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ceased at 116 days after sowing (DAS), apart from a small shower about 1 wk beforeheading (Fig. 2A). In 1994, rain also stopped about 1wk before heading at 70 DAT(for plots transplanted on 8 August), or 80 DAT (for plots transplanted on 29 July).During the 1994 WS, no dry spells longer than 5 d occurred between transplantingand the last rainshower before heading (Fig. 2B).In 1993, the water table depth stayed at less than 50 cm below the soil surface inmost of the fields from 85 to 130 DAS (some 10 d after panicle initiation). Such highgroundwater table levels mitigated the effects on the rice crop of the dry spells occurringduring that period. During the 1994 WS, the mean groundwater table depth roseto less than 20 cm from the soil surface from 15 d before transplanting and remainedat this level until 55 DAT (Fig. 2). The groundwater table remained within the top 40-cm depth up to the heading stage, but fell below 50 cm after flowering. This meansthat, in the 1994 WS, rice encountered water stress during grain filling.Effects of subsoil compaction on soil penetration resistanceand saturated conductivityIn the seven fields for which data were collected, subsoil compaction significantlyincreased soil penetration resistance to at least 75-cm depth, with the 15–65-cm layerbeing the most compacted (Fig. 3). Subsoil compaction effectively decreased saturatedhydraulic conductivity (by a factor of 2–3), but most of the postcompactionvalues remained higher than those commonly observed (10–20 cm d –1 ) in paddy fields(Fig. 4). The degree of subsoil compaction obtained on these very coarse-texturedsoils, without removing the topsoil, varied extensively, particularly according to thepercentage of clay in their subsoils. The observed effects on soil hydraulic conductiv-Soil depth (cm) Soil penetration resistance (kPa)0 100 200 300 400 500020406080100Uncompacted + shallow tillageCompacted + shallow tillageCompacted + deep tillage120Fig. 3. Soil penetration resistance profiles (mean ± SE values ofseven farmers’ fields) in different subsoil compaction and tillagetreatments, 1993 wet season, in Det Udom District, UbonRatchathani Province, lower northeast Thailand.Identifying soil suitability for subsoil compaction . . . 103