Identifying soil suitability for subsoil compaction to improve water-and nutrient-use efficiency in rainfed lowland rice

For coarse-textured soils of high subsoil permeability, research has demonstrated the benefits of subsoil compaction for improved water and nutrient-use efficiency in rainfed lowland rice (Oryza sativa L.). To better define soil conditions suited to this subsoil compaction, on-farm experiments were carried out on soils varying in subsoil clay content. In 1993 and 1994 rice (cv. KDML105) was grown in main plots comparing shallow dry tillage without compaction, shallow dry tillage with subsoil compaction, and deep dry tillage with subsoil compaction. Soil was compacted in seven farmers' fields in the south of Ubon Ratchathani Province, with 10 passes of a vibrating road roller on 19 and 23 May 1993. The effects of subsoil compaction on changes in soil physical and hydrological properties differed according to subsoil clay content, which ranged from 1.4% to 12.0%. Subsoil compaction decreased soil hydraulic conductivity sufficiently for fields with subsoil clay content greater than 2%. When subsoil clay content was higher than 10%, the justification for using relatively costly subsoil compaction was questionable, as the hydraulic conductiviy was already low and the gains in water-storing capacity seemed limited. Based on these results, proportions of soils with subsoils of <2%, 2-5%, 5-10%, or >10% day were mapped using geographic information systems for an area of about 40,000 ha within the Ubon Ratchathani Land Reform Area. About 40% of the mapped area had subsoils with clay % between 2 and 10, suggesting that substantial areas could be suitable for subsoil compaction. More investigations are needed to assess its economic and social acceptability, to better understand variability and probability of response, and to further refine soil suitability in relation to clay type, clay content and groundwater changes at the toposequence level. (Résumé d'auteur)