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 estimate the spatial means of several phosphate sorption characteristics in two areas with contrasting historical phosphate loads. The maps were used to stratify the areas for random sampling. This is a way of incorporating knowledge of spatial structure into a design-based sampling strategy. Three stratifications were evaluated, namely by land use, soil map unit and by both, in combination with three methods of allocating 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 sorption characteristics were: (a) the areic mass of P2O5 sorbed by soil, i.e the mass of P2O5 per M2 actually sorbed by soil above a reference depth; (b) the maximum areic mass Of P2O5 sorbed by soil, i.e the areic mass which can potentially be sorbed by soil above a reference depth; (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 phosphate, i.e. the fraction of an area with a relative mass of phosphate sorbed 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 soil map unit will be worthwhile in both areas. For the relative mass of phosphate sorbed by soil and the areal fraction of soil saturated with 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 stratifying also according to land use.