Contents & Foreword, Characterizing And ... - IRRI books

Clay classified≤ 2% 50 ha (2% but ≤ 5% 762 ha (2%)>5% but ≤ 10% 1,365 ha (37%)>10% 22,019 ha (60%)Fig. 7. Suitability for subsoil compaction in a subregion of the Ubon RatchathaniLand Reform Area. Subsoil compaction may be effective in reducing percolationin areas with clay content between 2% and 10%.ConclusionsVariations in ponded water depth and rice yield in response to subsoil compactionwere attributed to differences in subsoil texture and toposequence position, whichaffected the groundwater depth of the experimental fields. A minimum subsoil claycontent of 2% seemed to be necessary to ensure successful subsoil compaction of thecoarse-textured soils of the Nam Pong series. Even in such field situations, the infiltrationrates (which were largely affected by the observed hydraulic conductivity ofthe subsoil) in compacted plots were still relatively high; therefore, subsoil compactionwould not be expected to impede the production of annual crops such as grainlegumes. These findings need further confirmation.The study illustrated a methodology using GIS and available secondary data todelineate areas with appropriate subsoil clay contents for subsoil compaction. Carefulinterpretation of these areas is necessary, however, as the chosen approach toextrapolation did not address several characteristics that might prevent the successfuladoption of the technique. One important factor is the characteristic of the soil profiles.On-farm compaction trials were located in areas with deep sandy horizons, althoughmuch of the mapped area has compacted illuvial horizons in the 0.3–0.8-msoil depth. Gravelly layers of hardened iron oxide pebbles are also frequently foundjust below the topsoil; these might prevent successful compaction. Further on-farm108 Harnpichitvitaya et al