This paper investigated uplandization and mechanical working technology of an upland field that was converted from a heavy clay paddy. We set up an open ditch, a mole drain, and a sub-underdrain made from rice husks in an upland field that was converted from a heavy clay paddy. These drainage works appeared to be effective for improving soil physical conditions such as the uplandization index and pulverization ratio. They also increased the yield of vegetables. The 7 cm-long sub-underdrain,

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... rising rice husks, remained for four years after construction. The sub-underdrain maintained its drainage effects ever after circa 40% for decomposition of the rice husks through immersion in 2% NaOH.