Studies on the Straight Head of the Rice Plant (Preliminary report) : II Relation between the Properties of Soil and the Occurrence of Straight Head

Abstract

In order to find out the relation between the soil properties and the occurrence of straight head, we have determined the Eh values of the soil and the amount of NH₃-N produced during one or two weeks immediately after the water is given to the dried soil. In the case of two soils, rich in humus, taken from the fields where straight head often occurs, a large amount of NH₃-N was produced when the soil was dried to below 20 per cent in moisture content and then flooded. When these soils were brought to air-dried condition, the Eh value of them was lowered extremely and large amounts of NH₃-N were produced by supplying such a small quantity of water as enough to raise the soil moisture content to 40 per cent. While, in the case of other one, deficient in free iron, taken from the field where straight head seldom occurs, the Eh value of the soil descended very slowly and only a small amount of NH₃-N was formed when the dried soil was flooded. From these results it is suggested that both the extreme lowering of the Eh values of the soil and the production of a large amount of nitrogen caused by supplying water to the dried soil, have an intimate relationship to the occurrence of straight head.