Factors Affecting Phosphate Rock Dissolution in Acid Soil Amended with Liming Materials and Cellulose

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 CaCO₃ 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 (t₁/₂) 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 t₁/₂ 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 t₁/₂ and A could serve as an indicator of the NCPR dissolution potential in soils.