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Perception, understanding,and mapping of soil variabilityin the rainfed lowlandsof northeast ThailandT. Oberthür and S.P. KamPrevious studies that assessed the soil resources in northeast Thailand createthe impression that soils are universally infertile because of their lighttexture and low inherent nutrient contents. In reality, variations within microcatchmentsare sufficient to influence land productivity factors over shortdistances. This chapter describes a study carried out to examine soil variabilityin the rainfed lowlands of northeast Thailand and to develop quantitativemethods of spatial prediction that provide useful soil resource informationfor agronomic management. Our methodology is based on geostatisticalmapping, using soil data collected in soil surveys supplemented with lesscostly auxiliary information. The auxiliary information included knowledge oflocal farmers and soil experts about soil-landscape relations (providing a 5-category classification referred to in this study as the updated farmers’ fieldclassification, or UFFC). A soil sampling scheme was devised equivalent tothat employed for producing soil maps at a mapping scale of 1:50,000 to1:100,000. Hence, application of the proposed method in areas that havebeen mapped at these scales will only require a reanalysis of existing dataand collection of complementary data at lower costs.Soils of loamy sand or loam texture were found in many parts of thestudy region, indicating that the soil texture of these soils is not altogetherunsuitable for rice cultivation. Conventional statistical analysis of the soilsurvey data reveals very high spatial variability that cannot be ignored. Manytopsoil and subsoil properties related to nutrient availability (Bray-II P, cationexchange capacity, exchangeable bases) had large coefficients of variation(CVs), including those properties that are considered relatively stable, suchas organic matter and clay content. Further statistical analysis shows thatthis soil map accounts for less than only 8% of the variance in measured soilproperties, not enough to provide agronomically important information. Therewas no discernible distinction in soil fertility characteristics among the mappedsoil types. The UFFC accounted for higher proportions of the total variance(ranging from 0% to 43% for both soil depths), and indicated that soil productivitydeclines in the order alluvial fields > fields in low topographic position >fields in medium and high topographic position > fields with upper, deepsandy horizons.Perception, understanding, and mapping of soil variability . . . 75