The effects of moisture regime and incubation period on the dissolution of North Carolina phosphate rock in soil

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Abstract
The effect of soil water content on the dissolution of reactive North Carolina phosphate rock (PR) in a lateritic soil was investigated in a laboratory incubation study. Soil-PR and soil-HK2PO4 mixtures were incubated at different water contents for different periods and dissolution of PR was followed by measuring the increases in exchangeable Ca (.DELTA.Ca) in the soil. The phosphorus released (.DELTA.P) was calculated from the composition of the PR. Increases in bicarbonate-soluble P (.DELTA.Bic P) resulting from PR dissolution were also determined. Changes in both .DELTA.P and .DELTA.Bic P can be described by exponential functions of the form. .DELTA.P or .DELTA.Bic P = A - B exp(-CW), where W is the gravimetric water content of the soil and A, B and C are constants. Little additional dissolution occurred with W > 13% which was about the water content at free-draining field capacity. For an incubation period of 280 days, PR dissolution increased from 4% for air-dry soil to 13% W = 12.5%, increasing to about 17% for W = 100%. The .DELTA.Bic P/.DELTA.P ratio for KH2PO4-fertilized soil was initially much higher than that for PR, but decreased for both sources with period of incubation and values almost converged at 0.20 after 50 days. The effect of wet-dry treatments on .DELTA.P was also investigated for soil amended to pH values of 3.76 and 5.09. After a 33 day contact period including three 2-day drying periods, there was 40% and 38% of PR dissolved for continuously wet and wet-dry treatments, respectively, at pH 3.76. The corresponding values for pH 5.09 were 22% and 21%. Therefore, these data indicate that a short drying period does not substantially affect PR dissolution (.DELTA.P) and bicarbonate-soluble P (.DELTA.Bic P) levels.