Evaluation of use of fly ash-gypsum mixture for rice production at different nitrogen rates

Although a silicate fertilizer had been recommended for application at 4-year intervals by the Korean government to enhance rice production, since the quantity was not sufficient, it became necessary to identify an alternative source of cheaper silicate fertilizer. Fly ash, which has a high silicate content, was selected as an alternative. To improve fly ash, that is highly alkaline and has a high boron content, it was mixed with a by-product of gypsum (hereafter, referred to as gypsum) at the 75 : 25 ratio on a weight basis. A field experiment was carried out to evaluate the productivity of rice (Oryza sativa) on a silty clay loam to which 0 and 25 Mg ha(-1) of the fly ash-gypsum mixture had been applied. Under these two conditions, five levels of nitrogen (0, 40, 80, 120, and 160 kg ha(-1)) were set up to compare the rice response. The mixture increased the maximum grain yield by 8%. With increasing N uptake of rice, the mixture could decrease nitrogen application level to about 50 kg N ha(-1) to produce the target yield i.e., the maximum yield in the non-amendment treatment. The use of the mixture increased the uptake of silicate by rice, but did not result in an excessive uptake of heavy metals. The level of available B increased with the mixture application up to 1.5 mg kg(-1) in the surface soil at the initial stage, but did not reach toxicity levels. In conclusion, the mixture was considered to be a good alternative as a soil amendment to restore the nutrient balance in paddy soils and to reduce the nitrogen application rate of rice.