Growth Inhibition of Rice by Water Drainage during Fallow at IRRI

In a rice cultivation experiment at the International Rice Research Institute (IRRI), water drainage during the fallow period inhibited the growth of paddy rice, especially during the early stage of subsequent cultivation. This symptom became more prominent when rice straw was not applied, and when the amount of fertilizer applied was small. The application of a mixture of nitrogen, phosphorus, and potassium fertilizers reduced this inhibition. The amounts of phosphorus in the growth-inhibited rice and soil were smaller than those without growth inhibition. The findings suggest that one of the factors that inhibited rice growth was phosphorus deficiency. The inhibition was associated with high soil pH and low Fe(II) content in the problem soil. There was a negative correlation between soil pH and Fe(II) content. Water drainage during fallow decreased the amount of Fe(II) in the soil because Fe(II) was oxidized to Fe(III) under oxidative conditions. During the subsequent cultivation season, the amount of Fe(II) in the soil was low and the soil pH was high. The application of rice straw improved rice growth. The phosphorus deficiency of growth-inhibited rice was associated with a small amount of available phosphorus in soil presumably because of the high soil pH and the adsorptopn of phosphorus to Fe(III) oxides in the soil.