 50 40.0 42.2 35.0 3 51 to T0 61.6 66.0 53.0 50 T to 70 27.5 31.4 23.1 were removed by the plants, little or no change in amounts of active phos- phoric acid remaining in the soils was observed. In some cases, active phosphoric acid was slightly greater after cropping than before cropping. This may be caused by the roots of the plants in the soil making some of the phosphoric acid soluble in the acid, and is evident chiefly with the groups of soils containing less than 20 p.p.m. of active phosphoric acid. Either the phosphoric acid removed by the plants came from soil phos- phates other than the fraction which is soluble in 0.2 N nitric acid, or the action of the plants and the chemical changes in the soil caused a replen- ishment of the soluble fraction by phosphoric acid derived from more in- soluble compounds in the soil. Availability of Total Phosphoric Acid of Some Soils The availability of a plant nutrient in fertilizers has long been meas- ured by pot experiments in which definite amounts of the nutrient under study are added to soils containing abundant quantities of the other nu- trients. The effect of the nutrient under study was then measured by as- certaining the gain in weight of the crop or the increase in the quantity of nutrient removed by the crop. Examples of this type of work on fertilizers containing phosphoric acid have been reported by Ross and Jacobs (31) and by Haskins (29). This method has seldom been applied to soils for study- ing the availability of their constituents. However, the writers (26) have used it in a study of the availability of the potash in the soil. In a similar study on phosphoric acid, an amount of the soil under study calculated to contain 100 milligrams of total phosphoric acid was added to 5,000 grams of a Lufkin fine sandy soil which was known to respond to phosphate fertilizers, together with carriers of nitrogen and potash. The usual greenhouse procedure, previously described, was fol- lowed from this point. Superphosphate and rock phosphate were included for the purpose of comparing these phosphates with soil phosphates. The quantity of phosphoric acid taken up by the crops from the Lufkin fine sandy loam soil which did not receive phosphate was deducted from the quantities taken up by the crops on the soils which received the soil tested. FACTORS AFFECTING UTILIZATION OF PHOSPHORIC ACID BY PLANTS 23 Table 14. Phosphoric acid recovered by corn from additions which contained 100 milligrams of total phosphoric acid. Weight Per Phosphoric acid y o cent Removed Percentage Source of phosphoric acid crop in recovered gm. crop mgm. Basal soil 19.4 .17 33.6 0 Superphosphate 32.3 .21 67 .8 34.2 Rock phosphate 23.3 .21 47.8 14.2 Crockett very sfine sandy loam 19.2 .17 32.6 —1.0 Kirvin fine sandy loam 18.3 .19 34.8 1.2 Bell claY 19.9 .18 35.2 1.6 Wilson clay loam 18.6 .19 35.3 1.7 Webb fine sandy loam 18.3 .19 35.3 1.7 Wilson clay 18.6 .19 35.3 1.7 Abilene clay loam 20.7 .17 35.8 2.2 Miller clay 15.6 .24 36.9 3.3 Lufkin fine sandy loam 20.5 .18 36.9 3.3 Hunt black clay 23.1 .17 40.0 6.4 Trinity clay 16.3 .26 42.4 8.8 Ochlockonee fine sandy loam 27.0 .16 44.0 10.4 Houston clay 16.9 .26 44.5 10.9 Uvalde clay loam 14.4 . 1 44.6 a 11.0 Houston black clay 18.6 .25 45.8 12.2 Catalpa clay 16.9 .28 46.5 12.9 Denton clay 30.7 .18 54.3 20.7 The availability of the total phosphoric acid in the soils to corn grown in 1937 is shown in Table 14. The increase due to the phosphoric acid in the added soil ranged from 0 to 20.7 milligrams (equivalent to 20.7% of the total phosphoric acid added). The increase in phosphoric acid removed by the crop due to the addition of the soil being studied was low (less than 3.5 milligrams) in 9 of the 17 soils used. The change in phosphoric acid in the crops from these 9 soils ranged from a loss of 1.2 milligrams to a gain of 3.3 milligrams, with an average of 1.76 milligrams. The recovery from the remaining 8 soils was high, and averaged 11.66 milligrams. Of the 9 soils in which phosphoric acid had a low availability, 8 were low in basicity; of the 8 soils in‘ which the phosphoric acid had a high availability, 7 were high in basicity. The analyses of the soils in which the phosphoric acid had a low availability were averaged in one group, and those in which the availability was high were averaged in another group; these averages are given in Table 15. The Miller clay, a calcareous bottomland soil, was omitted from the low group. The soils from which the recovery of phosphoric acid was low averaged considerably lower in total nitrogen, total phosphoric acid, Table 15. Composition of soils with low availability of phosphoric acid compared with those with high availability. Low High availability availability Total nitrogen, per cent .087 .140 Total phosphoric acid, per cent .042 .103 Active phosphoric acid, p.p.m. 23 121 Total potash, per cent .73 1.14 Active potash, p.p.m. 245 317 Acid-soluble lime, per cent .46 8.91 Basicity, per cent .80 14.70 Number of soils 8 H Average availability, per cent 1.8 11.7 Js‘ ._v 24 BULLETIN NO. e47, TEXAS AGRICULTURAL EXPERIMENT STATION active phosphoric acid, total potash, and active potash than the group in which the recovery was high. The greatest differences, however, were in‘ acid-soluble lime and in basicity, in which the low recovery group was 1 much lower than the high recovery group. The fact that the corn removed “ larger quantities of phosphoric acid from soils which were high in total phosphoric acid indicated that the phosphoric acid is more available per.” unit in these soils. Only 5.5% of the total phosphoric acid in the low group was soluble in 0.2 N nitric acid (active), while 11.8% of that in the . high group was soluble. In all soils except one, the availability of the phosphoric acid was less g than that in rock phosphate. The phosphoric acid removed by the crops; was greater than the amount of active phosphoric acid in the soil in 4 of ' the 17 soils; all 4 of these soils were highly calcareous. The phosphoric]? acid removed, expressed as percentages of the active phosphoric acid in the l added soil, averaged 38% in soils of low basicity. Much of the phosphoric acid which was taken up by the corn plants from the soils of high basicity ‘; was from compounds in the soil which were insoluble in 0.2 N nitric acid. Another experiment was conducted in 1938 with 9 soils, of which 8 a were high in basicity. The results are given in Table 16. The quantity of _ phosphoric acid removed by the first crop, corn, from four of the soils , was less than that removed from the basal soil without any addition. The fact that the corn plants contained relatively high percentages of phos- ' phoric acid but were low in weight indicated that some factor other than phosphoric acid affected the growth. After the corn was harvested, milo _ was planted without any further addition of phosphoric acid. The milo re- A moved much more phosphoric acid than the corn. Because of irregularities in the growth of the corn and probably consequent irregularities in the milo, the discussion of results will be limited to the combined data for both crops. The quantity of phosphoric acid recovered from 8 of the 9 soils exceeded the quantity recovered from rock phosphate, and in 5 of 1i them, the recovery was over twice as large. The single exception was a l Lufkin fine sandy loam, the only soil of low basicity used in the series. The recovery of phosphoric acid from the soils was considerably less than that from superphosphate. Recovery from the soil of low basicity consid- erably exceeded the quantity of active phosphoric acid in the original soil. The results of this work show that soils differ markedly in the avail- ability of their total phosphoric acid; some of this variability may be at- tributed to basicity of the soils. Availability, per unit of phosphoric acid, is higher in soils which are high in total and active phosphoric acid, lime, basicity, and probably nitrogen. The quantity available in some calcareous soils may be considerably greater than is indicated by the quantity of active phosphoric acid. Phosphoric acid in some calcareous soils may be considerably more available than that in rock phosphate, but is always less than that in superphosphate. a » -.M~ » Wamaig“ Availability of Fixed Phosphoric Acid t The availability of the phosphate fixed by a number of soils was also j studied briefly. In this work, soils having a fixing power of over 80% for phosphoric acid were selected. The amount of soil which would fix 100 milligrams of phosphoric acid was treated with a weak phosphate solu- 25 FACTORS AFFECTING UTILIZATION OF PHOSPHORIC ACID BY PLANTS wém 1Q: 3:. mam in S. W3 w; Q5. 2.. Y: ma? >359 3m S; Em d8. ha. 5. N5 f? i.» wm. NAZ 52o c0325 in Q3 QC. m? ma... S. 75 o oi. s. wdfi >50 QEEm 5w gm fix. Q3 3m in m... 2N $7 0.3. 3. W3 >50 .352 3a ~15 W? Q2 W? “I. wém 5...: Ea 5. v.2 >30 E6059 fimw Em S...“ ma... gm Hm. mam g1 we” S. 2w 5x2 >30 @255 3N Q5 Ndm 2m wém mm. 9mm 9w: Em 3.. w.» >20 fiwfiiwm Q3 mAw ca... fiw v.5 S. i: W2 2m mm. 5: ma? Maxi fihfim 2 w? 3m m5 3a 2. wfi Nd 3:. 3. v.2 :52 >15...“ 25 EUGSQ “W: new Y?" f: 2m S. “i2 H4. 3a. g. Qwfi fifinwoan 18m N3 .22 ~25. ~22 ma.” 2. oi ma.» m? F”... méw wumsnmosnnwnwm x096 w? w? .620 Q8 3. v.5 v1.0.6 Q3. mm. T5 iow 13mm 19$ .EuE .Eu .EwE .EuE a» .Eu .EuE .EuE o.» .Eu 1253 wo>oE vmuouu 135E 3.3.5 193E owe éu LS 5n Em uou Bu: own-zines: u: QOHBQM éfloouoh 13PM usumw? 13cm. i??? 130M. asumo? Eon visa-mosh zufl Eva uicinuo-fm .39 Bu: via-awash PEG mncuo JEN Biz ECU .30: umuonamosn 13a ac mEdFMm-GE Q3 voEui-co 50E? E532. 1: EP-u mug E c=E 1E: F30 .3 wo>eE9~ Bu: oionauo-fw .3 03am. ‘ h ‘ _ a» v M‘ 11". ii; Lil-fzvapgv’ I V I 26 BULLETIN NO. 647, TEXAS AGRICULTURAL EXPERIMENT STATION tion. After 24 hours, with periodic shaking, the soil was allowed to settleé centrifuged, and the solution poured off. Water was added to equal original volume, and the soil suspension again centrifuged and separatedé The washing was repeated, the residual solution and washings were comli bined and made up to volume, and the phosphoric acid determined. The de-ITI crease of phosphoric acid in solution was taken to be the quantity fixed byilf the soil. T. The soil from this treatment (containing 100 milligrams of fixed phos-is phoric acid) was mixed with 5000 gms. of the basal soil. Equal quantitieajf of the original soil were likewise mixed with other lots of the basal soilg? and there were also check pots to which no soil was added. Ammonium}; nitrate and potassium sulphate were added to all pots and the work wasjifj completed as already described for pot experiments, with corn as the ex-I perimental crop. The average results from 3 pots of soil for each treat-é ment are given in Table 17. From 19% to 29% of the fixed phosphoric? Table 17. Recovery of fixed phosphoric acid by corn. iv . Phosphoric acid removed by crops Basal soil Basal soil Recovery Basicity pH Fixing plus un- plus soil of fixed i“; power treated soil with fixed phospho- of soil phosphoric ric acid acid “f? Soil type % % mgm. mgm. % Nacogdoches fine sandy loam 0.5 6.5 97 42.8 62.1 19.3 Houston black clay 5.6 8.0 88 57.8 86.9 29.1 "i Wilson clay loam 1.1 7.2 76 49.3 71.3 22.0 Susquehanna fine sandy loam 0.8 4.8 99 46.9 68.8 21.9 acid was taken up by the crops. A check test with superphosphate was not; made, but comparisons with Tables 15 and 16 show that the fixed phos-l?! phoric acid had a high availability. i" The fixing power of the Houston black clay and of the Wilson clay‘) loam is probably primarily due to limestone, so that the phosphorus is probably fixed as tricalcium phosphate. In the other two soils, the phos-‘i phorus was probably fixed in iron compounds. In all cases, the availability of the fixed phosphoric acid was comparatively high. Acknowledgement _ The work reported in this bulletin covered a period of over 20 years. In that time, a number of chemists have assisted in the laboratory analy-P ses and in the greenhouse. Their assistance, and particularly that of T. L. Ogier, W. H. Walker, S. E. Asbury, E. C. Carlyle, and P. F. Macy, is gratefully acknowledged. 7 Summary The quantities of phosphoric acid removed by one and by two crops of corn and milo or kafir and the weights of the crops produced in pot ex-’ periments in a greenhouse were determined with about 375 soils from va- "" rious parts of Texas. The average amount of phosphoric acid removed per crop ranged from 2.6 parts per million from soils which contained less than 10 p.p.m. of a ./' w. f, ._<\-.¢ FACTORS AFFECTING UTILIZATION OF PHOSPHORIC ACID BY PLANTS 27 active phosphoric acid to 18-.2 p.p.m. from soils which contained more than 400 p.p.m. of active phosphoric acid. The phosphoric acid (P205) dissolved by 0.2 N nitric acid is termed the active phosphoric acid. On an average, the quantities of the phosphoric acid removed by the crops increased as the active phosphoric acid in the soils increased. The phosphoric acid re- moved by crops from surface soils was greater than that removed from subsoils containing equal quantities of active phosphoric acid, but the cor- relation coefficients were nearly the same. From soils which contained similar amounts of active phosphoric acid, the average quantities of phosphoric acid removed per crop increased with increases in total phosphoric acid, total nitrogen, and active potash in the soils. With surface soils which contained from 31 to 100 p.p.m. of active phosphoric acid, the average phosphoric acid removed by the plants ranged from 5.2 p.p.m. when the total phosphoric acid was less than 0.25% to 24.2 p.p.m. when the total phosphoric acid was over .150%. The average phosphoric acid removed was 9.6 p.p.m. when the total nitrogen was less than 0.03% and 15.4 p.p.m. when the nitrogen was over 0.120%. When the soils, grouped according to their content of active phosphoric; acid, were then averaged in subgroups according to whether-the quantities of phosphoric acid removed from them by the crops were relatively low, intermediate, or high, the averages for total phosphoric acid, total nitro- gen, and active potash in the soils were also relatively low, intermediate, or high. When the soils were divided into groups based upon certain charac- teristics, the percentages of the soils within each group from which the quantity of phosphoric acid removed by the crops was relatively low were 54% for subsoils as compared with 20% for surface soils, 43% for soils from the subhumid section of the state as compared with 29% for soils from the humid section, 53% for soils of high basicityras compared with 27% for soils of low basicity, and 38% for soils of heavy texture as com- pared with 35% for soils of light texture. Coefficients of correlation between the quantities of active phosphoric acid in the soils and the quantities of phosphoric acid removed by the crops were higher when surface soils (.47) or subsoils (.49) were considered separately than when all soils were placed together (.37). The coefficients were much higher (.81) for soils with basicities below 2% than for those above 2% (.26). (The coefficients were high and significant for surface soils when the basicity was less than 2% from either humid or subhumid sections, but low and not significant when the basicity was over 2%. Two crops gave higher correlation coefficients than did one crop. Weights of crops gave lower correlation coefficients than the phosphoric acid removed by the crops. ' The correlation between the total phosphoric acid of the soil and the phosphoric acid removed by the crops was high for soils with basicity above 2% (.60). The correlation was not significant for humid soils of low basicity, either surface or subsoils, or for humid subsoils of high basicity. The correlation for total phosphoric acid is significant for both humid and subhumid surface soils with basicity exceeding 2%. 28 BULLETIN NO. 647, TEXAS AGRICULTURAL EXPERIMENT STATION Equations for simple regression, which give quantitative statements of A the relation between phosphoric acid removed and phosphoric acid in the soil, are given for different groups of soils. The significance of the re- ‘ gression coefficients was similar to that of the correlation coefficients. When the surface soils of basicity less than 2% were arranged in groups according to their content of active phosphoric acid, coefficients for correlation and characteristics for regression curves decreased in signifi- canec as the active phosphoric acid in the soils increased; significant char- acteristics were secured only with soils which contained less than 100 p.p.m. of active phosphoric acid. Coefficients of correlation for phosphoric acid in two crops decreased from .48, .46, and .45 in soils which contained 0-15, 16-30, and 31-100 p.p.m. active phosphoric acid, respectively, to 0 in soils which contained over 100 p.p.m. Constants in the regression curves for the lowest and highest groups of soils were 1.47 p.p.m. and 42.81 p.p.m., while the coefficients of active phosphoric acid were .618 and .00. Phosphoric acid removed by the crops came either directly or indi- rectly from compounds in which the phosphoric acid was insoluble in 0.2 N nitric acid. Active phosphoric acid in 50 soils averaged 27.5 p.p.m. be- fore cropping and 31.4 p.p.m. after cropping, although the crops removed phosphoric acid from the soil equivalent to 23.1 p.p.m. From additions of soil which contained 100 milligrams of total phos- phoric acid, crops removed from 0 to 20.7% of the added phosphoric acid, as compared with 34.2% from superphosphate and 14.2% from rock phos- phate. The availability of the soil phosphoric acid so measured was much higher in calcareous soils than in noncalcareous soils. From 20% to 30% of the phosphoric acid fixed from solution by 4 soils was removed by one crop of corn. IO U! 12. 13. 14. 15. 16. 30. 31. 32. 33. 34. . Association of Official Agricultural Chemists. . Fraps, G. . Fraps, . Fraps, . Fraps, . Fraps, . Fraps, G. S., FACTORS AFFECTING UTILIZATION OF PHOSPHORIC ACID BY PLANTS 29 REFERENCES 1940. Official and tentative methods of analysis. Fifth Edition. Washington, D . Black, C. A. 1941. The penetration of phosphate into the kaolinite crystal. Soil Sci. Soc. America Proc. 6 :157-161. . Fisher, R. A. 1928. Statistical methods for research workers. Second Edition. Oliver and Boyd, London, England. . Fraps, G. S. 1904. Factors of availability of plant food. Amer. Chem. Jour. 32:1-13. . Fraps, G. S. 1904. The availability of potash and phosphoric acid in soil. Twenty-seventh Annual Report, pp. 26-47. . Fraps, G. S. 1906. Availability of phosphoric acid of the soil. Jour. Amer. Chem. Soc. 28:823-834. . Fraps, G. S. 1907. The phosphoric acid of the soil. Proc. A.O.A.C., Bul. 116, Bureau of Chemistry, U.S.D.A., 95-6. . Fraps, G. S. 1907. Soil fertility and phosphoric acid. Transactions Tex. Academy of Sci., 10:41-45. . Fraps, G. S. 1908. The ammonia-soluble phosphoric acid of the soil. Amer. Chem. Jour. 39:579-586. S. 1909. Active phosphoric acid and its relation to the needs of the soil for phsophoric acid in pot experiments. Tex. Agr. Exp. Sta. Bul. 126. . Fraps, G. S. 1910. Relation of pot experiments to the active phosphoric acid of the soil. Jour. Ind. and Eng. Chem. 2:350-352. Fraps, G. S. 1911. Organic phosphoric acid in the soil. Tex. Agr. Exp. Sta. Bul. 136. . Fraps, G. S. 1911. Effect of ignition on solubility of soil phosphates. Jour. Ind. and Eng. Chem. 3:335 Fraps, G. S. 1912. Relation of active phosphoric acid and potash of the soil to pot and field experiments. Int. Cong. of App. Chem. 15:99-102. Fraps, G. S. 1915. Effect of the additions on availability of soil phosphates. Tex. Agr. Exp. Sta. Bul. 178. Fraps, G. S. 1917. The availability of phosphoric acid in rock phosphate. Tex. Agr. Exp. Sta. Bul. 212. . Fraps, G. S. 1917. Effects of lime and carbonate of lime on acid phosphate. Tex. Agr. Exp. Sta. Bul. 223. . Fraps, G. S. 1920. The relation of the phosphoric acid of the soil to pot experiments. Tex. Agr. Exp. Sta. Bul. 267. G‘. S. 1922. The effect of rock phosphate upon the corn possibility of the phosphoric acid of the soil. Tex. Agr. Exp. Sta. Bul. 289. G. S. 1922. The fixation of phosphoric acid by the soil. Tex. Agr. Exp. Sta. Bul. 304. G. S. 1924. The effect of cropping on the active potash of the soil. Tex. Agr. Exp. Sta. Bul. 325. G. S. 1931. How reliable are existing chemical methods for determining soil defi- ciencies in ash constituents of plants? Jour. Amer. Sov. Agron. 23:337-351. and Fudge, J . F. 1932. Relation of buffer capacity for acids to basicity and exchangeable bases of the soil. Tex. Agr. Exp. Sta. Bul. 442. . Fraps, G. S., and Fudge, J. F. 1933. Rapid chemical methods for the estimation of the capacity of the soil to supply phosphoric acid to plants. Jour. Amer. Soc. Agron. 25:217-230. . Fraps, G. S., and Fudge, J . F. 1937. Chemical composition of soils of Texas. Tex. Agr. Exp. Sta. Bul. 549. . Fraps, G. S., and Fudge, J . F. 1939. The availability of the potash of the soil as measured by pot experiments with corn. Soil. Sci. Soc. America Proc. 4:205-208. Fraps, G. S., Fudge, J . F., and Reynolds, E. B. 1937. Effect of fertilization on the chemi- cal composition of a Lufkin fine sandy loam and of oats grown on it. Jour. Amer. Soc. Agron. 29:990-996. Fraps, G. S., Fudge, J. F., and Reynolds, E. B. 1941. The effect of fertilization on the nitrogen, active phosphoric acid, and active potash of a Lake Charles clay loam. Jour. Amer. Soc. Agron. 33:130-134. . Haskins, H. D. 1924. Final report of the special committee of the Association of Official Agricultural Chemists on the availability of phosphoric acid in basic slag phosphates. Jour. A.O.A.C. 7z2l8-252. Murphy, H. F. 1939. The role of kaolinite in phosphate fixation. Hilgardia 12:341-382. Ross, W. H., and Jacob, K. D. 1931. Report on phosphoric acid. Availability of the re- verted phosphoric acid in ammoniated superphosphates. Jour. A.O.A.C. 14:182-196. Scarseth, G. D. 1935. The mechanism of phosphate retention by natural alumino-silicate colloids. Jour. Amer. Soc. Agron. 27:596-616. Snedecor, George W. 1938. Statistical methods. Collegiate Press, Inc. Ames, Iowa. Toth, S. J. 1937. Anion absorption oxides. Soil Sci. 44:299-314. by soil colloids in relation to changes in free iron