DAIRY MANURE INFLUENCE ON SOIL AND SEDIMENT COMPOSITION - IMPLICATIONS FOR PHOSPHORUS RETENTION

Manure-derived P can jeopardize surface water quality in regions domin ated by sandy soils, due to low P retention capacity of these soils. S urface (Ap) horizons from dairy-intensive areas in the Lake Okeechobee Basin were recently found to release P readily, despite abundant Ca a nd high pH. The purpose of this study was to determine the inorganic c omponents that influence or reflect the stability of P in these horizo ns and in a related stream sediment that released much less P than soi l material, despite comparable total P. Coarse fragments (>2 mm), sand , silt, and clay were separated by sieving and centrifugation. Whole s oil material and coarse fragments were examined using a dissecting mic roscope. Crystalline and noncrystalline components were identified usi ng a combination of optical microscopy, x-ray diffraction, scanning el ectron microscopy, energy dispersive x-ray analysis, electron micropro be analysis, thermogravimetry, density separation, and selective disso lution techniques. The >2-mum fraction of Ap horizons was dominated by quartz, but appreciable biogenic opal was present in the silt. Clay f rom these horizons was mainly noncrystalline Si (opal A), which persis ted after Ca and P were removed via selective dissolution. The clay-fr action Si had high CEC, abundant adsorbed water, amorphous morphology, and low affinity for P. It was probably a degradation product of opal ine forage phytoliths, since dried manure contained Si bodies similar in morphology to those found in silt and clay. Lack of Ca-P minerals s uggests that manure components (i.e., organic acids, Mg, Si, etc.) inh ibited crystallization of stable Ca-P, thereby maintaining high P solu bility. The stream sediment contained a calcium phosphate mineral rese mbling poorly crystalline apatite, and a ferrous phosphate mineral (vi vianite). The flux of P from dairy to aquatic systems in regions of do minantly sandy soils could be markedly reduced if the barrier to the c rystallization of Ca-P could be eliminated.