The role of subsoil on the crop production-subsoil edaphology.

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Subsoil, as well as surface soil, has an important role in supplying water and nutrients to crops. Fertilized nutrients easily migrate from surface layr to subsoil by rainfall in the humid regions such as Japan. The objectives of this study are to determine the effects of subsoil on the growth and nitrogen (N) uptake of crops in representative Japanese Andisols with different clay mineralogy, and the effect of controlled availability fertilizer (CAF) of N on the crop growth in soils inhibiting root development to subsoil. Weakly acidic allophanic Andisol and strongly acidic nonallophanic Andisol with 2 : 1 minerals were used in this study. Experimental treatments were surface soil plot where root elongation was limited to surface layr (15 cm depth) and subsoil plot where crop root can develop into subsoil (45 cm thickness). Total dry matter and grain yield of oats and barley in the subsoil plots were greater than those in the surface soil plots in both soils. Nitrogen uptake of both cr … More ops in subsoil plots were 1.7 to 3.3 times as much as in the surface soil plots. Oats absorbed about 70 % and 50 % of total uptake N from allophanic and nonallophanic subsoil, respectively. In the case of barley with sensitivity to Al toxicity, the ratios of N absorbed from subsoil to total uptake in nonallophanic soil (42%) were lower than those in allophanic soil (65%). Barley roots could not extend to deeper subsoil in nonallophanic soil with high toxic Al level due to presence of 2 : 1 minerals and low pH.Application of CAF-N increased growth and N uptake of crops in each surface soil plots.It was concluded that subsoil playd a signiflcant role in supplying fertilizer and soil-mineralized N to crops and in increasing the crop growth and the grain yield under humid climate of Japan. The contribution of subsoil to the crop growth was affected by soil clay mineralogy relating soil acidity. Application of CAF-N was effective for improving the crop growth and the grain yield in soils inhibiting root elongation into subsoil.Sandy paddy soil showed lower holding capacity of ammonium ion compared to clayy soil. Rice yield and utilization efficiency of fertilizer nitrogen was lower in sandy soil than clayy soil. Productivity of sandy paddy soil depended more strongly on nitrogen supplying ability of subsoil than clayy paddy soil. Less