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Table 3. Class medians were calculated for four mapping units of the soil map of Changprai et al (1971) and for five categories of the updated farmers’field classification (UFFC).Mapping units of 1971 soil taxonomic map a Categories of the UFFC bKO KO/PP RE RE/PH ABS LTP MTP HTP DUSHSoillayer Top Sub Top Sub Top Sub Top Sub Top Sub Top Sub Top Sub Top Sub Top SubClay 8.00 10.17 6.20 9.20 8.80 14.00 7.47 12.50 13.60 14.80 10.70 15.60 6.80 9.85 6.66 7.63 3.90 3.80Silt 18.09 17.93 18.20 16.29 18.90 19.20 21.93 20.07 21.90 18.30 24.10 24.10 17.93 16.50 15.37 14.29 11.00 12.10Sand 73.55 71.09 74.90 71.20 68.10 65.80 70.74 65.52 67.60 67.40 63.90 56.73 75.01 71.30 77.49 76.70 86.10 86.20pH 4.76 5.04 4.79 5.13 4.71 5.05 4.70 5.22 4.88 5.06 4.84 5.24 4.74 5.09 4.72 5.08 4.59 4.86EC 0.22 0.16 0.20 0.16 0.21 0.17 0.27 0.17 0.21 0.16 0.25 0.17 0.20 0.16 0.20 0.16 0.20 0.13OM 0.72 0.33 0.60 0.26 0.54 0.30 0.60 0.34 1.17 0.53 0.69 0.30 0.59 0.30 0.66 0.28 0.57 0.30P 4.00 1.53 4.00 1.00 4.00 1.00 4.00 1.00 6.86 2.04 4.00 1.00 4.00 1.53 4.00 1.84 4.00 2.65CEC 1.88 1.92 1.36 1.76 1.76 1.84 1.76 2.96 4.16 5.04 2.32 3.04 1.44 2.08 1.52 1.28 1.04 0.72Ca 0.24 0.22 0.16 0.21 0.18 0.23 0.16 0.32 0.55 0.52 0.34 0.51 0.16 0.20 0.16 0.16 0.07 0.06Mg 0.11 0.13 0.08 0.09 0.09 0.09 0.08 0.10 0.22 0.21 0.12 0.13 0.08 0.10 0.09 0.10 0.07 0.07K 0.03 0.03 0.03 0.02 0.03 0.03 0.03 0.04 0.09 0.08 0.04 0.04 0.03 0.03 0.03 0.02 0.02 0Na 0.05 0.05 0.05 0.06 0.04 0.06 0.06 0.09 0.05 0.06 0.07 0.12 0.04 0.07 0.04 0.03 0.04 0.05a KO = Korat (n = 98); KO/PP = Korat/Phon Phisay (n = 122); RE = Roi Et (n = 12); RE/PH = Roi Et/Phen (n = 15). Top = topsoil, Sub = subsoil.b ABS = alluvial sites on thebackslopes of the Lam Dom Yai River (n = 9); LTP = low topographic position (n = 69); MTP = medium topographic position (n = 107); HTP = high topographic position (n = 65);DUSH = upper sandy horizon deeper than 1.5 m (n = 23).Perception, understanding, and mapping of soil variability . . . 85