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Coal combustion by-product (CCBP; a wallboard-quality gypsum), limestone, and cellulose had been found to influence the dissolution of North Carolina phosphate rock (NCPR) in an acid Lily loam soil (fine-loamy, siliceous, mesic Typic Hapludult). Laboratory and incubation experiments were carried out in this study to elucidate the mechanisms responsible for the reduced dissolution of NCPR from CCBP application. The major factors responsible for the reduced PR dissolution were (i) an increase in exchangeable Ca + Mg derived from the CCBP, (ii) an increase in soil pH from the small amounts of CaC[O.sub.3] in the CCBP material; and (iii) sulfate released from the CCBP resulting in a decrease in P sorption capacity of the amended soil due to competitive sorption between sulfate and phosphate. The further reduction in NCPR dissolution by limestone addition was probably due to the increase in pH and exchangeable Ca relative to CCBP application alone. An increased NCPR dissolution from cellulose addition probably reflected an accelerated depletion of labile P by the cellulose-induced microbial growth and, hence, biomass production. The four kinetic dissolution parameters, i.e., the half-life ([t.sub.1/2]) from a first-order rate equation, the initial dissolution rate- and the average dissolution rate-related constants (A and b) from an Elovich equation, and the potential maximum dissolution of phosphate rock from a Langmuir equation were significantly correlated with pH, the content of exchangeable (Ca + Mg), and P sorption capacity of the CCBP-amended soil. The [t.sub.1/2] and A of NCPR in the acid soil could be predicted with 95% confidence by the three soil variables, i.e, soil pH, P sorption capacity, and exchangeable (Ca + Mg). Both [t.sub.1/2] and A could serve as an indicator of the NCPR dissolution potential in soils.

Phosphate rock (PR), coal combustion by-product (CCBP), limestone, and organic materials are promising amendments for low P, acid soils, and two or more of the amendments are commonly applied to improve crop yields in acid soil. However, the interactions between these amendments in acid soil are not fully understood. The effects of CCBP, limestone, and cellulose application on dissolution of North Carolina phosphate rock (NCPR) in an acid Lily loam (fine-loamy, siliceous, mesicTypic Hapludult) from West Virginia were examined in an incubation study. Based on the [Delta]P-NaOH estimation method, [approximately equal to]30% of the added NCPR was dissolved after 60 d of incubation. The NCPR dissolution decreased drastically with increasing rate of CCBP, particularly when it was applied together with limestone, whereas cellulose addition tended to enhance NCPR dissolution. Dissolution of NCPR was well described by first-order, Elovich, and Langmuir kinetic equations. The average [r.sup.2] values between the experimental data of NCPR dissolution and the three models were [0.829.sup.*], [0.851.sup.**], and [0.930.sup.**], respectively, for 17 soil samples with and without amendments. Half-life ([t.sub.1/2]) from the first-order reaction could be employed as a simple measure of the NCPR dissolution rate. Constants A and b derived from the Elovich equation were related to the initial and average dissolution rate of NCPR. The potential maximum dissolution of PR from the Langmuir equation provided an estimate of the dissolution extent of NCPR. All four kinetic parameters, particularly [t.sub.1/2] and A, were closely correlated with CCBP application rates. These parameters appeared useful in evaluating the effects of CCBP application on NCPR dissolution in the acid soil and might also be applicable to predicting PR dissolution for a wide range of soils with different properties.

Quantitative estimation of the enhancement effect of water-soluble P on P availability from phosphate rock (PR) has not been reported. The objective of this study was to use radioactive 32P as a tracer to distinguish P availability from soil, PR, and TSP so that P uptake by crops from PR in the presence of TSP could be estimated. Three sets of 4-kg soil samples of an acid Hartsells silt loam (fine-loamy, siliceous, thermic Typic Hapludult, pH 4.8) were mixed with the following treatments: (i) 32P solution and central Florida PR (CFPR), (ii) 32P-tagged TSP, and (iii) 32P-tagged TSP and CFPR at a P ratio of 50:50. The rates of P applied were 0, 12.5, 25, 50, 100, and 200 mg P [kg.sup.-1]. For treatment (iii), an additional rate of 400 mg P [kg.sup.-1] was also prepared. Maize (Zea mays L.) and cowpea (Vigna unguiculata [L.] Walp. ssp. unguiculata) were planted and harvested at 42 d after planting for maize and 45 d for cowpea. The effectiveness of P sources in terms of increasing dry-matter yield and P uptake followed the order of TSP [greater than or equal to] (CFPR + TSP) > CFPR for maize and TSP = (CFPR + TSP) > CFPR for cowpea. Phosphorus uptake from CFPR in the presence of TSP was higher than P uptake from CFPR applied alone, indicating an enhancement effect of TSP on the effectiveness of CFPR. The increase in P uptake from CFPR due to TSP influence, across all the PR rates applied, was 3.48 mg P [pot.sup.-1] for maize and 1.38 mg P [pot.sup.-1] for cowpea. With respect to P uptake from CFPR applied alone, the corresponding relative increase in P uptake from CFPR due to TSP influence was 165% for maize and 72% for cowpea.

A greenhouse experiment was conducted to study the influence of Fe and Al oxide in phosphate rocks (PR) on partially acidulated phosphate rock (PAPR) effectiveness and to evaluate compaction of PR with soluble phosphates as an alternate method of utilizing PR with high Fe and Al oxide. Five PR with Fe + Al oxide ranging from 19 to 320 g [kg.sup-1] were acidulated with 50% of the H2SO4 (PAPR, 50% required to fully convert the PR to triple superphosphate (TSP) or were compacted with TSP so that 50% of the total P was in TSP form (PR + TSP). They were applied to Hartsells silt loam (fine-loamy, siliceous, thermic Typic Hapluduh, pH 4.8) at rates 50, 100, 200, and 300 mg P [kg.sup-1]. Two successive crops of maize (Zea mays L.) and one of rice (Oryza sativa L.) were grown. Biomass yield of the first crop from PAPR-treated soils decreased from 86% of that from TSP to 16% and P uptake from 72 to 12% with increasing Fe and Al oxide from 1.9 to 32%. The decrease in effectiveness of PAPR was directly related to a decreage in water-soluble P in PAPR with increasing Fe and Al oxide content. The compacted PR = TSP were 86 to 104% as effective as TSP in increasing biomass yield and 78 to 94% as effective in increasing P uptake. There were no great differences between PAPR and the compacted PR + TSP in terms of residual effect on the second crop of maize, except for Matongo PR. The difference in residual effectiveness between PAPR and compacted PR + TSP was not as pronounced for rice as for maize.

Phosphate rock (PR) and pyrite deposits, indigenous to many African countries, may be useful to resource-poor farmers for increasing the P and S fertility of highly weathered tropical soils. Combination of PR and pyrite was examined as a means to increase the availability of P from five PR of African origin. Pyrite and PR were combined in several ratios (P/S) and incubated. Mixtures were periodically leached on a vacuum extractor with 0.01 M KC1 for a 6-mo period. Soluble P and pH of the leachate were measured. The pH of the leachates decreased with increasing levels of pyrite. In all cases, soluble P measured in the leachate increased with increasing levels of pyrite. Soluble P measured in the leachate was greatest from Togo and Uganda PR mixtures, much less from Zimbabwe PR, and virtually nil in all but the highest pyrite treatments for both Tanzania and Malawi PR mixtures. Citrate-soluble P was a less reliable predictor of P release than total P and the percentage of [CO.sib.2], A1, and Fe in the PR and associated minerals. High pyrite levels with low-quality rocks generated P release comparable with that from untreated high-quality rocks. The addition of Fe from the pyrite apparently did not lead to precipitation of substantial amounts of P as it was released from PR. The rocks responded very differently to the pyrite treatment. Although the method is promising for some rocks (Togo and Uganda), it is obviously not useful for other rocks Malawi).

The dissolution of two types of phosphate rock (PR) of different chemical reactivity in two systems was compared, and the effect of calcium (Ca) released from the phosphate rock on cation movement in soils was documented. Open-leaching systems provided a higher degree of dissolution for both PR materials than closed-incubation systems. This was attributed to removal of ionic calcium by leaching with near-complete dissolution of Christmas Island A phosphate rock. The Ca released from PR occupies exchange sites in the soil, thus increasing soil pH and cation-exchange capacity.