Is deep sowing beneficial for dry season cropping without irrigation on sandy soil with shallow water table?

Deep sowing (15 cm) on sands in the dry season is a practice used in post-rice sowing of legumes without irrigation, designed to increase moisture access for germination, growth and crops yield. However, with such deep sowing there can be a penalty for emergence and growth if there is abundant water stored in the upper soil profile during the growing season. Hence, there is a need to define the soil water regimes under which deep sowing is advantageous for different legumes. To investigate the adaptation of legume crop species to deep sowing, we studied their emergence, growth and yield on three deep soils (3-16% clay) with shallow water tables during two years in northeast Thailand. At site 1 and 2, peanut, cowpea, mungbean and soybean were sown shallow (~5 cm) or deep (~15 cm). At site 3, only cowpea and peanut were shallow or deep sown. Shallow water tables maintained soil water content (0-15 cm) above permanent wilting point throughout the growing season. Deep sowing of all legumes delayed emergence by 3-7 days at all locations. Shoot dry weight of legumes after deep sowing was mostly similar or lower than weight after shallow sowing. Yield and harvest index of legumes did not differ meaningfully among sowing depths. Therefore, deep sowing was not beneficial for dry season cropping without irrigation when there was a shallow water table and sufficient water for crop growth throughout soil profiles in the growing season. Taken together with previous studies, we conclude that shallow rather than deep sowing of legumes was preferred when the soil water content at 0-15-cm depth remained higher than permanent wilting point throughout the growing season due to shallow water table

Rooting patterns of four crop legumes in response to seed-placement depths in the dry season

On sandy paddy fields, key factors for successful crops in the dry season without irrigation are a shallow water table and practices such as deep seed-placement but only some legume species are adapted to such conditions. To understand the adaptation of legume species to deep seed-placement over shallow water tables, we studied their rooting patterns on two sandy soils. Cowpea (Vigna unguiculata), mungbean (Vigna radiata), peanut (Arachis hypogaea) and soybean (Glycine max) seeds were sown shallow (similar to 5 cm) or deep (similar to 15 cm) in deep sandy soils after harvesting rice in two shallow water table locations in north-east Thailand. The legumes depended mainly on capillary water rising from the water table and none experienced water deficit throughout the growing season. Generally, deeper seed-placement decreased overall root dry weight, but it increased the root surface area to weight ratio. Deep seed-placement promoted a greater fraction of root growth into the subsoil for cowpea (86-99% of total root length), mungbean (61-93% of total root length) and peanut (78-98% of total root length) where the soil contained more water throughout the growing season. Moreover, deep seed-placement at the site with the lower water table promoted deeper penetration of roots of cowpea (similar to 20 cm deeper), mungbean (similar to 20-40 cm deeper) and peanut (similar to 20-40 cm deeper) which improved water access, especially late during the growing season when topsoils dried to close to wilting point. Unlike other species, the soybean rooting pattern did not respond much to seed-placement depths, or soil moisture

Introducing mungbean as a preceding crop to enhance nitrogen uptake and yield of rainfed rice in the north-east of Thailand

One possible management option for farmers to improve the soil nitrogen (N) supply for rice production is the cultivation of a prior legume. The objective of this study was to investigate the value of such an option in the lowland of the north-east of Thailand. Two experiments were established in 2 typical locations in a split-plot design with 4 replicates. The main plots included 3 nitrogen levels (0, 30, and 60 kg N/ha) and the subplots, 4 pre-rice managements: (i) fallow with weeds removed (FW-); (ii) with weeds incorporated before the rice crop (FW+); (iii) mungbean incorporated at flowering as green manure (MGM); or (iv) incorporated after grains harvest (MR+). In both experiments the difference in rice yield between MGM and MR+ was not significant. In Expt 1, in contrast to Expt 2, the rice yield increase due to MR+ was significant and significantly higher than that due to application of 60 kg N/ha. Moreover, significantly higher apparent recovery of N (ANR(m), kg N uptake increase/kg N supplied by residues), probably due to the continuous flooding of the soil surface, was achieved in this experiment. The low values of internal efficiency of N (IEN, kg total grains/kg total N uptake), ANR(f) (Delta kg N uptake/kg N supplied by fertiliser), and of ANUE(f) (Delta kg grains/kg applied N fertiliser) recorded in the MR+ treatment of Expt 1, suggest that no application of N fertiliser is needed where the soil water conditions allow high recovery of the N supplied by a preceding mungbean crop

Water loss regulation in mature Hevea brasiliensis : effects of intermittent drought in the rainy season and hydraulic regulation

Effects of soil and atmospheric drought on whole-tree transpiration (E-T), leaf water potential (psi(L)) and whole-tree hydraulic conductance (K-T) were investigated in mature rubber trees (Hevea brasiliensis, clone RRIM 600) during the full canopy stage in the rainy season in a drought-prone area of northeast Thailand. Under well-watered soil conditions, transpiration was tightly regulated in response to high evaporative demand, i.e., above reference evapotranspiration (ETo) similar to 2.2 mm day(-1) or maximum vapor pressure deficit similar to 1.8 kPa. When the trees experienced intermittent soil drought E-T decreased sharply when relative extractable water in the top soil was < 0.4. The midday leaf water potential (psi(md)) on sunny days did not change as a function of soil drought and remained stable at approximately -1.95 MPa, i.e., displaying isohydric behavior. The decrease in E-T was mainly due to the change in K-T. K-T remained constant over a wide range of environmental conditions and decreased sharply at low soil water availability. A simple hydraulic model incorporating critical minimum water potential and the response of whole-tree hydraulic conductance to relative extractable water correctly simulated patterns of transpiration over 6 months. We conclude that an explicit and simplified framework of hydraulic limitation hypothesis was sufficient to describe water use regulation of a mature rubber tree stand in water-limited conditions. Given the complexity of constraints in the soil-plant-atmosphere pathway, our results confirm the relevance of this approach to synthesize the overall behavior of trees under drought