FACTORS AFFECTING PHOSPHATE ROCK DISSOLUTION IN ACID SOIL AMENDED WITH LIMING MATERIALS AND CELLULOSE

Coal combustion by-product (CCBP; a wallboard-quality gypsum), limesto ne, 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 experime nts were carried out in this study to elucidate the mechanisms respons ible for the reduced dissolution of NCPR from CCBP application. The ma jor factors responsible for the reduced PR dissolution were (i) an inc rease 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 sorpt ion capacity of the amended soil due to competitive sorption between s ulfate and phosphate. The further reduction in NCPR dissolution by lim estone addition was probably due to the increase in pH and exchangeabl e Ca relative to CCBP application alone. An increased NCPR dissolution from cellulose addition probably reflected an accelerated depletion o f labile P by the cellulose-induced microbial growth and, hence, bioma ss production. The four kinetic dissolution parameters, i.e., the half -life (t(1/2)) from a first-order rate equation, the initial dissoluti on rate- and the average dissolution rate-related constants (A and b) from an Elovich equation, and the potential maximum dissolution of pho sphate rock from a Langmuir equation were significantly correlated wit h pH, the content of exchangeable (Ca + Mg), and P sorption capacity o f the CCBP-amended soil. The t(1/2) and A of NCPR in the acid soil cou ld be predicted with 95% confidence by the three soil variables, i.e. soil pH, P sorption capacity, and exchangeable (Ca + Mg). Both t(1/2) and A could serve as an indicator of the NCPR dissolution potential in soils.