Fertilizer Research 44:107-112, 1996. 107 (~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.

Phosphorus sorption by three cultivated savanna Alfisols as influenced by pH

J.O. A g b e n i n Department of Soil Science, Institute for Agricultural Research, Ahmadu Bello University, Zaria, Nigeria

Received 10 November 1994; accepted in revised form 3 August 1995

Key words: exchangeable A1, exchangeable Ca, ion pairs, P sorption, pH, precipitation

Abstract

An earlier study of phosphate sorption by some savanna soils from Nigeria suggested that increased P sorption when pH was raised might be due to precipitation of exchangeable A1 as amorphous polymeric A1 species with increased sorpfion sites. But these savanna soils have Ca as the dominant cation in their exchange sites, and low exchangeable A1. The objective of this study was to determine the role played by Ca in pH-induced P sorption of three savanna soils under continuous cultivation. Phosphorus sorption increased when pH was raised from 4.5 to 7.0. Similarly, Ca retention increased with increasing pH. Regression of P sorpfion on Ca retention indicated a significant linear relationship in the three soils. Three possible mechanisms were proposed to explain the increasing P sorption with increasing pH: precipitation of Ca-phosphates, Ca-induced P sorption or co-adsorption of Ca and H2PO 4 or HPO4 2- as ion pairs or complexes. Available evidence suggests that all three mechanisms can operate together to enhance P retention as pH increases. The paper proposes that increased P sorption by savanna soils when pH is raised is likely to be related to the chemistry and retention of Ca rather than to hydrolytic reactions of A1.

Introduction

Low amounts of total and available P of Nigerian savanna soils make investigation into problems asso- ciated with P availability imperative. Many weathered soils with low pH and deficient in native P have high P sorption capacity. Management practices to increase P availability in such soils have received considerable attention. Sorption of P usually decreases with increas- es in soil pH (Parfitt, 1978), hence a tempting solution to the problem of P availability in weathered tropi- cal soils has been to lime to pH 6.5 and 7.0 (Sanchez, 1976). Savanna soils under continuous cultivation may have pH as low as 4.0 or less while the soils under natural vegetation may have pH of 6.0 and above as measured in water (Kowal & Kassam, 1978) exclud- ing salt-affected soils. Soil pH is a critical factor in P sorption. Two interacting effects ofpH can be delineat- ed. First, increasing pH increases net negative charge of variable-charge soils. Second, pH affects ionic P species involving H2PO 4 and HPO42-, and the latter may be preferentially adsorbed (Haynes, 1984).

Conflicting results have appeared in soil literature on the effect of pH on P sorption in weathered soils. Phosphorus sorption increased, decreased or showed no effect when pH was raised in soils (Haynes, 1983; Smyth & Sanchez, 1980). Decreasing P sorption with increasing pH may be related to electrostatic repul- sion of H2PO 4 and HPO42- from the surface due to increased negative charge on variable-charge colloids (Haynes & Swift, 1985). In four savanna soils con- sisting of Alfisols and Entisols, P sorption increased when pH was raised (Mokwunye, 1975), thus raising doubts about the effectiveness of liming as a manage- ment technique to increase P availability. Increasing P sorption with increasing pH has been attributed to precipitation of A1 as amorphous polymeric At cation species with active P sorption sites (Haynes & Swift, 1985; Murrman & Peech, 1969). Similarly, the hydrol- ysis of exchangeable A1 was also proposed to explain increased P sorption by four savanna soils when pH was raised (Mokwunye, 1975). But savanna soils have low exchangeable A1 because of rapid recycling of basic cations from mineralization of labile grass roots

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Table 1. Selected physico-chemicalproperties of the soils used for the study

Soil properties Soil- 1 Soil-2 Soil-3

Sand (g kg -l) 550 610 650 Silt (g kg -l) 360 260 120 Clay (g kg -I) 90 110 230 pH (CaC12) 5.2 5.5 4.6 Org. C (g kg -1) 3.4 4.9 4.4 Exch. Ca (cmol kg -1) 1.91 2.52 1.40 Exch. Mg (cmol kg -1) 0.59 0.89 0.26 Exch. K (cmol kg- 1) 0.26 0.26 0.18 Exch. Na (cmol kg -1) 0.03 0.03 0.02 Exch. acidity (mmol kg -1) 0.02 0.02 0.04 CEC (cmol kg-1) 2.80 3.72 2.26

or addition from ashes due to annual burning of vege- tation (Jones & Wild, 1975; Kowal & Kassam, 1978) combined with often very low CECs. This study reports that pH-induced P sorption by three cultivated Alfisols from Nigerian savanna may be closely related to the chemistry and retention of Ca, the dominant cation in the exchange sites of these soils.

Materials and methods

Soil sampling and characterization

The experimental soils were from Samaru, 11 ° 11' N and 70 and 38' E, in the Northern Guinea savanna zone of Nigeria. The soils are leached tropical ferruginuous soils classified as Typic Haplustalf in Soil Taxonomy or Acrisol in the FAO system (Valette & Ibanga, 1984). The soils are from the Loess Soil Unit covering an area of 43,000 km (Klinkenberg & Higgins, 1968), and are of major agricultural importance. The clay is mainly kaolinite, and the soils have characteristic low cation exchange capacity ranging from 1 to 5 cmole kg- 1.

Twenty core samples, from 0-20 cm depth, were taken from three cultivated fields, bulked and sub- sampled. Particle size analysis was determined by the pipette method after pre-treating the soils with 30% H202 to destroy organic matter. Soil pH was deter- mined in 0.01 M CaC12 suspension. Organic C was determined by Walkley Black procedure. Effective cation exchange capacity was determined by the sum- mation of basic cations and exchange acidity. Cations were displaced with unbuffered 1.0 M NH4OAc, and

exchange acidity was determined by shaking the soils with 1.0 M KC1 and titrating with 0.1 M NaOH. The Ca and Mg concentration in the extracts were deter- mined by atomic absorption spectrophotometry (AAS) and the exchangeable Na and K by flame photometry. Selected properties of the soils are given in Table 1.

Phosphorus sorption as influenced by pH

To determine the effect of pH on P sorption, 3 g of < 2 mm sieved soils were weighed into 50 ml centrifuge tubes to which were added 20 ml of 0.015 M KH2PO4 dissolved in O.01 M CaC12. The pH of the suspension was adjusted to between 4.5 to 7.0 using appropriate amounts of 0.1 M NaOH or 1.0 M HC1, and the amounts used to adjust pH did not exceed 2 ml for each tube. The soil suspensions were brought to 30 ml with appro- priate amounts of 0.01 M CaC12 giving a soil solution ratio of 1:10, and application rate of 10 mmol P 1-1. The tubes were shaken for 16 h, pH of the suspensions was measured to determine if variations occurred due to P sorption. Variations in pH of soil suspensions pri- or to and after equilibration were q- 0.2. The tubes were centrifuged at 6,000 g for 10 mins and filtered through Whatman No. 41 filter paper, and P in super- natant solution was determined by Murphy and Riley method (Murphy & Riley, 1962). Phosphorus sorbed was the difference between P added and P in solution. Calcium in solution was determined by atomic absorp- tion spectrophotometry, and chloride was determined by titrating to a permanent reddish brown with 0.1 M AgNO3. Thus Ca and C1 retained were the differences between Ca and C1 added as 0.01 M CaC12, and Ca and C1 in supernatant solution, allowing for CI introduced as 0.1 M HC1 to adjust soil pH.

Results and discussion

Phosphorus sorption by the soils increased when pH was raised from 4.5 to 7 (Fig. 1) consistent with the observations of Mokwunye (1975) for soils from the same region. The increasing P sorption with increas- ing pH is at variance with decreasing P sorption with increasing pH noted by some investigators for other soils (Muljadi et al., 1966; Syers et al., 1973). Howev- er, increased P sorption when pH was raised had also been observed in many soils. Increases in P sorption when pH was raised from 5.0 to pH 6.0 were reported for some acid soils (Anderson etal., 1974). Liming had also been observed to decrease soluble P concentration

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1 0 0

8 0 ' Soil -1

- - PO4 Ca

......... CI

g

i 40

"' 2 0

j " 0 i i f i

4.5 5.0 5.5 6.0 6.5 7.0

pH

100 1

Soil-2 80 ~ - ................

............ P04

'~ 60 " / . . . . . . a

/

~" 20"

i 0 4,5 5.0 5.5 6.0 6.5 7.0

pH

Fig.]. The trend in the sorption ofphosphorus, calcium and chloride by the soils at varying pH levels. Similar trend was observed for the third soil.

of acid soils (Murrmann & Peech, 1969). Similarly, there are reports that sorption capacities of limed soils were higher than unlimed soils (Amarasiri & Olsen, 1973; Haynes, 1983).

Increases in P sorption of soils when pH is raised or when soils are limed are often explained by possible precipitation of exchangeable A1 as amorphous poly- meric A1 species with active P sorption sites (Amarasiri & Olsen, 1973; Haynes, 1983) or possible occlusion of P in precipitating A1 and Fe oxides (Murrmann & Peech, 1969). In some savanna soils of Nigeria, pre- cipitation of A1 as amorphous polymeric A1 species was also given to explain increased P sorption when pH was raised (Mokwunye, 1975). In west African savanna soils, however, exchangeable Ca, rather than exchangeable A1, is the dominant cation in exchange sites (Table 1) even at pH below 5.0 (Jones & Wild, 1975; Kowal & Kassam, 1978). There is growing evi- dence that exchangeable Ca has a profound effect on P sorption of tropical soils so that modelling efforts must consider exchangeable Ca (Smillie et al., 1987).

Table 2. The relationship between P sorption and Ca retention by the soils estimated by linear regression mod-

els

Soil

Molar

Regression model r 2 P: Ca ratio

Soil-1 Y = 2.44 + 0.76 X 0.980 0.8

Soil-2 Y = 10.1 + 0.62 X 0.850 0.6

Soil-3 Y = 18.3 + 0.46 X 0.947 0.5

In view of the above, the retention of Ca 2+ and CI- by the soils was monitored with a view to determin- ing their relationship with P sorption. It is interesting that Ca 2+ retention increased with increasing pH, and C1- decreased slightly with increasing pH (Fig. 1) due probably to increases in pH-dependent negative charge of the soils (Jones & Wild, 1975). The possibil- ity of C1- adsorption as CaC1 + complex might partly explain why C1- retention decreased only slightly with increasing pH in these variable-charge soils. Sposito et

al. (1983) showed that adsorption of CaC1 + complex accounted for about 50% of exchangeable Ca in soil Na-Ca exchange phenomena in C1 media. Below pH 5.0, C1- retained was more than Ca 2+ retention proba- bly because of electrostatic attraction arising from sur- face positive charges, suggesting that the zero point of charge (ZPC) of the soils is near pH 5.0. This is consis- tent with ZPC of kaolinite at pH 4.6, the dominant clay mineral in savanna soils (Jones & Wild, 1975; Kowal & Kassam, 1978). However, C1- sorbed was signifi- cantly lower than P sorbed due probably to preferential sorption of P by the soils.

Sorption curves of P and Ca 2+ show remarkable similarity in all soils (Fig. 1). Regression ofP sorption on Ca retention indicated significant linear relationship with r 2 ranging from 0.850 to 0.980 (Table 2). Thus, the three possible mechanisms that may explain increasing P sorption with increasing pH are: 1. Ca retention may induce P sorption; 2. co-adsorption of Ca 2+ and HzPO 4 or HPO42- as ion-pairs or complexes; and 3. precipitation of Ca-phosphates. A discussion of these mechanisms is presented below to weigh the evidence for each of the mechanisms, since the experimental procedure is not designed to refute any.

Calcium-induced P sorption is a possible mecha- nism of increased P sorption when soil pH is raised. Calcium is a specifically adsorbed cation in west African savanna soils (Jones & Wild, 1975), and often constitutes the dominant cation in the cation exchange

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Table 3. The concentrations of Ca-orthophosphate complexes and Ca-phosphate precipitated in soil solution

Precipitated CaH2PO4 + CaHPO ] CaPOn" Ca-phosphate

Soil pH -log (mol 1- l )

Soil-I 5.0 3.55 4.41 7.85 3.52 (25) a 6.0 4.94 4.89 7.41 2.76 (79)

Soil-2 5.0 3.44 4.40 7.95 3.71 (42) 6.0 4.87 4.83 7.37 2.03 (60)

Soil-3 5.0 3.51 4.43 7.92 4.45 (31) 6.0 4.95 4.88 7.39 2.62 (58)

aValues in parentheses are percentages of Ca in solid form with phosphate, and are consistently higher at pH 6.0 than pH 5.0.

Table 4. Ion activity products (IAP) of selected Ca-phosphate solids in soil solution

Dicalcium phosphate Octocalcium phosphate /3-Tricalcium phosphate Hydroxyapatite Soil pH p(Ca)(HPO4) p(Ca)8(PO4)6(H)2 p(Ca)3(PO4)2 p(Ca)5(PO4)3(OH)

Soil-1 5.0 6.66 96.10 29.53 53.47 6.0 7.01 97.40 29.75 53.55

Soil-2 5.0 6.61 96.00 29.52 54.46 6.0 7.07 89.46 29.75 54.52

Soil-3 5.0 6.64 95.06 29.52 53.53 6.0 7.10 96.03 29.90 54.79

Reference pK a 6.90 93.96 28.92 58.20

aReference solubility constants of these Ca-phosphate minerals are from Lindsay and Vlek (1977).

sites. Specific adsorption of Ca usually creates positive charges on soil surfaces (Huang & Stumm, 1973) for the adsorption of anions such as HzPO 4 or HPO]- by electrostatic attraction. A similar mechanism has been proposed to explain an increased SO] - sorption by some soils when CaSO4 rather than K2SO4 was applied (Bolan et al . , 1993), and cannot be ruled out for P sorption.

It is also likely that Ca and H2PO 4 and HPO42- are sorbed together as ion-pairs or complexes either as CaH2PO4 + or CaHPO4 °. Increases in anion adsorption such as S O ] - and H2PO + in the presence of Ca z+ can be attributed to the formation of surface complexes between Ca 2+ and the anions, since Ca 2+ might be preferentially adsorbed as an ion-pair or a complex by soils and sediments. For instance, there is evidence of preferential adsorption of CaliCO + to Ca z+ by soil exchange complex (Griffoen & Appelo, 1993). The soil exchangers can be considered as softer Lewis bases than water molecules and thus would preferably bind to softer Lewis acids (Griffoen & Appelo, 1993; Pearson,

1963; Sullivan, 1977; Xu & Harsh, 1990). The soil exchangers, being softer Lewis bases, would prefer- ably bind CaH2PO + and/or CaHPO4 ° which are softer Lewis acids than Ca 2+ (Griffoen & Appelo, 1993; Pearson, 1963). Ionic speciation with SOILCHEM (Sposito & Coves, 1988) indicated substantial forma- tion of CaI-I2PO + relative to CaHPO ° in soil solution (Table 3). Equimolar concentration of Ca and P will be the main evidence of the co-adsorption of Ca and P as CaHPO4 ° by the soils. The regression coefficients which defined the molar P sorbed per unit change of molar Ca o?P/Ca ratio is less than one (Table 2), sug- gesting that there was relatively greater Ca adsorptio~ than E Probably, Ca 2+ is sorbed as CaH2PO + rather than as CaHPO4 ° at pH below 7.0.

Precipitation of Ca-phosphates was rejected by Ryden and Syers (1976) as a possible mechanism of increased P sorption with increasing Ca 2+ retention of soils based on two main considerations. Molar ratio of Ca/P was in a narrow range, and retained Ca 2+ was essentially recovered from washings with 1.0 M KC1

in contrast to fractional recovery of sorbed E In this study, I checked the possibi l i ty of formation or precip- itation of Ca-phosphates in the soils at pH 5,0 and 6.0 with SOILCHEM. The results indicated Ca-phosphate precipitat ion which was higher at pH 6.0 than at pH 5.0 (Table 3). Increasing Ca-phosphate precipitation at higher pH might explain why Ca-orthophosphate

complexes were lowered by a similar factor at pH 6.0 than pH 5.0 in the soil solutions (Table 3). Calculation of ion activity products (IAP) of some Ca-phosphate solids in soil solutions suggested that IAP values were consistent with the solubil i ty constants of dicalcium phosphate (Table 4) or its hydrated form: dicalcium phosphate dihydrate DCPDH with pK = 6.60 (Lindsay & Vlek, 1977). The slight variations in IAP of soil extracts and solubil i ty constants of DCP or DCPDH may suggest that true equil ibrium has not been attained

in the soil solution. Furthermore, initial precipitates of Ca-phosphate solids are l ikely to be amorphous,

and thus can show IAP that may be slightly below or above the solubil i ty constants of DCPDH and DCE Thus, enhanced P sorption with increasing pH might also be accounted for by increased precipitation of Ca- phosphate solids at higher pH.

In conclusion, it is proposed that increased P sorp- tion by savanna soils when pH is raised may be related to the chemistry and retention of Ca 2+ consistent with

a recent report in which there was retardation of P dif- fusion in cation exchange-resins saturated with Ca due

to precipitat ion of Ca-phosphates (Akinremi & Cho, 1991). This is akin to cation exchange sites dominat-

ed by Ca. Previous work (Mokwunye, 1975) did not recognize the role of Ca on P sorption and immobi- l ization in savanna soils. The precipitation of A1 as amorphous polymeric A1 species with active P sorp- tion sites as the hypothetical mechanism of increased P sorption with increasing pH seems most unlikely in savanna soils for two reasons: 1. The soils have low amounts of exchangeable A1 even when all exchange acidity of the soils is attributed to exchangeable A1 (Table 1), and 2. the soils have no history of crop responses to l iming even at pH below 4.5 (Kowal & Kassam, 1978), suggesting that soluble A1 or toxicity of A1 has not been a problem under low pH or acid conditions. The potential danger of soil acidity might l ikely come from deficiency of molybdenum (Heath- cote, 1972; Heathcote & Fowler, 1977), or possible toxicity of micronutrients. However, from the results of this study, and previous study (Mokwunye, 1975), l iming seems to carry the potential risk of severely cur-

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tailing P availabili ty in these soils due to increased P retention.

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