Soil Science Society of America Journal 65:147-152 (2001)
© 2001 Soil Science Society of America
DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION
Lime-Induced Changes in Indices of Soil Phosphate Availability
D. Curtina and
J.K. Syersb
a New Zealand Institute for Crop and Food Research Limited, Private Bag 4704, Christchurch, New Zealand
b Department of Natural Resources and Environmental Sciences, Naresuan University, Phitsanulok 65000, Thailand
Corresponding author (keiths{at}ncl.ac.uk)
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ABSTRACT
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Increases in soil P availability due to liming have been reported in a number of glasshouse and field trials, but the mechanism responsible for this effect has not been identified definitely. In a laboratory study, we examined the effects of lime on labile P fractions in six New Zealand soils that varied in P-retention capacity. The soils (5.1–5.5 initial pH in water) were incubated with four rates of CaCO3 to raise pH incrementally to a maximum of 
6.5. Subsequently, P (as KH2PO4) was applied to give three P levels in each soil. Liming generally decreased Olsen bicarbonate values, with the effect being largest at the highest rate of P addition. Averaged across P treatments, the decrease in Olsen P for a unit increase in pH ranged from 3 to 7 mg kg-1. Liming also tended to depress water-extractable P. Decreases in extractable P suggest that liming increased phosphate adsorption. When data for the lime and P treatments were combined, water-extractable P and Olsen P were well correlated, although each soil showed a different relationship. Phosphate-retention capacity appeared to have a strong influence on the relationship between water-extractable P and Olsen P, with the high P retention soils having relatively low proportions of water-extractable P. When exchangeable cations were replaced with Na, soils that had been limed released significantly more P to distilled water than their unlimed counterparts. The results confirm that the nature of the exchangeable cation suite has a major influence on the pH-dependence of the phosphate adsorption–desorption equilibrium. In limed soil, exchangeable Ca and pH increase simultaneously so that shifts in this equilibrium may be small and unpredictable.
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INTRODUCTION
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APPLICATION OF LIME to acid soil can stimulate crop growth by eliminating toxicities (particularly Al and Mn toxicity) and by increasing the availability of certain plant nutrients (e.g., Ca, N, Mo) (Adams, 1984). Several laboratory and field studies have been undertaken to determine how phosphate (P) availability responds to lime addition (Mansell et al., 1984; Naidu et al., 1990; Curtin et al., 1993; Holford et al., 1994; Agbenin, 1996). The results have been notoriously inconsistent. Liming has been reported to increase, decrease, or have no effect on P availability (Haynes, 1982).
The supply of P to plants is largely controlled by adsorption–desorption reactions, which regulate the concentration of P in the soil solution. These reactions may be influenced by lime-induced increases in pH and Ca (Barrow, 1984; Curtin et al., 1993; Agbenin, 1996). An increase in pH will change two key factors that underpin the adsorption reactions, the speciation of phosphate and the electrostatic potential of the adsorbing surfaces (Barrow, 1984). When pH is increased, the proportion of the divalent phosphate ion 
, the P species considered to be adsorbed (Barrow, 1984), is also increased. This change in phosphate speciation promotes adsorption, but, at the same time, surface electrostatic potential becomes more negative as pH increases and thus less attractive to phosphate ions. The result of these competing two tendencies is expected to determine whether lime will increase or decrease P adsorption. Barrow (1984) points out that quite small differences in the rate of decrease in surface potential can explain why P adsorption decreases as pH increases in one soil but increases in another. The pH dependence of surface potential is sensitive to factors such as exchangeable cation composition and the ionic strength of the soil solution (Barrow, 1984; Curtin et al., 1993), both of which change when lime is added.
Other possible effects of liming on P availability include precipitation of P as Ca phosphate (Naidu et al., 1990) and changes in the rate of organic P mineralization due to altered rates of microbial activity (Haynes, 1982). Because there are no models capable of predicting the various chemical and biological interactions that determine how plant-available P will respond to lime addition in a particular soil, laboratory and field experiments have been essential sources of information and guidance. In New Zealand, results from 25 field experiments on a range of soil types showed that lime had a beneficial effect (albeit a small one) on P availability to pasture in 50% of cases (Mansell et al., 1984). The mechanism responsible for the so-called P sparing effect of lime was not identified. Studies of P chemistry in two New Zealand soils by Sorn-Srivichai et al. (1984) showed that Olsen P values decreased following lime addition, but pasture production was unaffected. The effect of lime was considered to be an artifact resulting from the precipitation of Ca phosphate in Olsen extracts of limed soils.
Laboratory evaluation of lime effects on P chemistry can be approached directly by measuring changes of P adsorption or desorption. Because the desorption or release of P is of most practical relevance in relation to the supply of P to plants, one of our objectives was to determine effects of liming on P release from a range of soils.
The magnitude of the pH effect on P desorption depends on the composition of the exchangeable cation suite (Curtin et al., 1993). Specifically, desorption is less sensitive to pH when cation-exchange sites are occupied by Ca than when a monovalent cation is present. In this study we attempted a simple partitioning of lime effects on P release into a pH effect per se and the effect of increased exchangeable Ca. The role of Ca in the P chemistry of limed soil was further examined by measuring the effects of applied P and lime on concentrations of soluble Ca in saturated paste extracts of the soils.
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MATERIALS AND METHODS
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Six soils, of contrasting P status and P-retention capacity, were sampled (0–15 cm depth) in the North Island of New Zealand (Table 1). The soils were derived from a variety of parent materials, including andesitic volcanic ash (Egmont loam), olivine basalt (Okaihau gravelly clay), siliceous loess (Tokomaru and Kairanga silt loams), recently deposited river alluvium (Twyford silt loam), and siliceous loess containing volcanic ash (Ramiha silt loam). Soils derived from volcanic ash contained allophane and short-range order (amorphous) Fe components, resulting in high P retention values. The mineralogy of soils derived from sedimentary materials (loess and alluvium) is dominated by 2:1 clays (illite and vermiculite) with halloysite and kaolinite also present. The Okaihau soil is high in gibbsite and crystalline Fe oxide. Phosphate retention was measured according to the procedure of Saunders (1965), which was designed to fully saturate the soil with P. This is achieved by shaking soil with a NaOAc–acetic acid solution (buffered at pH 4.6) containing a high concentration of P (5000 mg P kg-1) for 24 h. Phosphate retention measured in this way is insensitive to soil P status and P fertilizer history, and the values (Table 1) provide a good indication of the amounts of hydrous metal oxides and allophanic material in the soils (Saunders, 1965). Soil pH was measured in water (1:2 soil/water ratio). Exchangeable Al was extracted in 1 M KCl and measured by the aluminon method (Hsu, 1963).
Subsamples (<2 mm) of the soils were treated with four rates of CaCO3 (analytical grade) to give a range of pH values with a maximum of 6.5. The lime rates were determined in preliminary tests by equlibrating soil–water suspensions with Ca(OH)2 on an end-over-end shaker for 16 h. Duplicate samples of limed and unlimed soil were incubated in polyethylene bags (room temperature and 90% of field capacity) for 4 wk. Distilled water was added to compensate for evaporative losses. The soils were then treated with three rates of P, as KH2PO4 solution, and reincubated for a further 4 wk. The P application rates varied depending on soil P status and P-retention capacity. Two soils with large P retention values (Ramiha and Egmont; Table 1) received P at rates of 0, 250, and 500 mg kg-1. For the Twyford, Kairanga, and Okaihau soils, the P rates were 0, 50, and 100 mg kg-1; for the Tokomaru soil, they were 0, 25, and 50 mg kg-1. Following incubation, the soils were air dried, and sieved (<2 mm). Soil pH and exchangeable Al were measured as described above.
Olsen P was determined by extracting 2 g of soil with 40 mL of 0.5 M NaHCO3 for 30 min (Olsen et al., 1954). Water-extractable P was determined by shaking 1-g soil samples with 40 mL of distilled water for 1 h. Resin-extractable P was measured using a double resin system consisting of an anion exchange resin in the OH form combined with a cation-exchange resin in the H form (Curtin et al., 1987). The cation- and anion-exchange resins were enclosed in separate nylon mesh bags (1 g of resin per bag). Soil samples (1 g) were shaken for 16 h with the resins in 35 mL of distilled water. The bag containing the anion-exchange resin was then washed free of soil using distilled water and shaken for 1 h in 40 mL of 1 M NaCl to displace P (preliminary tests showed that this extraction procedure effectively recovered all of the P adsorbed by the resin). Phosphate in all extracts was determined by the method of Murphy and Riley (1962).
To determine the extent to which lime-induced increases in exchangeable Ca influence P release, measurements of water-extractable P were also made after replacement of exchangeable cations by Na. Unlimed and limed samples (1 g) of selected soils were washed with 40 mL of 1 M NaCl to saturate exchange sites with Na. The Na-saturated soil was then suspended in 40 mL of distilled water and shaken for 24 h. The suspensions were centrifuged for 15 min in 12000 g and the supernatant solutions collected for P determination following filtration. A fresh aliquot (40 mL) of water was added and the procedure was repeated. Each sample was extracted a total of nine times with distilled water.
The effect of lime and P on soluble cations was assessed by measuring the composition of saturated paste extracts. The pastes were prepared as follows: duplicate soil samples (100 g) were wetted to saturated with distilled water, equilibrated overnight at room temperature, and transferred to Buchner funnels for vacuum extraction of the solution. After filtration, the solutions were analyzed for cations (Ca, Mg, K, Mn) by atomic absorption spectrophotometry and Si by the method of Pruden and King (1969).
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RESULTS AND DISCUSSION
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The initial pH values of the soils used in this study were in the narrow range 5.1 to 5.5, and exchangeable acidity (Al) was relatively low in all cases (Table 1). Decreases in pH of 0.1 to 0.4 units occurred when unamended soils (no lime or P added) were incubated for 8 wk, probably because of acidity generated by mineralization of organic matter (Curtin and Smillie, 1995). This acidification resulted in increases in exchangeable Al in four of the soils. Other changes noted in unamended soils during incubation were decreases in extractable P (Olsen and water), particularly in soils of high P status (Table 1). This suggests that P added in the field prior to sampling had not completely reacted and continued to be absorbed after the soil was incubated in the laboratory.
Effect of Lime on Indices of Phosphorus Status
The pH values of soils incubated with lime were close to the target values. At the highest rate of lime addition, for example, soil pH was within 0.2 units of the target pH of 6.5 (Fig. 1)
. As expected, Olsen P was increased by P addition in all soils. In five of the six soils, liming significantly (P < 0.05) decreased Olsen P, with the effect generally being largest at the highest rate of P addition (Fig. 1). In the remaining soil (Kairanga), the low and medium lime rates decreased Olsen P, but there was a tendency for Olsen P to increase at pH > 6.
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Fig. 1. Changes in Olsen P in response to increasing pH by liming for six soils treated with three rates of P. Circle, zero P; triangle, low P rate; square, high P rate. The low and high P rates for each soil are given in the Materials and Methods section. Vertical bars represent LSDs at 
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Olsen P was generally well described by regression equations having as independent variables the amount of P added and soil pH (Table 2). Except for the Kairanga and Tokomaru soils, there was a significant (P < 0.05) negative interaction between pH and added P, confirming that the pH-dependence of Olsen P became larger as the P application rate increased. Averaged across P addition rates, the decline in Olsen P (in mg kg-1) for a unit increase in pH was 3.2 for Tokomaru, 4.3 for Egmont, 4.4 for Twyford, 5.2 for Ramiha, and 6.7 for the Okaihau soil with a high Olsen P. As a result of the nonlinear relationship between pH and Olsen P in the Kairanga soil, the pH term was not significant (P > 0.05) when the data were analyzed by linear regression (Table 2).
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Table 2. Regression equations relating indices of P availability to amount of added P (X1), soil pH (X2), and their interaction
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To further explore the effect of initial P status on the pH dependence of the Olsen test, we calculated the decrease in Olsen P for a unit increase in pH [
Olsen (pH unit)-1] at each level of P addition to Tokomaru, Twyford, Egmont, Ramiha, and Okaihau soils by linear regression. When these Olsen (pH unit)-1 values were regressed against initial Olsen values (the Olsen value of unlimed soil was regarded as the initial Olsen P value, designated Olseni in Eq. [1]) the following equation was obtained:
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According to this relationship, liming agricultural soils, which normally have Olsen values in the 5 to 30 mg P kg-1 range, would decrease Olsen P by 2 to 5 mg kg-1 per unit increase in soil pH. Given that Eq. [1] explained less than one-half of the variation in pH dependence of the Olsen test, caution is clearly needed in extrapolating the relationship to other soils. However, the equation was successful in predicting lime-induced reductions in Olsen P observed by Sorn-Srivichai et al. (1984) in a study with two soils whose P status was varied by addition of phosphate in the laboratory. Their data show decreases in Olsen P of 2 to 6 mg kg-1 per unit increase in pH, which are very similar to those observed in our study. The magnitude of the pH effect increased by a factor of 0.14 for an increase in initial Olsen P of 1 mg kg-1, which is consistent with Eq. [1]. Although our results and those of Sorn-Srivichai et al. (1984) suggest that Olsen P is commonly depressed when New Zealand soils are limed, there are likely to be exceptions, as indicated by data for the Kairanga soil (Fig. 1), where there was a tendency for Olsen P to increase in the upper pH range.
The response of water-extractable P to liming was not consistent across soils (Fig. 2)
. Water-extractable P tended to decrease as lime rate increased from Ramiha, Kairanga, Twyford, and Okaihau soils. As with Olsen P, the response was largest at the highest rate of P application (Table 2). Water-extractable P was unaffected by lime in Egmont, and lime-induced increases were observed in the Kairanga soil. The change in water-extractable P per unit increase in pH (averaged across P treatments) ranged from an increase of 0.3 mg kg-1 (Kairanga) to a decrease of 2 mg kg-1 (Okaihau). It is noteworthy that, in the mineralogically similar Tokomaru and Kairanga soils, water-extractable P exhibited opposite responses to lime (Fig. 2). These results indicate the difficulty of predicting the effect of lime on P availability.
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Fig. 2. Changes in water-extractable P in response to increasing pH by liming for six soils treated with three rates of P. Circle, zero P; triangle, low P rate; square, high P rate. The low and high P rates for each soil are given in the Materials and Methods section. Vertical bars represent LSDs at
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Overall, our results do not appear to support the conclusion of Sorn-Srivichai et al. (1984) that lime-induced decreases are less likely with water-extractable P than with Olsen P. They attributed decreases in Olsen P in limed soil to precipitation of Ca phosphates in the bicarbonate extract as a result of the combination of high pH (8.5) and high Ca concentration. They argued that extraction with water is preferable for assessing the response of available P to lime because lower Ca and pH values in the water extract make it a less favorable medium than the bicarbonate extractant for Ca phosphate precipitation. In our study, water-extractable P and Olsen P generally exhibited similar pH trends (Table 2), suggesting that use of these extractants yields comparable information for the effect of lime addition on P availability. It seems reasonable to interpret trends exhibited by most soils for water-extractable P and Olsen P to decrease in response to lime addition as evidence that liming increased phosphate adsorption.
When data for all lime and P treatments were combined, water-extractable P and Olsen P were well correlated, although each soil gave a different relationship (Fig. 3)
. The slope of the water-extractable P vs. Olsen P relationship tended to decrease as soil P-retention capacity increased. Thus, in soils with high P-retention capacity (Egmont and Ramiha), water-extractable P represented a relatively small proportion of Olsen P.
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Fig. 3. Relationship between Olsen P and water-extractable P for soils incubated with lime and P. Values beside the regression lines are coefficients of determination (R2), all of which were significant at P < 0.001. Data points marked with arrows are preincubation values for soils not amended with lime or P
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The Olsen bicarbonate reagent extracts labile, surface-adsorbed P that is in equilibrium with P in the soil solution, which is the immediate P source for plant growth. The relationships shown in Fig. 3 imply that, at a given Olsen P value, soils can differ substantially in the level of P they maintain in solution, and, consequently, Olsen values for optimum growth could vary from soil to soil. Also, losses of soluble inorganic P in surface runoff, which can make an important contribution to surface water eutrophication (Sumner and McLaughlin, 1996), are unlikely to be predicted adequately from Olsen P values (Haygarth et al., 1998). Similarly, because of the lack of a consistent relationship between Olsen P and water-extractable P for different soils, it seems unlikely that P leaching can be predicted from a common Olsen P value, as suggested by Heckrath et al. (1995).
The relationship between water-extractable P and Olsen P in our soils showed no consistent effect of liming. In three of the soils, the ratio of water-extractable P to Olsen P was increased by P addition, but the effect was relatively small. As discussed above, untreated soils (not limed or fertilized with P) showed large decreases in Olsen P and water-extractable P during 8 wk of laboratory incubation (Table 1). Nevertheless, ratios of water-extractable P to Olsen P in the nonincubated soils were consistent with relationships between these P fractions in soils incubated with P and lime (Fig. 3). Our results suggest that the relationship between water-extractable P and Olsen P is mainly a function of intrinsic soil properties that determine P-retention capacity, with effects of P addition, liming, and incubation treatments being secondary. Information on the amount of water-soluble P associated with a given Olsen value should be useful in assessing the rate of P transfer from soil to plant roots and surface runoff, and the likely extent of P leaching. Further work is needed to identify soil factors (other than P retention capacity) that underpin the relationship between water-extractable P and Olsen P.
Effect of Lime on Resin-Extractable Phosphorus
Phosphate extracted using the mixed anion–cation resin system tended to increase, especially in P-treated soil, when soil pH was raised (Fig. 4)
. The resin combination that was used causes extensive removal of exchangeable cations, which are replaced by H+ from the cation resin, resulting in acidification of the soil (Curtin et al., 1987). These changes in soil exchangeable cation composition probably had a significant influence on the observed resin-P trends. As shown below, removal of divalent cations may promote P release, particularly in limed soil with elevated exchangeable Ca levels. The mixed resin also acts as a weak-acid extractant. There is evidence in the literature that acidic extractants may be more effective in extracting P from limed than from unlimed soil. For example, Rhue and Hensel (1983) found that, although liming a Florida soil from pH 4.8 to 6.5 induced P deficiency in potato (Solanum tuberosum L.) extractability of P in dilute acid (Mehlich 1 test) was increased by lime addition.
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Fig. 4. Effect of lime (pH) and P on amounts of P extracted with mixed anion–cation resin from three soils at three levels of applied P. Circle, zero P; triangle, low P rate; square, high P rate. The low and high P rates for each soil are given in the Materials and Methods section. Significance (probability level) of lime (L), added P (P), and L x P effects are shown for each soil
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Soluble Cation Concentrations and Cation Effects on Phosphorus Release
Addition of KH2PO4, as used in this study, was expected to increase soluble Ca and Mg as a result of the displacement of these cations from exchange sites by K. However, significant decreases in divalent cation concentrations in saturated paste extracts were observed in most soils and, where high rates of P were added (Ramiha and Egmont soils), P-induced decreases in soluble Ca and Mg were particularly large (Fig. 5)
. Addition of P also decreased Mn concentration (Fig. 5). Mechanisms that might account for the decrease in divalent cation solubility in P-treated soils include coadsorption with phosphate or precipitation as sparingly soluble Ca, Mg, and Mn phosphate compounds. Because addition of P decreased Ca, Mg, and Mn, even in unlimed soil (Fig. 5), where precipitation was unlikely, coadsorption was considered to be the most plausible mechanism. Phosphate-induced adsorption of Ca has been reported previously by Ryden and Syers (1976) and Agbenin (1996). Concentrations of soluble Si increased in response to P addition, indicating displacement of adsorbed silicate (Fig. 5). However, silicate displacement was small relative to amounts of P adsorbed. In the Egmont soil, for example, the addition of 500 mg P kg-1 increased soluble Si by 1 to 3 mg L-1, depending on lime rate. These values represent increases in soluble Si of only 0.8 to 2.5 mg kg-1, indicating that silicate displacement was a minor mechanism associated with phosphate adsorption. As found in other studies (Curtin and Smillie, 1983), liming decreased soluble Si due to enhanced silicate adsorption in the upper pH range.
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Fig. 5. Concentrations of Ca, Mg, Si, and Mn in saturated paste extracts of Egmont soil, as influenced by lime and P application. Circle, zero P; triangle, 250 mg P kg-1; square, 500 mg P kg-1. Significance (probability level) of lime (L), added P (P), and L x P effects are shown for each element
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Following the replacement of divalent cations by Na, P release to distilled water increased systematically with increases in pH or lime rate (Table 3). Lime-enhanced P release from Na-saturated soil was observed both when measured by a single water extraction or a sequence of nine successive water extractions. These trends are similar to those observed for P extracted by the mixed resin, which also removes exchangeable Ca and Mg. Our data confirm that the interaction between pH and exchangeable cations exerts a major influence on the equilibrium between adsorbed P and P in solution. Increasing the pH could substantially increase P solubility when Na is a dominant exchangeable cation. Compared with Na, Ca tends to decrease P release, through its effect on surface electrostatic potential or by coadsorbing with phosphate. The simultaneous increase in pH and exchangeable Ca in limed soil has the effect of damping shifts in the P adsorption–desorption equilibrium, resulting in lime-induced changes in P solubility being relatively small and difficult to predict.
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Table 3. Effect of lime on P extracted by water from three soils after exchangeable divalent cations were displaced by washing with 1 M NaCl
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CONCLUSIONS
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Measurements of Olsen P and water-extractable P for a range of New Zealand soils suggest that the extractability of P is generally decreased slightly by lime application. The decline in Olsen P for a unit increase in pH ranged from 4 to 7 mg kg-1. Lime-induced decreases in extractability of P in water and Olsen bicarbonate were attributed to enhanced phosphate adsorption following liming. Shifts in the P adsorption–desorption equilibrium due to lime application are likely to be small in soils dominated by divalent exchangeable cations. Further, the direction of the lime response (positive or negative) is not predictable, and soils with similar mineralogy can show quite different P release responses. Thus, based on the results of this laboratory study, there are no grounds for recommending a decrease in P inputs following lime application to the soil studied. Only in special circumstances (i.e., in Na-affected soils) is an increase in pH likely to produce a consistent improvement in P availability.
Received for publication November 15, 1999.
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REFERENCES
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ABSTRACT
INTRODUCTION
