QUANTIFICATION OF SOIL TEXTURAL FRACTIONS OF BAS-ZAIRE USING SOIL MAP POLYGONS AND OR POINT OBSERVATIONS

Reconnaissance soil surveys typically produce qualitative choropleth maps and descriptions of sampled soil profiles, These can be converted to quantitative information directly. Alternatively quantitative predictors can be made by interpolating between the point observations, or the two can be combined. Three methods to predict the sand and clay content of Bas-Zaire using a reconnaissance soil map and 151 soil profiles are compared. A cross-validation showed that the combination method produced the most precise predictions with an acceptably small bias. This result was obtained only after verifying and correcting the soil surveyor's estimations of the soil map prediction variances and stratifying the area to acquire a relative pooled within-stratum variogram.