Assessing the vertical movement of phosphorus in manure-impacted soil

Soil phosphorus (P) concentrations in agricultural lands have increased over the years, increasing the potential for P loss to surface waters and the risk of eutrophication. Sandy soils present high risks of P leaching, especially where intensive animal agriculture is taking place. The purpose of this study is to evaluate the potential for P leaching from dairy and poultry manure-impacted sites of the. Suwannee River Basin. Soil samples were collected from two dairy and two poultry sites during three periods of time. Different physical and chemical characteristics of the soil were determined. The P saturation ratio (PSR) was calculated as the molar ratio of extractable P to extractable Fe and Al. The soil P storage capacity (SPSC) was calculated using a PSR threshold of 0.15. A low soil:water ratio (5:1) was used to determine water soluble P (WSP) in order to minimize solubility differences in different manure types. For evaluation of the leaching potential in a column experiment, forty soil samples representing A, E and Bt horizons were leached using unsaturated flow. The amounts of P in the sandy (A and E) and in the loamy and clayey (Bt, Btg, etc) horizons were calculated over the three periods as well as the SPSC of the sandy horizons.; Results showed that relationships between PSR and WSP 5:1 were consistent for dairy and poultry manure-impacted soils. The 5:1 soil:water ratio, which is closer to field moisture conditions, appeared to be a better indicator of P leaching from manure-amended soils and is highly comparable to P concentrations in the column leachales. SPSC predictions of P storage capacity following P loading closely correspond to observed values, especially for low-impacted soils. The analysis of the P contents of the sandy and the loamy and clayey horizons showed increases for the low-impacted soils over the years, with corresponding decreases in the P storage capacity, probably due to intense P loadings. Statistical analysis of the SPSC of the sandy horizons showed that all the main effects and interactions were significant: time, type of operation (dairy or poultry) and impact level (minimally or highly impacted by manure additions).