Based on the logistic theory on the plant growth, the analysis was made on the mechanism of soil moisture on the growth of upland rice plant. Seeds were sown on June 10, 1968 in the plastic Wagner pots (1/2000 a) and after 24 days the soil moisture was regulated to about 10 different levels. Treatments were continued till the harvest time in glass house, of which air temperature were exceedingly high (about 2-8°C higher than those of outdoor) throughout growing season. Average weight of dry matter of upland rice plant on the 37th, 70th and 119th day from sowing were used for the present analysis.
The results were summarized as follows;
(1) The effect of soil moisture on the plant growth were the optimum factor in the logistic theory (Fig. 3 and 4).
(2) According to the logistic theory, the effect of available soil moisture saticefied the following optimum factor equation,
1/w=Af+A′/f+B
where, w is the total dry weight per plant at a certain time (t) and f is the amount of a factor supplied (soil moisture), A, A′ and B are the functions of time as defined by Eq. (2).
In this experiment, the fitness of the equation with the experimental results was satisfactory (Fig. 5).
(3) Soil moisture must be expressed by the available moisture (% in dry base) which was subtracted by the moisture of the initial wilting point for 37 and 70th day, and of the permanent wilting point for 119th.
(4) The optimum soil moisture changed from lower to higher with plant growth.
(5) Soil moisture affected mainly the growth of top parts of plant throughout whole growing season, and in the leaf area at middle stage.