PHOSPHORUS RETENTION AS RELATED TO MORPHOLOGY OF SANDY COASTAL-PLAIN SOIL MATERIALS

Phosphorus retention in sandy coastal plain soils can be low enough th at surface water quality is jeopardized by agricultural P loadings. Ob jectives of this study were to: (i) determine if discrete morphologica l characteristics could effectively differentiate sandy soils with res pect to P retention and (ii) test the efficacy of a rapid chemical ass essment of relative P adsorption (single-point isotherm) for sandy mat erials. Soil samples from 96 surface and subsurface horizons of random ly selected Alaquod, Quartzipsamment, and Paleudult pedons were used. These pedons had previously been described, sampled, and characterized as part of the Florida Cooperative Soil Survey Program. Two groups of uncoated Quartzipsamments (<5% silt plus clay) were distinguished: th ose having ''clean'' (coating-free) and ''slightly coated'' grains. El uvial horizons from Alaquods were also designated as clean because of a dominance of coating-free sand grains. Single-point P adsorption iso therms were obtained for all samples, and multipoint adsorption and de sorption isotherms for 21 of the 96 samples. Single-point isotherms ef fectively arrayed sandy material with respect to relative P adsorption . Sand-grain coatings significantly enhanced P adsorption and resistan ce to desorption. All clean samples readily desorbed P regardless of o rigin or amount adsorbed. Thus clean sands pose a greater hazard for P leaching than sands with grain coatings. Clay content was closely rel ated to P adsorption, but silt content was not. The P-retention distin ction between clean and other Quartzipsamments is more marked than the ''uncoated vs. coated'' USDA family criterion, the latter being confo unded by the inclusion of sift content.