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

Rainfed lowland rice, growing with an average yield of only 1.5 t ha –1 on some 4.5million ha of coarse-textured and low-fertility soils in northeastern Thailand, maysuffer from water stress as soon as rains are interrupted for about 1 wk (Sharma 1992).Percolation rate can be as high as a few centimeters d –1 . Puddling is not fully effectivedue to high sand content. Efficient management of rainwater and nutrients is a keyfactor in improving the productivity and stability of rice. On-station experiments,carried out at the Ubon Ratchathani Rice Research Center on a soil in the Ubon serieswith a clay content of 12% in the subsoil (30–60 cm), showed that water- and nutrient-useefficiency may be improved by reducing percolation through the creation of asubsurface barrier (Garrity et al 1992). Subsequent field-oriented research to identifypractical and cost-effective techniques for decreasing subsoil permeability showedthat multiple passes of a 12-t conventional road roller with vibration action (DynapacCA25) that compacts the subsoil to 75-cm depth was most effective (Sharma et al1995a,b, Trébuil et al 1998). As subsoil compaction was observed to concentrate riceroots in the plow layer above the compacted zone, a deeper (15–20 cm) tillage depththan the one currently achieved in farmers’ fields (7–10 cm) was found to be desirable.All previous studies were carried out at the experiment station of the Ubon RiceResearch Center. The effects of subsoil compaction across a range of subsoil claycontents have not been investigated, and the combined effects of soil compaction andgreen manuring have not been investigated either.The objectives of the on-farm study were1. To quantify the effects of subsoil compaction and tillage depth on changes insandy soil physical and hydrological properties across a range of subsoilclay contents.2. To quantify the interactions between subsoil compaction and green manurefertilization and their effects on the productivity of rainfed lowland rice.3. To define soil suitability for subsoil compaction by using geographic informationsystems (GIS) techniques applied to the Ubon Ratchathani Land ReformArea.Materials and methodsExperimental siteTwo-factor on-farm field experiments were conducted in the 1993 and 1994 wet seasonsin Ban Klang and Ban Mak Mai villages of Klang subdistrict of Det UdomDistrict, Ubon Ratchathani Province, lower northeast Thailand. The experimental sandysoils (Table 1) belonged to the Nam Phong series and were acidic, low in organicmatter content and available N, P, and K, and with a high concentration of ferrousiron in some fields.Experimental treatmentsWe selected seven farmers’ fields for this study. The elementary experimental plotsize varied from 100 to 200 m 2 . Each plot was surrounded by a single levee, approximately20 cm high and 40 cm wide.98 Harnpichitvitaya et al