Phosphorus Retention as Related to Morphology of Sandy Coastal Plain Soil Materials

Phosphorus retention in sandy coastal plain soils can be low enough that surface water quality is jeopardized by agricultural P loadings. Objectives of this study were to: (i) determine if discrete morphological characteristics could effectively differentiate sandy soils with respect to P retention and (ii) test the efficacy of a rapid chemical assessment of relative P adsorption (single-point isotherm) for sandy materials. Soil samples from 96 surface and subsurface horizons of randomly 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: those having clean (coating-free) and slightly coated grains. Eluvial horizons from Alaquods were also designated as clean because of a dominance of coating-free sand grains. Single-point P adsorption isotherms were obtained for all samples, and multipoint adsorption and desorption isotherms for 21 of the 96 samples. Single-point isotherms effectively arrayed sandy material with respect to relative P adsorption. Sand-grain coatings significantly enhanced P adsorption and resistance to desorption. All clean samples readily desorbed P regardless of origin or amount adsorbed. Thus clean sands pose a greater hazard for P leaching than sands with grain coatings. Clay content was closely related to P adsorption, but silt content was not. The P-retention distinction between clean and other Quartzipsamments is more marked than the uncoated vs. coated USDA family criterion, the latter being confounded by the inclusion of silt content.