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Saturated hydraulic conductivity (cm d –1 )600500400513UncompactedCompacted3002001000101.4 2.1 2.3 2.6 4.4 4.6 10.9 12.0Subsoil clay content (%)Fig. 4. Relationship between percentage of subsoil clay content andsubsoil compaction effects on saturated hydraulic conductivity ofseven farmers’ fields and one field within the Ubon Rice ResearchCenter (with subsoil clay content of 12%), Ubon Ratchathani Province,lower northeast Thailand.ity suggested that subsoil compaction was not effective in fields with less than 2%clay in the subsoil, where the postcompaction conductivity remained exceedinglyhigh (more than 200 cm d –1 ). Soil hydraulic conductivity decreased significantly infields having a subsoil clay content of more than 2% but less than 5%, but not up to anoptimum of less than 10–20 cm d –1 . On the other hand, where subsoil clay contentwas higher than 10%, the uncompacted hydraulic conductivity was already low. Wheresubsoil clay exceeded 10%, the technical, and probably the economic, justificationfor using subsoil compaction could be questionable, as the benefits of reducing percolationrate were limited. Farmers’ fields with subsoil clay contents between 2% and10% seemed to be suitable for subsoil compaction. Since data points were few, thisrelationship requires further study.Effects of subsoil compaction on field hydrological conditionsPonded water depth and duration. In the 1993 WS, no ponded water was observed inany plot before 82 DAS. Subsoil compaction did not increase the duration of floodwater,and had only a limited effect on floodwater depth during 82–120 DAS in shallowtillage plots (Fig. 5A). In all but one field, the groundwater table was within 20–30-cm depth from 85 to 130 DAS. Shallow groundwater was a major factor maskingthe effect of subsoil compaction on floodwater duration in the 1993 WS. In the otherfield where the groundwater table was mostly below 50–100-cm depth during 80–130 DAS, the differences in floodwater depth clearly displayed the effect of subsoilcompaction on water retention above the soil surface (data not shown). Since the104 Harnpichitvitaya et al