Effects of two-year crop rotation with soybeans [Glycine max] on successively grown paddy rice [Oryza sativa] and wheat [Triticum aestivum]

Organic matter continuous use resulted in differences in field fertility. Soybean were planted in the fields of different fertility levels for two consecutive years for crop rotation to study how successive crops were affected in yield and quality and changes in the soil physics and chemistry characteristics of the fields. 1. Wheat planted in the first cultivation after soybean had longer lengths of the culm, larger numbers of ears, and a heavier weight of the dry straw than wheat following paddy rice. Consequently, the wheat yield after soybeans was greater than wheat yields after paddy rice by more than 100kg/10a regardless of organic matter use conditions. 2. In a high fertility field due to continuous manure applications, wheat planted in the first year after soybeans delivered a higher yield than wheat grown in an artificial manure ward and a straw reduction ward given standard fertilizer applications even when the amount of nitrogen fertilizer application was decreased by 20%. 3. Paddy rice planted in the second cultivation after soybeans had a little longer lengths of the culm than paddy rice following paddy rice, decreasing in yield due to lodging in such high fertility fields as a manure application ward and a manure and straw ward. However, 30% less nitrogen fertilizer applications reduced lodging, increased yield, and, also, suppressed increase in the protein content of the brown rice. Yield also increased in such low fertility fields as the artificial manure ward and the straw reduction ward. 4. Wheat planted in the third cultivation after soybeans had a little shorter lengths of the culm than wheat following paddy rice. Yield also tended to decrease. It was considered necessary to study whether larger amounts of fertilizers should be applied. 5. The growth and yield of paddy rice planted in the fourth cultivation after soybeans were close to those of paddy rice following paddy rice. The effects of the two-year crop rotation with soybeans were judged to have mostly disappeared. 6. The soil physics and chemistry characteristics after soybeans showed no large differences from those after paddy rice in a field of the same organic matter continuous use. Available nitrogen after soybeans measured by an autoclave method or a culture method was close to or even less than available nitrogen after paddy rice.