Effects of continuous application of farmyard manure on physico-chemical characteristics of the soil and the root system, nutrient absorption, yield of rice cultured with minimal agricultural chemicals

From 1991 to 2000, a rice cultivar Koshihikari was grown in the field, with either farmyard manure or chemical fertilizer continuously applied using a minimum of agricultural chemicals. After six years of continuous application of the manure, grain yield was the highest (63.7 kg/a) in the field supplemented with a small amount of chemical fertilizer. Continuous application of the farmyard manure increased the pH, available phosphorus content and gas phase of the soil, and also the uptake of phosphorus and potassium by plants although, it slightly decreased the nitrogen absorption. Extension of the period of continuous application of the farmyard manure beyond five years did not change the soil properties or nutrient absorption by plants, but decreased the grain yield. Continuous application of 200 kg/a manure is inadequate for raising the soil fertility, but it may be possible to increase the yield by applying nitrogen at appropriate stages, because the continuous application of farmyard manure improved the soil conditions and increased the uptake of phosphorus and potassium by plants.