The Effect of Soil Clay Content on Phosphorus Uptake1
FRANCISCO BALDoviNOS2 AND GRANT W. THOMAS"
ABSTRACTObservations have been made that plants remove more P
from soil high in clay than from sandy soils at a similar level of P.These observations led to a study of P uptake by snap beans(Phaseolus vulgaris) as a function of clay content of the soil.Three Virginia soils, with 10, 22.5 and 63.5% clay, were used.Snap beans were grown in a growth chamber and the tops wereanalyzed for P. It was found that the amount of P absorbed bythe plants at a given soil solution level increased with clay con-tent. The similarity of the results with acid soils to those ob-tained with calcareous soils by Olsen and Watanabe suggestedthat their prediction value for uptake might be employed. Theagreement between relative uptake and predicted relative uptakewas rather good. Clay content is indicated as a prime factor inP availability.
Additional Key Words for Indexing: soil texture, phosphorusavailability.
OBSERVATIONS made on clay soils of the southeastern USAsuggest that although high rates of P have been applied,
this applied P is not reflected in the soil test. The soil testused in the Piedmont region from Maryland to Alabama isthat developed by Mehlich at North Carolina and consists ofextracting the soil with a mixture of 0.025./V H2S04 and 0.05NHC1 (6).
A part of the problem is that the dilute acid is consumed bythe positively charged iron hydroxide compounds on the clay,so that with the "Mehlich" test very little acid remains toextract P. This problem can be solved by the use of stronger
extracting solutions or large soil-solution ratios. However, itis still found that good growth of plants occurs in clay soilswhere P extractable with a solution as Q.5N NH4F is lowcompared to that found in fertilized sandy soils. (T. A.Singh, 1964. Unpublished work, Virginia Polytechnic Institute.).
In calcareous soils, Olsen and Watanabe (5) have shownthat P uptake by plants at a given soil solution concentrationincreases as clay content increases. They have specificallydeclined to extrapolate their results to acid soils, w _ere,presumably, the presence of iron oxides would result inreactions different from those found in calcareous soils. Thispaper represents an application of the findings of Olsen andWatanabe to three soils typical of the Southeast USA. Theresults obtained indicate that their work has wideapplicability.
MATERIALS AND METHODSThree Virginia surface soils, a Davidson clay from Orange, a
Congaree fine sandy loam from Snowville and a Wellston sandyclay loam from Blacksburg were used in the experiment. David-son represents a group of clayey, red, productive soils which makea rather large acreage of the Piedmont region of Virginia andother Southeastern states. Congaree is a highly productive well-drained first bottom soil formed in alluvium coming from crystal-line rocks. Wellston is a moderately deep mountain soil formedfrom shale and sandstone, and is similar chemically to many ofthe other soils of the southern Appalachians. The clay mineralsin all three soils are a mixture of interlayered vermiculite andkaolinite.
These soils were ground and passed through a 2-mm sieve,limed to pH 6 with CaCO3, placed in painted tin cans and fer-tilized with O, 20, 40, 80, 160 and 320 ppm of P as Ca(H2PO4VH2O. All pots received 100 ppm K as KOI and 100 ppm of N asNEUNOs. The weight of soil ia each pot was 1.5 kg. In all cases,fertilizer treatments were mixed thoroughly with the soils. Thepots were planted to snap beans (Phaseolus vulgaris) and placedin a Scherrer controlled environment chamber. Three replica-tions were used. Light intensity was kept at 1,500-foot candlesfor 14 hours each day. Daytime temperature was maintained at25C and nighttime temperature at 15C. After thinning, 5 plantswere allowed to grow for 30 days before the vegetative growth washarvested, dried and weighed. Ashed samples of the plants were
BALDOV1XOS AND THOMAS: EFFECT OF CLAY CONTENT ON P UPTAKE 681
Table 1—Vegetativejyield and uptake of P by snap beans grownin three Virginia soils as affected by P application
Davidson c
ppin0
204080
IRO820
Yield
g/pot4.774.004.525.776.427.14
Uptake
ing/pot6.005.638.52
. 12.8321.7937.89
Wellston sci
Yield
g/pot4.405.720.517.638.829.14
Uptake
mg/pot5.85
10.2415.7322.9437.9056.83
Cong;
Yield
g/POt
3.794.085.556.436.067.09
aree si
Uptake
mg/pot3.737.00
11.2517.1320.0945.30
Table 2—The effect of P additions on the P extracted by 0.05 NHC1 + 0.025 JV H2SO4 solution
0204080
100320
Davidson c
1.50.91.42.89.0
34.9
pprn extracted
Wcllston sci
0.63.07.2
20.748.081.1
Congaree si
17.221.025.743.470.293.2
analyzed for P by the ammonium molybdate-vandate method,using a Beckman Model B Spectrophotometer.
Phosphorus in soils was estimated by the HC1-H2SO4 methodused at the Virginia soil testing laboratory (6), with a soil-solu-tion ratio of 1: 4 and shaking time of 5 minutes, by extractionwith 0.5N neutral NH4F (1), and by determination of P in thesoil solution using an isobutyl alcohol extraction described byLueck and Boltz (4). Estimations of clay content in the soilwere made by the hydrometer method of Bouyucos (2). Thegeneral approach of predicting P uptake and the estimates ofporous diffusion coefficients were taken entirely from a paper byOlsen and Watanabe (5).
RESULTS AND DISCUSSIONThe vegetative yields and the uptake of P by snap bean
tops are shown in Table 1. Good response of beans to Papplications was obtained for each of the three soils. Theyields of plants grown on Wellston sci were considerablyhigher than those grown on the other two soils. The authorscan offer no explanation for this. Uptake of P was comparablebetween soils and continued to increase even at the highestlevel of P application. The plant yield was nearly doubledwith addition of O to 320 ppm P, whereas the uptake of P wasincreased 6-fold in the Davidson, 9 in the Wellston and 12times in the Congaree soil.
The effect of P rate on HC1-H»S04 extractable P for thethree soils is shown in Table 2. The effect of soil on the Precovered by HC1-H2S04 extraction is striking. Phosphorusextracted from Davidson clay remained low until 320 ppm wasadded. Amounts of P extracted from Congaree and Wellstonsoils were comparable. It is apparent, from these results, thata large amount of P (80 ppm) would have to be applied toDavidson clay before the soil test level would change verymuch.
Uptake of P by plants was plotted over a range of soilsolutions where the three soils were comparable. For David-son clay, these represent the three highest rates, since P insoil solution did not change appreciably at lower rates of P.The results are shown in Fig. 1. For the same soil solutionconcentration, uptake from Davidson clay is about four timesthat from Congaree si and more than twice that from Wellstonsci. These curves are very similar to those shown by Olsenand Watanabe for calcareous soils (5). This led to theconclusion that the use of the porous diffusion data of Olsenand Watanabe would describe uptake from acid as well ascalcareous soils.
Capacity factors ("b" values) were computed in a mannersimilar to that used by Olsen and Watanabe (5). TheirZ>-vaJues are the slopes of the curves obtained when 32P-ex-
0.1 0.2 0.3
ppm P in SOIL SOLUTIONFig. 1—The uptake of P from three soils by snap beans over a
comparable range of soil solution P.
ppm NH4F-Extractable P
125-
100
75
50
25
,ODAVIDSONb-543
CONGAREEb-125
WELLSTONb-214
O.I 0.2 0.3ppm P in Solution
Fig. 2—Calculated "b" values for three soils (b= Ay/Ax).
682 SOIL SCI. SOC. AMER. PROC., VOL. 31, 1967
Table 3—Predicted and measured relative uptake of P by snap beans from three Virginia soils
Soil %Clay P cone, in soil sol'n. Capacity factor (b) Diffusion Coeff. (A>|.) ———Relative uptake rate*
Actual Calculated
Davidson c
\Vellson sci
Conjraree al
OS. 5 0.0.0
22. 5 000.
10 00.U.
.1(1
.15
.20
.10
. ].i
.20
.10
.1520
( X 10/cmVsec)
543 8.20
214 2 .UO 000,
125 1.0 00,0,
111
.43
.48
.51
.18
.28
.30
0000,00
111
.35
.33
.35
.17
.17
.17
* from —Dpb.
changeable P is plotted against soil solution P. Dunbar andBaker (3), in soils similar to the ones used here, showed thatNH4F-extractable P was more nearly equilibrated with 32Pthan were the NaOH or H2S04 fractions. Therefore, since32P exchangeable P was not determined, P extracted withneutral 0.5iV NH4F was used to approximate "surface" P.Comparison of our NHjF-extractable P with "surface" Pdetermined by Woodruff and Kamprath (7) on North Carolinasoils showed very similar values. The approximation ofsurface P plotted against P in the soil solution was used tocalculate a b value or capacity factor for each soil. Curvesfitted to the jicints by using the method of least squares areshown in Fig. 2.
Porous diffusion coefficients were estimated from a graphof Dp vs. percent clay in soils, taken from Olsen and Watanabe(5). The value for a clay content as low as the Congaree soilwas not available from their graph, so the curve was ex-trapolated to a Dp of zero at zero clay percentage, whichcertainly is not an overestimation. The estimated valueobtained for Congaree was 1.0 X 10~7 cm2/sec.
The square root of the product of 6 (capacity factor) andDp (porous diffusion coefficient) has been used by Olsen andWatanabe (5) to estimate relative uptake of P by plants grownin soils of varying clay content. Their results showedreasonably good agreement between predicted and actualuptake. The predicted and actual uptake for the three acidsoils used in this study are shown in Table 3. The agreementis rather good at a soil solution concentration of 0.1 ppm, butdifferences between predicted and actual values increase as Pconcentration in solution increases to 0.15 and 0.2 ppm.Agreement with prediction is still fairly good in Wellston, butpoor in the case of Congaree.
Considering the assumptions made in calculating b, the use
of the Dp vs. percent clay from the calcareous soils graph toobtain Dp's for acid soils, the use of extrapolation to obtainDp for the Congaree soil and the fact that iron oxide contentvaries from about 2 to 15% between Congaree and Davidsonsoils, the agreement is quite astounding.
The results of this study encourage the authors to believethat the approach of Olsen and Watanabe, heretofore usedonly in alkaline soils, is useful in acid soils as well. Assump-tions made in this paper may or may not be valid, but itappears that the clay content of these soils has a larger effecton P uptake than any other measurable factor. Theseobservations should be helpful in devising an appropriatecalibration of soil test values based on the percentage of clayin the soil.
