PHOSPHATE FIXATION IN SOIL AND ITS PRACTICAL CONTROL FIRMAN E. BEAR AND STEPHEN J. TOTH Rutgers University. N m Brunswick. N. J. RESENT concepts of phosP phate fixation have out of the need to explain four grown
observations of agronomists: 1. The recovery of a plied phosphate in the crop tgat is planted immediately after its a p plicstion amounts to only 10 to 30 oer cent of the anantitv added ti ihe soil. 2. Whenphos hateisa plied to unplowed Ian$ most o! that not accounted for I the harvested crom will be &und in the lop t w o 06 i l i m inches of soil. :1. \\'herr water-suluble hos nhaic is sodied t u Ihwedyand; :t is muci bore efiictive in inercaciny crop \bids if applied in bttmli tllrm I f distributed broadFYIII nnd - mixed with the soil. 4. The loss of applied phosphate in the drainage water fmm soils o t l w than sands is very
rmsll.
iron and aluminum of soils, use is made of data previously reported (6) in connection with the study of s hydrogensaturated Colts Neck sandy loam soil which had a fixing powerof 12tonsofP205(equivalent h 60 tons of 20 per cent superphosphate) per 2 million pounds of soil. Electrodialysis of a field sample of this soil (pH 4.8) for a period of 142 hours resulted in the release of 56 pounds of iron and 34 pounds of aluminum per 2 million pounds of soil. Such t r e a t m e n t w u found to be more drastic than digestion of the soil with a hydrochloric acid solution of pH 4.0, the ultimate pH value of the electrodialyzed sample. These amounts of iron and aluminum, in the fonn of FePO, and AIPO,, would account for less than 200 pounds of P,O, out of the total of 12 tons fixed,and would leave over 11.9 tons of fixed P206tobeaccounted for. Even if one assumes that additional iron and aluminum would be released into the soil solution as the original supply of these cations is removed by precipitation as phosphates, it is inconceivable that this would explain any very large part in the fixation observed. The phosphate-Exing capacity of the hydrogen colloid of this Colts Neck soil decreased with rise in DH resnlting from additions ditions of ammonium hydroxide.
Microbiological consumption, chemical precipitation, and physicochemical adsorption am responsible for phosphate-hxation in mils. The amounts of phosphate eonsumed by soil microorganisms are relatively small, most of the fixation being the result of precipitation and adsorption. Iron and aluminum serve as precipitating agqnts a t pH ~alueabelow 5.5, calcium plays a dominant mle at pH 6.5, and magnesium entem the picture a t 7.5. But precipitation by thions is inadequate to explain the high phosphate fixationwhich normally ocoum and which, for Penn silt loam, may amount to 125 tons of 20 per cent superpboephatc equiralent per 2 million pounds of mil. In soils of Buch high fixing capacity, mo-t of the phosphate is EOIloid-bound or saloid-bound, the colloid-bound pbospbate being replaceable by hydroxyl, humate, and qilieate ions, and the doid-bound phosphate by sulfate, chloride, citrate, and tartrate ions. Excessive fixation can be avoided by placing pbospbate in bands or by the use of granular forms. As tbe quantity of phosphate applied is increased because of greater agricultural intensity, a change from the shallow along-the-row method of application appeara advisable to avoid positional unavailability. Deep placement of most of the phosphate, below the zone affected by cultivation and summer drought, is suggested. Heavy phosphating, such as is required on acid potato soils, increases their exchange capacity and lowers the pH at which iron and aluminum beeome soluble.
These phenomena have been explained (S) by assuming either that the unrecovered 70 to 90 per cent of the applied phosphate wa8 eonsumed by soil microorganisms, was precipitated by soluble cations in the soil solution, or was adsorbed by the soil complex. It is nowknown that the~part played by soil microorganisms in phosphate fixation is relatively minor, and that chemical precipitation and physiccchemical adsorption play the major roles in the fixation Drocess.
Chemical Precipitation For phosphate phosphate fixation fixation by by chemical chemical precipitation precipitation to to occur, occur, For cations capable of forming insoluble phosphates must be present in the soil solution or in the exchange complex of the soil. Normally these cations would be mostly those of iron, aluminum. calcium. and maenesium. The ouestion arises: IS there a su5ciedt concentration of any or all of these cations in the soil solution to explain the lixation of such large amounts of phosphate as is known to occur? Examination of soil solutions has revealed that iron and aluminum are the dominating ions in phosphate precipitation at pH values below 5.5, that calcium becomes operative as the pH approaches 6 and is dominant at pH 6.5, and that magnesium enters the picture at a pH of approximately 7.5 (1). In estimating the phosphateprecipitating capacity of the
D E of SuDernatant Liquid
P10i Fired-
3.2
9.26 7.0s
6.U
3.48
4.4 4.4 ~
In mi,liequivalenta per 1o Brems of
One mieht assume that this decrease in fixation with increasing pH a;s merely the result of the removal of iron and aluminum ions from solution. However, a similar reduction in phosphate-fixing capacity occurs when liming materials are employed to raise the pH of soils under field conditions. These liming materials supply an abundance of calcium which reacts with soluble phosphate to form the insoluble tricalcium phosphate. It is evident, therefore, that reduction in the phosphahiking power of normal field soils with rise in pH cannot be explained on the hasis of a lack of precipitating cations. 49
50
I N D U S T R I A L A N D ENGi I N E E R I N G C H E M I S T R Y
Vol. 34, No. 1
mmpl.33 of soil in 1Mknl. poFtim of dutions c€&&ing M ) TAB- 1. P~ofmurnRxw~ CAPACITY 0 ) T m Nmw J ~ B U Y and 100 m i n i e q w of P & respectnsb, in the form of Rosa monocalcium phosphafe (diamslonium phorphste if the fidd pH was 5.8 or sbove). At the end of a %day period, the soil WBB Gltered by suction and washed with five &d. portions of distilled water. After air drying, the total P;O, was detrsmined both on the phosphated and unphosphated mil, the difremnca W l g the m o u n t fix@. The phosphate-king capaoity of mil colloida is dcdy related to their SiO;:R&, ratios. Under wnditions of low p a , soils having a low ratio (containing large amounts of f&e oxide and alumina) fix more phosphates than thorn havingahighratio. Under wnditionsofhighpH, thenverse may be true, because thoas having a high SiO;:Iko, ratio also The precipitation theory of phosphate fixation having have a high cationaxohange capacity, and the exohange calfailed to wwunt either for the extramely high fixing capity oium functions as a fixing agent. Yellow mils have a grester of very acid mils, or for the marked reduction in fixing fixing power than red mils containing the w e a m o m capacity that o o m following the use of lime, alternative of iron oxidea, because of the greater degree of hydration of h r i e a of phosphate fixation had to be devised. One of their oxides. these dthat the hydrated oxidea of iron and aluminum Efiect of Horizon play dominant mles in the fixation proteas. The mechanism of thin fixation was believed to be that of the fohnation of The piumphabfixing power of a soil tan& to inoream with definite chemical combmtions betweem the oxides and the depth in the pmfle thrsugh the B horizon (Table 11). phwphate. This wumption has b a n d e d by x-ray examinations of the miner& thus formed.
Physioochemical Adsorption The w a n d alternative theory of phosphate fixation (a) aesumed that d u b l e phosphate is adnorbed by the wlloidal partidea of the soil. It in now known that this process takes plsce nnd that the adsorbed phosphate ions may either be colloid-bound or doid-bound. If colloid-bound, the phab phate ions may be displaced either by hydroxyl, humate, or silicate ions, and must be oonsidered 88 an integral part of the soil complex. If doid-bound, the phosphate is present as H30‘ ions in the ion atmosphere 8munding the soil psi' tides, rather than as a de6uite part of the soil complex. Saloid-bound ions a eaeily be displaced by obloride, sulfate, citrate, and tartrate ions. However, no sharp line of dis tinction om be drawn between colloid-bound and doidbound phosphstea. The extant to which each of the phosphate-fixing meahsnisms operates in any Boil depandu upon the nature and condition of that soil. Chemical precipitation by iron and aluminum may be entirely adequate to explain the fixation of d the phosphate applied in ordinary field practice in whioh the rate of application normally liea between 20 and 160 pounds of P;O, per acre. It does not explain the high p h w phakfixing capaoity of the Colts Neck sandy loam soil. This mil wntaina 15 per cent hydrated iron oxides which enter into direct chemioal combmtion with applied phos phate. The reduction in phosphate fixation whioh normally acwmpaniea the UBB of liming materials on this and most other soils must be scwunted for by the incresse in the number of hydroxyl ions, and their competition with phosphate ions for positions in the wlloidal wmplex. In soils having extremelJT high phosphatefixing ospscitiea, probably all of the fixation p r o m operate to a marM degree. But the phosphateking oapacity of the Colts Ne& sandy loam is relatively low compared with that of the Penn silt loam soil (Table I). The Wmncea in the phosphate-king powers of the ten New Jerseysoilslisted are closelyrelated40 their textures. The ssnd fixed the least, and the ailt losms the most, the sandy loams and loams being intermediste in their capacity to remove phosphate from dution. Such exceptions as ocow om be lsgely acwunted for by diffemncea in the h l d pH of these soils. The method emplosea in determining the phospbt&xing CsPSCity of these EO& Wnsisted in Buspending 26.gram
ey,
4.40 0.10 4.4s 0.01 4.7s 1.48 4.76 1.10 C a*/. 4.70 0.82 TOW a In term ol20 per &Ut SuperDhWPht..
2
:
m
7
4.20
80.4 B.S
4.m
6i.a 7.7
4.w
a
7 17 6%
4.10
4.70
19.4
TOW1%
A Bomewbat better pioture of the relative phosphate-king
capacities of these two soil typee, in their hydmgenaaturated
and calcium-eaturated states, is obtained by examination of the data in Table 111, in which the calcnlationsare made on the basic of equal quantitiea of soil.
TABLB 111. PEOBPEATE-FIXINQ CAPACITY‘ or 2 WON P o m s O? &n. 0.10
0.10
o.9a
0.46
0.02
0.60
0.01
0.16
ia.6 16.7 aa.8 i8.a 7.7
6.a 7.7
12.2
8.1 4.0
Knowledge of the relative king capacities of soils is highly important in relation to fertiliim practice. For soils of very low fixing capacity-Lakewwd sand, for e x a m p w i t would seem advhble to apply repated d doses of soluhle phosphates Cm wnjunction with water) during the growing 8888011, rather than to use one large doee at the kgiming of the muon. For soils of very high phosphablixing capacity, such as the Penn silt loam, heavy applications of phosphata are eaeential; and if water-soluble phosphate is ~ m p b p d , CBIY) must be taken not to mlx it with the soil but rather to apply it in ban&, or in granular form, as a means of reducing fixstion. Where soils are podsolised, as t h y are in New Jeaaes; iron and aluminum tend to concentrate in the very soid B horiron, with the result that this horiron has an extrrrmely high
laaauy, 1942
INDUSTBIAL A N D ENGINE%BING CHEMISTRY
51
6xing capcity for phosphate. If soluble phosphate is to be applied deeply in such soila, particularly where a lslge part of the A horizon haa been m v e d t k m g h erosion, it is important that the pH of thin lower horizon, to the depth to which the phosphate is to be applied, be raked to a suitable level in ordes to overcome ex&ve Eation. This d s for plowisg under liming materide. I
stepped up to WO-loOO pounds per acre, where should the eatrs fertdkr be placed? Four possibilities suggeet themselvPeandareb4ngexplordxploredinatest; tha one-ywr data from it are reaorded in Table W . The heat fertilim placement on this 8assafras loamwil I I appesred to be that aoaomplished by the uee of a disk drill whioh drop@ the fmtihr in s’treems 7 inchea apart and F O l m 1. b C I ’ I 0 N A L VISW 01 PLANr Roolr, AND BANW deep, Mow the level of the shovela of the OIF~STIWZBIIAS WR~AN~T ~ B ~ ~ ~ P L A N T U ? ~ T about O B A4~ inchea ~ AND Tom~m~s cultivator that was later employed in keeping the corn free Thi.h d v h - t i.daiaod to r h e a v b b r i e .nid btior.. of w d . The &&a of the residual fertilizer on the wheat and the alfalfa crop that are to followremain to be determined. The land will be prepar2d for wheat by diaking Since phosphata is p r a o t i d y immobile in mwt soils, a wcond problem is m t e d in that a surf- application of phosphate, or one which in plseed a t depths of only 1 or 2 inchea, becomea phaitionsus unavailable, even though the phosphste haa been pmteated again& Eaticm by tieing a p plied in bands or in grsnular form. The &ace soil tends to dry out in nwnmer, and no abmrbing mot hairs may BUTvive it. Positional unavailability can he overcome by dividing the phosphate application, p h h g part of it in a hand that is nesr the seed or mot% and locating the remainder at mme greater depth in the soil to which the mots will later penetrate and where there is sufficient moisture for the ahmrbingroot hnira to continue to function wen during periods of drought. Pho~phateEation CSJI slso be mtIy reduced by haFQrating large amount8 of organic matter into the soil. In the humic acid Ate, organic matter rea& with soluble iron and aluminum to form humates which are less soluble than the phosphates of theea elements. Like the hydroxyl ion, humic acids &so function as replacing sgenta for adsorbed phosphate W may be used as partial mhatitutm for liming
meterials and orgauic matter. For crops requiring acid-aoil conditions,there are important pomibilities in the use of the relstively insduble phosphates, such M bone meal, calcined phosphate, trioslcium phwphate, and possibly phosphate rook. While dl of theea materisls tend to raise the pH of the mil, the rate and extent of the change in pH efiected by them, in the quantities u%ually a p plied, is notgreat enough to dinqualify them from we on acidsoil crop. On- the soil is well supplied with phosphate in one or another of them forma, plants may be able to eatisfy their phosphate needs by direct “fmding” on the phoephste Weka. However, these relatively insoluble phosphates should probably be supplemented by bands of soluble p h w uhates placed dong the row to give the young plants a qui& t.
Placement of Fertilizer A m applications of f m t i h r are tending to inmeaae, and the question hsa ariaen whether theae hrgw amounts of fertili.a, partiddy the phosphate portion of them, &odd not be difrerentIy placed than were the d e r quanti* previouuly employed. conaidering the corn crop, for example, an dong-themw hand placement is entirely satis faotary, when the rate of application is be@veen100 and 200 pounds per am. If, however, the qwti6y applied is
rather than plowing, so the fmtihr, other than that applied dong the row, will remain whem it ass origin& placed. The wheat will receive 200 poundn of &10-10 at wding tj, but no additional fertilim will he applied to the &E&. This mop will be d d to ramain on the land untiI it8 g m d ooverage is reduced to 50 per osnt. Th8oontinUolIa use Oflarpp arlI0nnia of soluble paospheaa a a soil d t a in marked changea in the p h y d d d d
INDUSTRIAL AND ENGINEERING CHEMISTRY
PmpertieS of that soil. Among thesa cbangee s;ya an iliqresm in exchange capsoity (4) and a lowering of the pH at which iron and duminum come into solution. Any inin exchange capacity d t a in a -tar retention of the nutrient cations which will have been applied in oombination or in association with tbe phwphate. The downward shift in pH at which the iron and duminum come into solution m a y be 88 much 88 0.5. Theaa facte help explain why it is @le to continue producing bigh yields of potatma on soils which must be maintained at pH vsluas approximating 5.2 for the c o n t d ofsaab. The potato farmer accompliahea hi^ immediate purpose by wing bbably twica 88 much phoephste a~ would be raquired on aoiLe of pH 6.6, Ultimately he may develop a bigh-pbwphate soil, having a relatidy high exchange eapacity and B relatively low content of soluble ir0n and duminum for ita pH. An ides of the d b l e effect of the long-continued w of large amounta of soluble phospaste in incmwing tbe exchange capacity of soils is obtained fmm Table V.
End of
.. 0.0 0.4 0.6
0.55 0.82
1.0.
o.n
0.9
0.65
0.m
VoL34,No.l
... 60
70 1W 151
Symposium
~~
Nomograph for Flow from Partially Filled Pipes D. S. DAWS W a p e UnirSnity, Detroit, Mieh.
REvE1 G . flow in partislly filled
proposed an qua-
tioq for
---
horizontal pipes, Q 9.48 Z P W K1.u w h Q rata af flow oubiofeet D K
-
per e n d inner &meterof ipe, fwt (limited toi.10.6 foot
fraction of vertical diameter under 0uid (limited to 0.2-0.8)
The method of measuring tlow is simple, inexpensive, and =tisfaotory for rough e&.u~~tions gocdtosbout10perEent. The nemogrsph which facilitatenthe dculation is lwsed on the equiv& lent equation:
--
p
where q d
7.8 d1.U g l . 8 ,
r8ta.d How, gauonS per
rmntlte inner diamaqu of pipe aO-kta2-8 @ch standard iron pips
The index h e
&om
that 24
om per minute flow through
a horizontal %inch 8tsndard iron pipe when 0.4 of the inner vertid diameter is under the fluid. Bull. R v l u a Unb.. IS, Ne. 6 (1928). Bull. 8%
,
