Dairy Manure Influence on Soil and Sediment Composition: Implications for Phosphorus Retention

Manure-derived P can jeopardize surface water quality in regions dominated by sandy soils, due to low P retention capacity of these soils. Surface (Ap) horizons from dairy-intensive areas in the Lake Okeechobee Basin were recently found to release P readily, despite abundant Ca and high pH. The purpose of this study was to determine the inorganic components that influence or reflect the stability of P in these horizons and in a related stream sediment that released much less P than soil material, despite comparable total P. Coarse fragments (>2 mm), sand, silt, and clay were separated by sieving and centrifugation. Whole soil material and coarse fragments were examined using a dissecting microscope. Crystalline and noncrystalline components were identified using a combination of optical microscopy, x-ray diffraction, scanning electron microscopy, energy dispersive x-ray analysis, electron microprobe analysis, thermogravimetry, density separation, and selective dissolution techniques. The >2-μm fraction of Ap horizons was dominated by quartz, but appreciable biogenic opal was present in the silt. Clay from these horizons was mainly noncrystalline Si (opal A), which persisted after Ca and P were removed via selective dissolution. The clay-fraction Si had high CEC, abundant adsorbed water, amorphous morphology, and low affinity for P. It was probably a degradation product of opaline forage phytoliths, since dried manure contained Si bodies similar in morphology to those found in silt and clay. Lack of Ca-P minerals suggests that manure components (i.e., organic acids, Mg, Si, etc.) inhibited crystallization of stable Ca-P, thereby maintaining high P solubility. The stream sediment contained a calcium phosphate mineral resembling poorly crystalline apatite, and a ferrous phosphate mineral (vivianite). The flux of P from dairy to aquatic systems in regions of dominantly sandy soils could be markedly reduced if the barrier to the crystallization of Ca-P could be eliminated.