Characterization and Spatial Variability of a Phosphate Minesoil in Central Florida

Abstract:
As of 1978, 20:^ of Polk County, Florida, was owned by phosphate mining companies. Seventy percent of that land was either being mined or had been mined. Mine lands exist predominantly in agricultural areas, so characterization of minesoils will be vital in planning for future agricultural uses. The present soil characterization system is based primarily on types of minesoil "parent materials," including overburden (spoil), sand tailings, clay, or mixes of these materials.
Initial reconnaissance of spoil indicated that conventional soil characterization techniques might not be adequate to assess differences in spoil types. Therefore, this study was designed to assess both minesoil characteristics and spatial variability of the spoil. Samples were taken from two grids and two transects, all nested. Points were spaced 50 x 200 ra, 10 x 10 m and 1 m apart. Average values of the surface 25 cm were H^O-pH 5.5, KCl-pH 5.5, sand 83%, organic carbon (OC) 0.5%, cation exchange capacity (CEC) 10 raeq/100 g, extractable K 0.096 meq/ 100 g, extractable Mg 2.1 meq/100 g, and extractable acidity 4.7 meq/100 g. All values except extractable K appeared to depend on position with respect to spoil islands. Average subsurface values were H2O-PH 5.9, KCl-pH 5.6, sand 84%, OC 0.35%, CEC 9.8 meq/100 g, extractable K 0.03 meq/100 g, extractable Mg 2.4 raeq/100 g, and extractable acidity 3.8 raeq/100 g.
Direction-dependent and direction-independent seraivariograms were calculated for each parameter. Except for elevation, all exhibited a nugget variance and sill. The nuggets of sand, CEC, OC, extractable acidity, and K were greater than 40% of the respective sill values. Organic carbon had the shortest range (90 ra) and extractable K exhibited the longest (400 m).
The combination of long ranges, large nuggets, and placement of sampling grids contributed to imprecision of parameter contour mapping over the field. Kriged and non- Kriged maps of sand and OC were compared. Both types of maps were relatively inaccurate; neither map was superior to the other. Increasing the density of Kriged points from the same number of measured points does not result in more accurate contour mapping. It is recommended that a sampling scheme have relatively equal numbers of points spaced at varying distances apart, spread evenly over the study site to maximize understanding of spatial variability.
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