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Table 2. The coefficient of variation (%) was calculated for four mapping units of the soil map of Changprai et al (1971) and for five categories of theupdated 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 70 78 80 76 91 86 68 70 56 48 70 64 55 68 73 70 30 78Silt 51 44 54 50 52 58 35 39 25 46 33 36 57 49 49 48 35 49Sand 22 24 20 22 32 37 21 23 25 28 24 28 20 20 16 16 5 10pH 9 9 8 9 5 12 5 16 6 4 8 10 8 10 9 8 7 7EC 31 79 45 73 55 33 59 89 28 16 50 80 32 72 36 69 28 109OM 44 57 35 59 57 40 31 53 50 59 45 48 36 60 41 54 46 73P 114 81 79 93 29 77 73 83 60 84 87 64 55 70 78 96 161 292CEC 75 93 96 101 80 89 81 51 69 58 75 74 59 85 78 118 49 87Ca 108 144 127 164 92 120 100 177 97 115 88 101 73 193 182 230 70 172Mg 64 103 40 49 69 86 44 23 53 48 54 60 47 57 71 135 35 41K 95 119 89 137 114 133 76 71 41 68 78 79 104 138 96 113 107 122Na 117 273 100 269 127 195 130 233 133 72 101 214 135 243 141 322 73 89a 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).84 Oberthür and Kam