We have investigated the effect of puddling on percolation rate, water pressure distribution and quality of percolating water in a flooded rice field on diluvial plateau covered by Kanto loam of volcanic ash soil.'The field is well-drained type. Irrigation period is from May to August. In May, field is flooded by irrigation water and rice is transplanted. In September rice is harvested. After the rice, there is no crop in the field. Soil is usually puddled before transplantation. However in 1988, we did not puddle in order to compare the results measured in 1987 with puddling. We measured :
1) Rainfall, the change of the level of surface water, soil temperature by automatic recorder.
2) Evapotranspiration, daily water requirement in depth, and percolation rate.
3) Water pressures were measured by tensiometers and piezometers in the depth of 10, 20, 30, 40, 60, 80cm.
4) Percolating waters in the depth of 10, 20, 30, 40, 60, 80cm were sampled and the concentration of NO3-N, NH4
-N and EC, pH, ORP were measured.
5) SoiFs component phases and hydraulic conductivities of each layer were measured at each stage. Average percolation rate measured inside in 1987 was 0.6cm/d, but in 1988 the value of inside increased to 4.5 cm/d due to nonpuddling as shown in Fig.1. Pressure distribution of percolating water also changed due to puddling (Fig. 4).
The concentration of NH4 -N in percolating water of 20cm depth increased to 4 mg/1 at the period of basal apply of chemical fertilizer in May, and after that it gradually decreased. In August it became zero. In the subsoil, the value of NH4-N concentration in 1987 was also zero. However in 1988, the concentration increased to about 3 mg/1. It is considered that NH4 flow down due to large percolation rate.
View full abstract