Field Water Regimes
The unique physical and chemical properties that influence the field water regime of volcanic ash soils are due mainly to the presence of allophane. Many measurements have been made of water movement in volcanic ash soils, especially in Kanto loam, a typical volcanic ash soil in Japan. In Japan, there are many clayey and swampy paddy fields where it is impossible to use combine harvesters and tractors due to low bearing capacity of the soils. Low bearing capacity results in decreased trafficability. Drainage of a clayey paddy field is delayed by the large water holding capacity and low hydraulic conductivity. The hydraulic conductivity of clayey soil is less than 10-5 cm/sec, although once the soil is dried, water flows easily through cracks. Puddling of paddy fields is important to make soil soft for transplanting and impervious for percolation control. The soil surface should remain flat to keep a uniform water depth and to drain surface water rapidly at harvest time.