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Influence of soil constituents on soil phosphorussorption and desorptionRichard McDowell a & Leo Condron ba Department of Geography , University of Cambridge , Downing Place, Cambridge CB23EN, UKb Soil, Plant & Ecological Sciences Division, Lincoln University , P.O. Box 84, Canterbury,New ZealandPublished online: 05 Feb 2007.

To cite this article: Richard McDowell & Leo Condron (2001) Influence of soil constituents on soil phosphorus sorption anddesorption, Communications in Soil Science and Plant Analysis, 32:15-16, 2531-2547, DOI: 10.1081/CSS-120000389

To link to this article: http://dx.doi.org/10.1081/CSS-120000389

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INFLUENCE OF SOIL CONSTITUENTS ONSOIL PHOSPHORUS SORPTION AND

DESORPTION

Richard McDowell1,* and Leo Condron2

1Department of Geography, University of Cambridge,

Downing Place, Cambridge CB2 3EN, UK2Soil, Plant & Ecological Sciences Division, P.O. Box 84,

Lincoln University, Canterbury, New Zealand

ABSTRACT

A study was conducted to examine phosphorus (P) sorption and

immediate desorption in 0.01 M CaCl2 in unmanured and manured

grassland and arable soils after chemical treatment designed to

remove organic matter [sodium hypochlorite (NaOCl)], aluminum

(Al) and iron (Fe) oxides [dithionite-citrate], and acid soluble

materials [hydrochloric acid (HCl)], either in combination or

individually. Removal of Al and Fe oxides had the greatest effect in

decreasing P sorption and increasing P desorption relative to the

fractionofPpreviouslysorbed.SorptionofPwasapproximately2 to

5 times greater in soils extracted with HCl than in dithionite-citrate

treated soils, while P desorption as a fraction of P sorbed was

approximately one-seventh that in dithionite-citrate treated soils.

The effect of HCl pretreatment was more pronounced in arable soils

compared to grassland soils, which reflected the influence of Ca on P

2531

Copyright q 2001 by Marcel Dekker, Inc. www.dekker.com

*Corresponding author. Current address: USDA-ARS Pasture Systems and Watershed

Management Research Laboratory, Curtin Road, University Park, PA 16802-3702.

COMMUN. SOIL SCI. PLANT ANAL., 32(15&16), 2531–2547 (2001)

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solubility. Following pretreatment with NaOCl, P sorption was

similar inmanuredandunmanuredarablesoils.Thiswasnot thecase

in the corresponding grassland soils which was attributed to either

physical occlusion of sorption sites by organic matter and/or

differences in the chemical natureandstabilityof soil organic matter

in the grassland compared to arable soils.

INTRODUCTION

The application of fertilizers and manure in excess of plant requirements

increases soil phosphorus (P) concentrations and its potential mobility. Reddy

et al. (1) showed P movement to a depth of 0.75 m below the soil surface

following the application of 322 kg ha21 y21 of swine lagoon waste to a loamy

sand soil (Typic Paleudult) over 5 years. Similarly, Mozaffari and Sims (2) found

elevated levels of P at 0.4 m following the application of poultry manure to soil

which was accompanied by a decrease in P sorption index. On the other hand,

Eghball et al. (3) found that the Langmuir P adsorption maximum and P sorption

index were unrelated to P movement in soils receiving cattle feedlot manure and

they suggested that P had moved in organic forms. A number of factors are known

to influence P sorption-desorption processes in soil. Soil P is associated with

aluminum (Al) and iron (Fe) in mineral form or specifically sorbed onto Al and

Fe oxides by exchange with hydroxyl groups during the formation of binuclear

surface complexes (4). In near neutral and calcareous soils, P is also associated

with calcium (Ca) in mineral form or adsorbed onto calcium carbonate minerals

(5). Organic matter, being largely negatively charged, may compete for sorption

sites on mineral surfaces and decrease P sorption (6,7). However, soil P sorption

may increase when organic matter is associated with Ca, Al and Fe (8,9). Bhatti

et al. (10) found that the removal of organic matter from a Forrested spodic

horizon (Ultic Alaquod) by hydrogen peroxide (H2O2) oxidation greatly

increased P sorption. In contrast, Borggaard et al. (11) found that removing

organic matter by H2O2 oxidation in five sandy soils that were either cultivated

(limed/fertilized) or forested had no direct influence on P sorption, while Afif

et al. (12) showed that organic matter delayed but did not prevent P sorption in 12

soils (mainly Oxisols).

Differences in soil sorption-desorption characteristics may also occur in

soils under different land uses. For example, amounts and forms of organic matter

are likely to be different in grassland soils compared to cultivated (arable) soils.

There is a need to improve our understanding of the relative influence of soil

constituents in determining P sorption and release in soils under contrasting

management regimes. The objective of this chapter was to compare P-sorption

MCDOWELL AND CONDRON2532

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and desorption processes in selected manured and unmanured grassland and

arable soils using selective extraction to remove various soil constituents.

MATERIALS AND METHODS

Soils

Two soil types were examined in this investigation. The first soil (Udic

Ustochrept) from a long-term trial at the Winchmore Irrigation Research Station

Canterbury, New Zealand, which had been under permanent pasture since 1952.

Soil (0–7.5 cm) was sampled in 1998 from replicate plots which had received no

P inputs since 1951 (unmanured grassland). Additional samples from the same

depth and soil type were taken from permanent pasture adjacent to an abattoir

which had received fellmongery (organic) effluent since 1938 (manured

grassland) (13). The pH of the grassland unmanured and manured grassland soils

were 5.4 and 5.5, respectively (1:2.5 soil to water ratio). The second soil was

obtained from Bedfordshire, United Kingdom (Typic Hapludalf) which had been

continuously cropped for approximately 25 years. Samples (0–23 cm) were taken

from fields which had received occasional liming and either no P inputs

(unmanured arable) or a combination of farmyard manure at approximately

35 t ha21 y21 and approximately 35 kg superphosphate ha21 y21 (manured

arable). The pH of the arable unmanured and manured arable soils was 7.6 and

7.2, respectively (1:2.5 soil to water ratio). All soil samples were air-dried,

ground to ,2 mm and stored until used.

Extractions

Each soil was sub-sampled in triplicate for extraction with one of a

combination of the following reagents to remove organic matter, Al and Fe and

Ca constituents.

Organic matter was removed by shaking 10 g of soil with 30 ml 6% sodium

hypochlorite (NaOCl), adjusted to pH 8.5 overnight, centrifuging the suspension

and discarding the solution. The extraction was repeated twice and the remaining

soil washed twice with distilled water. Sodium hypochlorite has several advantages

over H2O2 for the removal of soil organic matter. Lavkulich and Wiens (14) found

that NaOCl destroyed more organic matter than H2O2 with a minimal loss of Al, Fe,

Si, and Mn and little alteration of sesquioxides. There is also some evidence to show

that NaOCl does not dissolve carbonate minerals (15).

A fraction of Fe and Al compounds were removed by shaking 10 g of

untreated or NaOCl pretreated soil with 30 ml dithionite-citrate solution

SOIL P SORPTION AND DESORPTION 2533

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overnight, centrifuging the suspension and discarding the solution. This

extraction was repeated twice and the remaining soil washed twice with distilled

water. Sheldrick and McKeague (16) showed that an overnight extraction gave

similar results to the dithionite-citrate-bicarbonate method of Mehra and Jackson

(17). Dithionite-citrate will remove finely divided hematite, geothite,

lepidocrocite, ferrihydrate and noncrystalline Fe oxides in addition to organic-

complexed Fe and Al (16).

Acid soluble compounds were removed by shaking 10 g of untreated and/or

NaOCl and/or dithionite-citrate pretreated soil with 30 mL of 1.0 M hydrochloric

acid (HCl) overnight, centrifuging the suspension at 5,000 rpm for 10 min and

discarding the supernatent. This extraction was repeated and the remaining soil

washed twice with distilled water.

Analyses

Following extraction, each treated soil was air-dried and sub-sampled for

the determination of total Al, Fe, Ca and P by hydrochloric-nitric acid digestion

(18) and organic C by ignition (19).

Phosphate sorption was determined by shaking 1 g of each soil for 48 h with

solutions of 0.01 M CaCl2 containing different concentrations of P added as

potassium dihydrogen phosphate (KH2PO4) ranging from 0 to 50 mg P kg21.

After shaking, suspensions were centrifuged at 5,000 rpm for 10 min, filtered

through Whatman no. 42 filter paper and an aliquot taken for P determination.

The remaining solution was discarded and the soil allowed to air dry. The

Freundlich equation was fitted to sorption isotherms. Desorption of added P was

studied by re-suspending the soil in 0.01 M CaCl2, shaking for 48 h, centrifuged at

5,000 rpm for 10 min, filtered through Whatman no. 42 filter paper and taking an

aliquot for P determination.

The amount of P sorbed in each sample, x (mg P kg21), from an addition of

1.5 g P kg soil21 (as KH2PO4 in distilled water) was determined on a filtered

sample after shaking for 24 h at a soil to solution ratio of 1:20. This data was used

to calculate the P sorption index (PSI):

PSI ¼ x log C21 ð1Þ

where C is the solution concentration in the filtrate (mg L21). This quotient is

highly correlated to the P sorption maximum in a wide range of soils (20).

A one-way analysis of variance was conducted to test variance between

treatment means and within treatments each with three replicates using Statistical

Package for the Social Sciences (21).

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RESULTS AND DISCUSSION

Effect of Extraction on Soil Constituents

Selected chemical analyses of manured and unmanured grassland and

arable soils with and without pretreatment are shown in Tables 1 and 2. A

preliminary analysis of variance for organic matter, PSI and total Al, Fe, and P

showed that all treatments and soils were significantly different from each other

ðp , 0:05Þ: Total P concentrations ranged from 286 mg kg21 in the NaOCl 1

dithionite–citrate 1 acid treated unmanured grassland soil (soil 8) to

1766 mg kg21 in the untreated arable manured soil (Soil 1). Most P was removed

by treatment of the unmanured arable soil with acid, followed by dithionite-

citrate and then NaOCl (compare treatments 1 to 5, and 3 to 2 respectively;

Table 2). In the manured arable soil, more P was removed by dithionite-citrate

than NaOCl or acid. In the grassland unmanured and manured soils, most P was

removed by dithionite-citrate. In the manured grassland soil, NaOCl removed

more P than acid, whereas similar amounts were removed by NaOCl and acid

treatment in the unmanured soil.

The efficiency of organic matter removal with NaOCl treatment was greater

for the arable soils compared to the grassland soils, and greater in unmanured

compared to manured soils (Tables 1 and 2). This in-turn suggests that physical

protection of organic constituents within aggregates may have been greater in

manured and uncultivated soils (22). In general, treatment of the soil with

dithionite-citrate removed more Fe and Al than either NaOCl or acid (Tables 1

and 2). The effect of acid treatment on the removal of Ca was variable. Although

more Ca was removed by acid than either dithionite-citrate or NaOCl, the

amounts removed were large enough to suggest that, contrary to previous findings

(15), a proportion of exchangeable Ca and carbonates may have been removed

during NaOCl oxidation (Tables 1 and 2). This effect was more pronounced in the

grassland soils compared with the arable soils.

Phosphate Sorption

Sorption of P was affected more by the removal of Al/Fe oxides than either

organic matter or acid soluble material. Removal of the latter greatly increased P

sorption, especially when organic matter was removed beforehand (Tables 1 and

2). This may have been caused by the destruction of the crystalline lattice of clay

minerals (23) exposing new surfaces for sorption. In higher organic matter soils

this effect was less noticeable probably due to their enhanced aggregate stability

(i.e., manured soils were less affected than unmanured soils and grassland soil

were less affected than arable soils). The PSI of soils treated to remove organic

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Table 1. Composition and Sorption Characteristics (P Sorption Index, PSI) for Unmanured and Manured Grassland Soils

Soil and Treatment† Total Remaining After Pretreatment Freundlich Isotherm‡

Soil NaOCl Di/Cit§ HCl

Al

(g kg21)

Ca

(g kg21)

Fe

(g kg21)

P

(mg kg21)

Molar Ratio of P

to the Sum of

Al 1 Ca 1 Fe

Sum of Al, Ca,

& Fe (g kg21)

K

(mg kg21)

n

(mg kg21) r 2 PSI{

Organic

Matter (%)

Grassland unmanured

1 2 2 2 34.2 10.5 36.1 1151 0.0170 80.8 367 0.45 0.991 1871 5.38

2 + 2 2 31.4 5.8 26.5 782 0.0141 63.6 339 0.45 0.991 1758 3.69

3 2 + 2 29.7 4.8 18.9 622 0.0128 53.5 62 0.62 0.994 425 3.87

4 + + 2 29.2 5.3 19.3 380 0.0078 53.8 95 0.48 0.971 578 0.63

5 2 2 + 22.5 5.4 20.4 705 0.0170 48.3 1432 0.42 0.994 7209 4.31

6 + 2 + 28.2 4.9 25.6 691 0.0137 58.7 1656 0.58 0.989 4696 2.18

7 2 + + 28.7 5.1 16.9 487 0.0105 50.8 139 0.50 0.993 601 3.75

8 + + + 25.0 4.7 16.1 286 0.0069 46.0 193 0.39 0.991 693 0.88

Grassland manured

1 2 2 2 33.3 8.6 37.9 1693 0.0256 79.9 226 0.78 0.994 1669 10.39

2 + 2 2 25.5 5.6 23.2 1363 0.0293 54.3 222 0.78 0.993 1629 7.67

3 2 + 2 23.7 4.4 15.7 611 0.0155 43.9 60 0.57 0.976 375 8.66

4 + + 2 22.7 4.4 16.5 489 0.0126 43.6 71 0.56 0.993 536 4.15

5 2 2 + 26.0 5.2 27.3 1550 0.0315 58.6 937 0.51 0.998 3101 9.74

6 + 2 + 29.3 4.9 25.6 1108 0.0214 59.8 933 0.44 0.994 3332 5.40

7 2 + + 22.6 5.1 14.8 545 0.0143 42.5 121 0.55 0.986 573 7.00

8 + + + 24.2 4.4 14.6 413 0.0105 43.2 121 0.53 0.991 626 4.19

† Refers to soils with (+) or without (2) pretreatment.

‡ Coefficients and r 2 of the Freundlich equation q ¼ Kc n; where q is the quantity of P sorbed, C is the equilibrium P concentration in

solution and K and n are constants.

{PSI ¼ x log C21; where x is the amount of P sorbed in each sample (mg P kg21) and C is the solution concentration in the filtrate (mg

L21).

§ Di/Cit = pretreatment with dithionite-citrate.

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Table 2. Composition and Sorption Characteristics (P Sorption Index, PSI) for Unmanured and Manured Arable Soils

Soil and Treatment† Total Remaining After Pretreatment Freundlich Isotherm‡

Soil NaOCl Di/Cit§ HCl

Al

(g kg21)

Ca

(g kg21)

Fe

(g kg21)

P

(mg kg21)

Molar Ratio of

P to the Sum of

Al 1 Ca 1 Fe

Sum of Al, Ca,

& Fe (g kg21)

K

(mg kg21)

n

(mg kg21) r 2 PSI{

Organic

Matter (%)

Arable unmanured

1 2 2 2 36.7 66.3 39.6 1081 0.0156 83.0 408 0.38 0.996 1878 4.10

2 + 2 2 29.5 57.3 27.8 835 0.0155 63.1 574 0.40 0.996 2100 0.80

3 2 + 2 26.9 25.3 17.1 684 0.0161 46.7 188 0.50 0.998 614 1.59

4 + + 2 26.4 30.6 14.9 518 0.0126 44.4 341 0.43 0.996 928 0.55

5 2 2 + 28.7 21.8 31.5 534 0.0102 62.5 1635 0.42 0.990 11381 3.45

6 + 2 + 28.3 18.7 25.1 608 0.0126 55.3 1426 0.44 0.993 4287 0.63

7 2 + + 26.9 26.1 20.1 484 0.0109 49.7 103 0.52 0.997 502 1.01

8 + + + 25.4 16.3 14.5 348 0.0090 41.5 115 0.46 0.992 596 0.18

Arable manured

1 2 2 2 39.2 84.6 40.7 1766 0.0237 88.5 233 0.64 0.980 1658 4.85

2 + 2 2 27.6 25.7 32.1 1130 0.0219 62.2 416 0.48 0.998 2048 2.10

3 2 + 2 35.3 29.7 15.6 665 0.0129 53.8 116 0.70 0.947 335 2.55

4 + + 2 33.5 15.2 15.1 530 0.0110 50.2 205 0.36 0.982 710 0.57

5 2 2 + 37.7 66.5 37.5 1225 0.0177 81.9 1650 0.47 0.993 9313 4.13

6 + 2 + 32.9 37.5 43.6 1013 0.0156 80.2 1660 0.47 0.995 8094 0.75

7 2 + + 28.8 29.9 16.8 645 0.0144 48.7 86 0.73 0.994 481 2.47

8 + + + 26.3 18.1 14.7 371 0.0093 42.8 88 0.48 0.974 549 0.52

† Refers to soils with (+) or without (2) pretreatment.

‡ Coefficients and r 2 of the Freundlich equation q ¼ Kc n; where q is the quantity of P sorbed, C is the equilibrium P concentration in

solution and K and n are constants.

{ PSI ¼ x log C21; where x is the amount of P sorbed in each sample (mg P kg21) and C is the solution concentration in the filtrate (mg

L21).

§ Di/Cit = pretreatment with dithionite-citrate.

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matter suggested that organic matter increased sorption (compare treatments 1

and 2). In contrast, less P was sorbed in manured soils compared to unmanured

soils, which can be attributed to the saturation of P sorption sites by previous

manurial P inputs.

Sorption data fitted the Freundlich equation well ðr 2 . 0:910Þ:Excluding soils from treatments 5 and 6, the Freundlich parameter K was

significantly correlated to the PSI in both grassland ðPSI ¼ 380 K 1 320; r 2 ¼

0:909; p , 0:001Þ and arable ðPSI ¼ 171 K 2 813; r 2 ¼ 0:997; p , 0:001Þ

soils. Results presented in Table 3 shows that for grassland and arable soils,

the PSI was also strongly correlated with total P and Fe, the molar ratio of

total P to the sum of total Al, Ca, and Fe and occasionally correlated to total

Al and organic matter. The PSI for arable soils was also strongly correlated to

total Ca. In contrast, the Freundlich parameter K was only weekly correlated to

total Fe in grassland and arable soils. The Freundlich parameter n showed a

weak correlation with total P, the molar ratio of total P to the sum of total Al,

Ca, and Fe and organic matter.

The results show that following the removal of organic matter in the arable

soil PSI increases while in the grassland soils PSI decreases. In addition, the

proportion of organic matter removed by NaOCl in the unmanured (80%) and

manured (57%) arable soils was substantially greater than either the unmanured

(31%) or manured (26%) grassland soils (Tables 1 and 2). These differences are

most probably related to the nature of organic matter (grassland or arable and

type of organic inputs) and its physical protection of aggregates upon NaOCl

treatment. Previous findings have shown organic matter to decrease and increase

P sorption depending upon the type of organic matter and soil properties. For

example, Singh and Jones (24) found that organic residues containing less than

0.3% P increased P sorption and those with more than 0.3% P decreased P

sorption.

It is unlikely that organic matter binds directly with P, rather with

associated Al and Fe. Haynes and Swift (25) showed that Al-organic matter

complexes were associated with an increase in P sorption. This was attributed to

the condensation of organic matter upon drying, leading to an increased

accessibility of P sorption sites. In contrast, de Mesquita Fillho and Torrent (26)

found P sorption in tropical soils (mainly Oxisols) from Brazil increased after

removing organic matter with H2O2 and the ratio of P sorbed by treated soil to

untreated soil was related to organic matter. However, it was later found that

organic matter only delayed and did not prevent P sorption in the long-term

(256 d) (12). These results could mean, that in order to act as a competitor for

sorption sites, organic matter must be added in conjunction with P, otherwise it

has a limited affect. Furthermore, it appears that the effect of organic matter will

depend more upon the chemical composition of organic matter (especially

complexes with P-reacting cations) than the quantity of organic matter present.

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Table 3. Correlation Coefficients Between Soil Properties and P Sorption Parameters for Grassland and Arable Soils

Total Remaining After Pretreatment

Sorption Parameter

and Soil

Al

(mg kg21)

Ca

(mg kg21)

Fe

(mg kg21)

P

(mg kg21)

Ratio of P Concentration

to Sum of Al, Ca, and

Fe Concentrations

Organic

Matter (%)

Grassland

PSI 0.376*† ns‡ 0.663*** 0.760*** 0.735*** 0.413**

K ns ns 0.289* ns Ns ns

n ns ns ns 0.485*** 0.492*** 0.522***

Arable

PSI 0.326* 0.675*** 0.848*** 0.707*** 0.696*** 0.436***

K ns ns 0.611*** ns ns ns

n ns ns ns ns ns ns

† Indicates significance at the p , 0:05 (*), 0.01 (**) and 0.001(***) level.

‡ ns = not significant at p , 0:05:

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Organic matter had much less influence on P sorption than Al/Fe oxides or

acid soluble soil constituents. Both Al and Fe oxides sorb P by ligand exchange,

where P replaces hydroxyl groups on the oxide surface. The capacity to sorb P is

related to the surface area of the oxides but independent of its mineral form (27).

By changing the surface area of an oxide, organic constituents may be able to

influence P sorption. Regression analysis showed that the relationship between

organic matter and total Al and Fe left after citrate-dithionite treatment was not

significant ðp . 0:05; r 2 ¼ 0:284Þ: The amount of P that was sorbed after

removal of organic matter concentration changed far less relative to other

treatments much. This suggests that organic matter only affects P sorption

indirectly by altering the crystallinity and surface area of oxides and does not

compete for sorption sites.

Total Al and Fe account for most variation in P sorption. Jørgensen and

Borggaard (28) found the Langmuir P sorption capacity could be accounted for

by Al and Fe oxides extracted by dithionite-citrate and ammonium oxalate. In this

study, Fe was present in greater quantities and removed more efficiently than Al

(Tables 1 and 2), while the quantity of Fe remaining (and to a lesser extent Al)

was significantly correlated to the PSI in all soils (Table 3). It is possible that

more of the variability in the PSI may have been accounted for if an ammonium-

oxalate extraction (which removes amorphous Al involved in P sorption) had

been included in this study.

Phosphate Desorption

Sorption and desorption isotherms for the unmanured grassland soil are

shown in Fig. 1. In this soil most P was sorbed after acid pretreatment, while the

least amount of P was sorbed following dithionite-citrate pretreatment.

Increasing the amount of oxides in a soil is known to decrease P desorption

(29). Hence, relative to the amount of P sorbed more P was desorbed from

dithionite-citrate pretreatment, compared to acid pretreated soil (Fig. 1). This

hysteresis effect is well documented, and can be affected by such factors as

incubation time, temperature and soil to solution ratio (30). However, since these

parameters were kept constant for both sorption and desorption phases it is more

Figure 1. Sorption (solid symbols) and desorption (outline symbols) isotherms fitted to

the Freundlich isotherm (solid and dashed lines for sorption and desorption isotherms

respectively) for the grassland unmanured control soil and soil pretreated with NaOCl,

dithionite-citrate or HCl to remove organic matter, Al/Fe oxides and acid soluble

materials, respectively.

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Figure 2. Desorption of P from unmanured (A) and manured (B) grassland soils as a

percentage of adsorbed P against the initial P added (numbers refer to treatments listed in

Table 1).

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Figure 3. Desorption of P from unmanured (A) and manured (B) arable soils as a

percentage of adsorbed P against the initial P added (numbers refer to treatments as listed

in Table 1).

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likely that hysteresis was caused by a combination of P diffusion away from the

initial sorption site and resorption of P during the desorption phase (30).

Desorption data are shown in Figs. 2 and 3 as a percentage of adsorption. A

value of 50% means half of that adsorbed is readily desorbed. For all soils,

removing Al and Fe oxides greatly increased the quantity of adsorbed P that could

be easily desorbed (e.g., compare treatments 1, 2, 5, and 6 with 3, 4, 7, and 8).

This effect was more pronounced in the manured soils than in the unmanured

soils (compare treatments 1 and 3). Where Al/Fe oxides were present, the level of

organic matter only slightly increased the amount of sorbed P that could be

desorbed (Figs. 1–3). However, organic matter appeared to increase P desorption

when Al/Fe oxides were removed, especially in the manured soils and more so in

grassland soils compared with arable soils (compare treatments 3 and 4 in Figs. 2

and 3). Acid treatment of soil caused a large increase in P sorption followed by a

decrease in P desorption (Figs. 2 and 3).

In soils receiving long-term application of manure, the potential for P

movement cannot be related to P sorption alone due to possible precipitation of P

with Al, Fe and Ca (1). If P is applied at small amounts relative to the potential

sorbing surface then sorption maintains low concentrations of P the soil solution.

However, as more manurial P is applied in excess of the effective sorbing surface

(as also affected by organic matter in manure), P begins to form precipitates with

Al, Fe, and Ca. Consequently, increasing concentrations of desorbed P from soils

that received increasing P inputs either as manure or in the laboratory sorption

phase, probably originated from both sorbed and precipitated forms. In neutral to

alkaline soils, Ca is the first to precipitate with P, and in acid soils Al-P and Fe-P

are more likely (31). This is in agreement with the correlation of P sorption to Ca

in the neutral arable soils and to Fe and Al in the acidic grassland soils (Table 3).

Holford and Mattingly (5) found that P sorption was related to CaCO3 content in

the same soil type as used in this study (arable soil). Elsewhere, Raven and

Hossner (32) found that P desorption was not related to any particular soil

constituent except CaCO3 and clay content in calcareous soils. However, in this

study it is unclear whether the Ca originated from CaCO3 or was in exchangeable

form. Acid treatment of soil is likely to dissolve more than just carbonates and

apatite like minerals (33), and may not be used to explain large differences in P

sorption between untreated soil and acid only treated soils (Tables 1 and 2).

CONCLUSIONS

After pretreating soils with dithionite-citrate to remove Al and Fe oxides P

sorption decreased which, in-turn, decreased P desorption. Compared to

dithionite-citrate, pretreating soils with HCl to remove acid soluble materials

markedly increased P sorption while P desorption relative to that sorbed was

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minimal. The effect of HCl pretreatment was more pronounced in the arable soils

compared to grassland soils which were partly attributed to the influence of pH

and Ca on P solubility. Following pretreatment with NaOCl to remove organic

matter, P sorption was similar in the unmanured and manured arable soils. In

contrast, P sorption was greater in the unmanured grassland soil compared to the

manured grassland soil following removal of organic matter. This may be

attributed to differences in the chemical nature of organic matter in the grassland

and arable soils and/or the removal of labile organic matter that did not affect P

sorption. The findings of this study showed that Al and Fe hydrous-oxides have

the greatest impact on soil P sorption and desorption. However, Ca and organic

matter also had a significant effect in the higher pH soils and in manured soils.

Further research is required to clarify the role of Ca and organic matter in

determining the nature and dynamics of P sorption and desorption processes in

soil and the effect of different management practices.

ACKNOWLEDGMENTS

Financial assistance was provided by St John’s College, Cambridge (UK),

the Agricultural and Marketing Research and Development Trust (New Zealand),

and Lincoln University (New Zealand). We also wish to thank AgResearch (New

Zealand), the Primary Production Co-operative Society (New Zealand) and L

Raith & Sons (UK) for providing soils for this study.

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