Permeability Reduction Caused by Surface Compaction in Large Dry Paddy Fields under Direct-seeding Rice Cultivation

Abstract

Experiments assessing direct-seeding rice cultivation using plowing and compaction were conducted in large Muck soil paddy fields (soil texture of plow lyre: lite clay) of 3 ha scale to establish a method of reducing paddy field water requirements of rice-cultivation methods using compaction. A laboratory compaction experiment based on the soil hardness index clarified the soil hardness necessary to reduce the water requirement. Results show that saturated hydraulic conductivity decreased to 2.3×10^-5 cm s^-1 by compacting soil to 20 mm of soil hardness with more than 34% soil water content. After four repetitions of whole field surface compaction using a Cambridge-roller in the field experiment, the soil hardness at 5 cm under the field surface increased to about 20 mm, as measured using a soil hardness meter. Results show that saturated hydraulic conductivity at 5 cm under the field surface decreased to the targeted level of 2.3×10^-5 cm s^-1. The water requirement in experiment fields compacted with the Cambridge-roller decreased to about 10 mm d^-1, which is lower than our targeted value of 20 mm d^-1. Investigation of the water level at the paddy surface and in the subsurface drain pipe revealed that the paddy surface water level in compacted paddy field was maintained by the compacted soil layer at 5 cm depth under the field surface because of a discontinuity of water levels between the paddy surface and the subsurface drain pipe. Results show that field surface compaction with index value of soil hardness reduces the water requirements of a large paddy field.