HOW GOOD IS A RECONNAISSANCE SOIL MAP FOR AGRONOMIC PURPOSES

Information about the soil fertility status in irrigated ricelands at regional scales (1:50000-1:250 000) is commonly not contained in class ical soil maps. To assess the agronomic suitability of two different r econnaissance soil maps, we conducted a detailed soil survey in the Nu eva Ecija province, Philippines. Soil samples were collected from 384 farmers' fields, and soil properties were measured for topsoil and sub soil samples. For most soil properties, a soil map made in 1940 (1:125 000) had within-map unit variances that were smaller than the total v ariance, whereas a new soil map of 1992 (1:50 000) did not significant ly reduce the within-class variance. In both soil maps, classification into mapping units accounted for 0-40% of the variance of 14 agronomi cally important soil properties and large within-map unit variabilitie s were found. Underlying strategies of classical soil survey supported the partition of variance for relatively stable soil properties, such as soil texture, CEC, and organic matter. If reconnaissance soil maps are used in quantitative land evaluation studies, existing maps requi re upgrading by adding quantitative information about relevant soil pr operties and their within-map unit variability. The sampling demand fo r upgrading a reconnaissance soil map was large, but pedotransfer func tions can be used as cost-saving tools. Measures of soil nutrient stat us were highly variable within all mapping units and differences among farmers were much greater than the differences between soil types. Th erefore, nutrient management in the study region should be based on in dividual field or farm recommendations rather than on soil-map based r ecommendations.