Abstract
When paddy rice plants were irrigated with low concentration of nitric acid (18-45 ppm), the yiedld increased mainly due to the increase in number of tillering. When the concentration of nitric acid was slightly increased (100 ppm), the tendency of the plant to be lodgea increased. These results indicated thau up to this concentration of nitric acid, amount of nitrogen was the main factor for affecting growth and yield of rice. Howevr, when nitric acid concentration was further increased (300-675 ppm), toxicty of H ion became the main effect. In the soil with pH value lower than 4.5, growth of rice plant was inhibited, and in that of pH lower than 3.5, yield was decreased with the increasing number of degenerated spikeles caused by the injury at time of meiosis, and in that of pH as low as 3.0, yield was severely decreased due to the withering and death which occurred soon after heading. Nitrite was only slightly injurious to the rice. Raw straw application inhibited the growth of early stage. However at the later stage, the difference in Eh between the soil of straw-applied and non-straw-applied plots became small and as the result, number of spikelets per panicle increased and that affected favourably on the yield. Raw straw application reduced the injurious action of irrigation with higher concentration of nitric acid to the early growing stage of rice. It was likely that the buffering action of straw, which was favourable for growth, surpassed the harmful effect of straw to lower Eh of soil. When low concentration of cyanide, sulfulic acid and raw straw were added to the irrigating water of soil of normally fertilized pot, the additional yield was culculated as the difference between positive yield by favourable materials and the negative yield by unfavourable ones. The actual yield accurately coincided with the culculated value when nitric acid was not present in irrigating water. But when nitric acid was present, some discrepancies between the actual yield and calculated value were observed. The rooting of rice seedlings was promoted by the irrigating with very low concentration of nitric acid (18 ppm). It was inhibited by the concentration above 45 ppm. This inhibition was also reduced by the application of raw atraw.
