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was approximately 2,000 m with a total of 91 sampling locations in the area. Positionaldeviations from the original grid node locations were due to site inaccessibility.Second, to depict local variations, 117 sampling sites were located within 29 transects(Set 2). Each transect comprised three to five sampling locations with 50 m to 150 mspacing. Transects were positioned along the slopes of micro-catchments. Third, avalidation set (Set 3) with 70 locations was obtained using a stratified random samplingdesign. The area was split into 14 strata of equal size and five locations wererandomly selected for each stratum. For all three sampling sets, each sampling siterepresented a rice field. Soil samples were collected from 0–0.15-m depth (topsoil;soil monolith 0.2 × 0.2 × 0.15 m by spade) and from 0.15–0.4-m depth (subsoil;Dutch auger with 0.1-m diameter). Five soil samples were bulked, one from the centerof the field and four samples within a 6-m radius around the center of the field.The samples were air-dried, ground to pass through a 2-mm sieve, and analyzed (Table1). Qualitative soil profile descriptions were conducted on auger borings at each location.Where possible, interviews were conducted with the farmers of the sampledfields to record their perceptions on soil fertility of their fields (ranked on an ordinalscale: very low, low, medium, high, very high), the probability that water is sufficientfor rice production (two, four, six, eight, or ten out of ten years), and the likelihood ofiron toxicity (high, medium, low, very low, none). Interviews also revealed informationabout the field’s topographical position in the toposequence (low, medium, high).Farmers make use of this information and combine it with knowledge about the performanceof different rice varieties to build strategies to minimize risks of crop failure.Survey observations suggested that fields with an upper sandy horizon deeperthan 1.5 m and fields frequently rejuvenated by alluvial sediments of the Lam DomYai River should be added to the farmers’ field classification (FFC) of the three topographicalpositions. This updated classification is referred to as the UFFC. Exhaustivecoverage with the UFFC was achieved in an 18,000-ha subarea of the URLRA,delineated by a rectangle in Figure 1. Scanned, pan-chromatic aerial photographs(1:4,000) for this subarea were provided by the Thailand Agricultural Land ReformOffice. Within a GIS, a 250 × 250-m grid was generated for the subarea and one of theUFFC classes was assigned to each of the 2,864 grid nodes, visually aided by thescanned aerial photographs and an elevation map. The procedure was validated in1997 when locations of 100 randomly selected grid nodes were visited in the field.Classification accuracy of 96% was obtained.Summary statisticsAbout 50% of the samples had more than 70% sand and less than 7% clay in thetopsoil. Clay contents increased slightly in the subsoil. However, soil texture wasloamy sand or loam in many parts of the study region because silt contents reachedalmost 20% in about 50% of the samples (topsoil and subsoil). About 25% of alltopsoil samples had organic carbon contents of more than 8.2 g kg –1 . Subsoil organiccarbon values were very low, however, with three-quarters of the samples having lessthan 4.4 g kg –1 . Bray-II P content was more than 1.1 mg kg –1 and more than 4 mgPerception, understanding, and mapping of soil variability . . . 81