Tools for Predicting Phosphorus Release from Soils of Animal Agricultural Production Systems.

Phosphorus (P) loss from manure-impacted sandy soils resulting in eutrophication of water bodies has been a major concern for the past several years. It is necessary to develop easily available and cost-effective tools to identify locations within animal operations that pose a threat to water quality. A threshold PSR (molar ratio of P to [Fe+Al] in an oxalate or a soil test solution; 0.1 for Florida soils) has been identified as the PSR at which P release from a soil increases abruptly. The soil P storage capacity (SPSC) which indicates the amount of P a soil can hold before becoming an environmental risk can be calculated as SPSC = (Threshold PSR-Soil PSR)*(Fe+Al)*31 mg kg^-1.Equilibrium models often use the strength of P bonding, K_L, or the Freundlich adsorption coefficient, K_F obtained from traditional isotherms, as an input to predict P loss. Our hypothesis is that K_L and K_F would be variable below the threshold PSR and would tend toward zero as the threshold PSR is approached. The PSR below the threshold value will be related to SPSC since SPSC is a function of Fe and Al. The objective was to relate PSR and SPSC to isotherm parameters (K_L and K_F) for each horizon. Soils from four location (three replicates within each horizon) within dairy manure-impacted sites were collected by horizon – A, E and Bt. The PSR, SPSC, K_L and K_F were determined for all soils and relationships between the various parameters were evaluated. Surface horizons generally had low K_L/ K_F values with high PSR and low SPSC; E horizons had higher K_L/ K_F values, lower PSR and higher SPSC with the Bt horizons having the highest K_L/ K_F lowest PSR and highest SPSC for this set of soil samples. The relationship obtained suggest that estimated K_L/and K_F from soil test data can be used as input into P loss predictive models.