IMPROVING DESIGN-BASED ESTIMATION OF SPATIAL MEANS BY SOIL MAP STRATIFICATION - A CASE-STUDY OF PHOSPHATE SATURATION

The usefulness of soil maps and maps of land use was evaluated to esti mate the spatial means of several phosphate sorption characteristics i n two areas with contrasting historical phosphate loads. The maps were used to stratify the areas for random sampling. This is a way of inco rporating knowledge of spatial structure into a design-based sampling strategy. Three stratifications were evaluated, namely by land use, so il map unit and by both, in combination with three methods of allocati ng sample points to the strata: proportional, optimum and near-optimum . The efficiency of various stratified simple random sampling designs was calculated from data of one sample from each area. The phosphate s orption characteristics were: (a) the areic mass of P2O5 sorbed by soi l, i.e the mass of P2O5 per M2 actually sorbed by soil above a referen ce depth; (b) the maximum areic mass Of P2O5 sorbed by soil, i.e the a reic mass which can potentially be sorbed by soil above a reference de pth; (c) the relative mass of phosphate sorbed by soil, i.e. the ratio of (a) and (b); (d) the areal fraction of soil saturated with phospha te, i.e. the fraction of an area with a relative mass of phosphate sor bed by soil greater than a critical value. For the maximum areic mass of P2O5 and the areic mass Of P2O5 sorbed by soil, stratification by s oil map unit will be worthwhile in both areas. For the relative mass o f phosphate sorbed by soil and the areal fraction of soil saturated wi th phosphate there will be a gain only where the historical phosphate load is small. The gain for the areal fraction of soil saturated with phosphate depends Strongly on the critical value of the relative mass of phosphate sorbed by soil. This gain may be further increased by str atifying also according to land use.