Dissolution and effectiveness of phosphate rock in acidic soil amended with cattle manure

Phosphate rock (PR) and organic manure (OM) are promising amendments to acidic soils to correct P deficiency and Al/Mn toxicities. The interaction between PR and OM and consequent effects in acidic soil are not fully understood. The effects of OM on the dissolution of two types of PR (NCPR, North Carolina PR; SYPR, Syrian PR) and dry matter production and nutrient uptake efficiency in an acidic Lily loamy soil from West Virginia were examined in an incubation and plant pot studies. NCPR and SYPR were supplied at a rate of 500 mg P kg(-1) and organic manure at a rate of 30 g kg(-1). Observations were made over 126 days at 21-day intervals. The dissolution of applied NCPR and SYPR was calculated based on the estimation of ?Pi and ?TP-NaOH method. Dissolution of NCPR and SYPR calculated by DeltaTP-NaOH was quick during the initial 21 day (similar to20%), and thereafter increased slowly reaching 42 and 40% at 126 day for NCPR and SYPR, respectively. On the other hand, percent dissolution calculated based on DeltaPi-NaOH, while fluctuating within +/-5% between intervals, were less by 10 and 15% at 126 day for NCPR and SYPR compared to dissolutions estimated based on Delta-TP-NaOH. OM generally reduced PR dissolution, and to a 11-14% lesser extend in the presence of plant roots, presumably due to plant roots depleting P and Ca from the dissolution zone. OM increased dry matter production (DM) of switch grass, increased specific accumulation rates (SAcR, mg or mug g(-1) DM) of P and Mg in shoots, decreased SAcRs of Al and Mn in shoots DM, and increased use of P from NCPR and SYPR. Approximately 29% of dissolved P from applied NCPR or SYPR was Bray-1 extractable-Pi and increasing to 45% when OM was also amended to the soil. This increase in dissolved P probably explains the enhanced growth and P concentration in switch grass DM.