Abstract
Extreme sterillity so-called "straight head"of rice often occurs in farmers fields under dry weather in Japan. This straight head seems to be a sort of drought damages, though the mechanism of its occurrences is still unknown. In order to find out the mechanism of the occurrence and to establish the preventive culture method against it, the writer has conducted a series of experiments from several years ago. According to the results already obtained in these experiments the occurrence of straight head appeared to be closely related to the following two soil properties; the manuring effect of soil by drying and the amount of active iron in soil. Therefore in the present experiment these two soil properties and other physical soil properties were measured with many kinds of soils obtained from various districts where the straight head often occurs or not. Results of the experimene are as follows:-- (1) Close relationship was not found between the occurrence of straight head and the following two soil properies; the content ot sand or clay and the water holding capacity. (2) The soil of the fields where straight head often occurs showed either high manuring effect of soil drying or deficiency or active iron. And if A denotes the value of the manuring effect of soil drying as experessed by the ammonium nitrogen in mgr per 100gr dry soil and B denotes the amounts of active iron expressed by free Fe2O3 in mgr per 1gr dry soil, close relationship was found between the occurrence of straight head and the value of A/√B of the soil. In general, the value of where the straght head often occurs was greater than 5 and that of the soil A/√B of the soil where the straight head often occurs greater than 5 and that of the soil where the straighr head never occurs was less than 3. (3) The fact that straight head often occurs where the soil is rich in humus or in the fields under stagnant water all the year round and it seldom occurs in the ordinary well drained fields or in the field used to be exposed to drought condition seems to be responsible for the fact that the former fields show higher manuring effect of soil by drying than the latter. (4) From these results, both the manuring effect of soil drying and the amount of active iron in soil are considered to be essential among factors which determine whether straight head actually occurs or not under drought condition.
