Effect of byproduct, nitrogen fertilizer, and zeolite on phosphate rock dissolution and extractable phosphorus in acid soil

Coal combustion by products (BP) and phosphate rock (PR) have been widely used as cost-effective amendments for acid soils. Information is needed on the proper combination of BP with chemical fertilizers or other organic and inorganic amendments to improve the productivity of acid soils. Chemical analysis and soil incubation studies were carried out to examine the effect of BP, N fertilizers, and zeolite on dissolution of PR and on the status of extractable P in acid soil. Several kinetic models were compared for describing PR dissolution in acid soils that received different amounts of BP, different forms of N fertilizer, and zeolite. PR dissolution in acid soil measured by 0.5 M NaOH extraction was best described by a Langmuir kinetic model (r2=0.988**, followed by an Elovich (r2=0.950**), a two-constant rate (r2=0.947**), a parabolic diffusion (r2=0.905**), and a first-order reaction equation (r2=0.637*). A second-order reaction equation was the poorest among various models tested (r2=0.484). Addition of BP, N fertilizers, and zeolite to the PR-amended soil did not affect the good fitness of PR dissolution to these kinetic equations. Increasing BP addition decreased initial and average dissolution rate and potential maximum dissolution of PR during the incubation period of 132 days as calculated from the Elovich and Langmuir kinetic models. In general, NH4NO3 and (NH4)2SO4 increased the initial rate and decreased slightly the average PR dissolution rate due to a rapid but short-term acidifying effect. On the other hand, urea and zeolite decreased the initial rate of PR dissolution due to higher pH and increased the average PR dissolution rate because of long and persistent acidification by urea and slow but continued removal of Ca by zeolite. The effect of N fertilizers and zeolite on the potential maximum dissolution of PR was related to amounts of BP added. Extractable P in the PR-amended soils as determined by 0.5 M NaHCO3 was closely correlated with P released by PR dissolution. The ratio of increased NaHCO3-extractable P due to PR application divided by the total amount of P released from PR dissolution measured by NaOH extraction might reflect relative availability of P from PR dissolution. This ratio was increased by addition of BP, urea, and zeolite but decreased by NH4NO3 and (NH4)2SO4.