Contents & Foreword, Characterizing And ... - IRRI books

and rice yield at regional or more detailed scales are needed to identify problem areasas a basis for making strategic agronomic decisions. This chapter discusses this problemand suggests solutions to improve the availability of soil data and characterizethe associated uncertainty whereby GIS tools are used in conjunction with relatedtechniques such as geostatistics.Perception of soil variabilityPrevious studies that assessed the soil resources in northeast Thailand created theimpression that soils are universally infertile because of their light texture and lowinherent nutrient contents (Ragland and Boonpuckdee 1988). These generalizationsmight be grounded in the fact that the finite elements of the assessment were soilseries or soil groups; see, for instance, Suddhiprakarn and Kheoruenromne (1987) orPatcharapreecha (1988). In reality, however, variations within micro-catchments aresufficient to influence land productivity factors over short distances. This short-rangevariation of soil fertility indicators within the landscape has long been recognized asimportant in studies on land resource assessment (Craig and Pisone 1988, Grandstaff1988) but is often ignored or smoothed out in conventional soil maps. Acceptance ofMoormann’s soil development theory (Moormann et al 1964), which implied gradualchanges in soil properties over long distances, has contributed to this practice.Land formation and soil genesis in northeast Thailand have been the subject ofcontroversial discussions. Pendleton and Montrakun (1960) favored an in situ soildevelopment of particular soil types at certain topographic positions. Moormann et al(1964) refuted the in situ soil genesis hypothesis and linked soil formation to vastPleistocene sediments deposited by the Mekong River and its tributaries. Accordingto Moormann et al (1964), deposition of sediments and soil formation occurred infour distinct phases, resulting in the high, middle, and lower terraces and the presentfloodplain. Recently, however, several workers have independently questioned thistheory. Paiboon et al (1985), Tamura (1986), Mitsuchi et al (1989), and Miura (1990)all support a combination of in situ soil development, mainly through lessivage, andsoil development by colluvial processes over short distances. The topography thatresembles bedrock relief and soil texture distribution within the soil matrix supportthis hypothesis. Research by Michael (1982), Löffler et al (1984), and Kubiniok (1990)revealed strong relationships among regional tectonics, sedimentation, climatic conditions,and soil genesis. Most soils of the Korat Plateau more likely developed on anerosional relief over different rock formations. Two relief generations, which haveformed under different climatic conditions, can be distinguished. Initially, Early TertiaryRed Soil (Oxisols) covered most of the plateau. These soils developed underhumid tropical conditions and are currently found as isolated remnants, mainly onhills. Subsequently, increasing Miocene tectonic activity destroyed the old relief.Furthermore, the climate became increasingly drier with the glacial maximum of theLate Pleistocene. Consequently, yellowish, brownish soils (Oxisols, Ultisols, Alfisols)developed on the now gently sloping landscape. Quaternary weakly developed alluvialsoils (Entisols and Inceptisols) are found in the alluvial plains of the Mun RiverPerception, understanding, and mapping of soil variability . . . 77