Analysis of Yield-Determining Process and Its Application to Yield-Rrediction and Culture Improvement of Lowland Rice : LXXIX. An experiment of maximizing yield based on ideal plant types by using a circulating irrigation method applied with nitrate nitrogen

Abstract

In the previous papers (Report nos. 74, 69, 67), Matsushima et al. stressed the necessity of the ideal plant types for maximizing grain-yields of rice. In the present paper the authors further studied a method no obtain high yields by making the plant into ideal plant types. Two conditions are subjected to making the ideal plant type, i.e. (1) ensuring the number of spikelets per unit area corresponding to a target yield by the time of 69 leaf-number index (which is the time just before the neck-node differentiation and roughly corresponding to 43 days before heading) and (2) restricting nitrogen supply during the period from 69 to 93 in the leaf-number index (the period is approximately corresponding to the duration from 43 days to 20 days before heading) in which the plant type is definitely predetermined. For ensuring the number of spikelets correspondig to high yields, much ammonium nitrogen must usually be supplied, but when much ammonium nitrogen has been used in a paddy field, the control of nitrogen absorption by the plant during the critical period in which the plant type is definitely predetermined is hardly possible on account of the ammonium nitrogen having been adsorbed by the soil. On the othr hand, when nitrate nitrogen is applied to a paddy field, it is by no means adsorbed by the soil, but it is always dissolved in irrigation water so far as the irrigation water remains in the field. In this occasion, if one wants to stop the supply of nitrogen, one can easily do so at any time by changing the irrigation water which remains in the paddy field for new water which contains no nitrogen. Getting a hint from a circulating irrigation method in which identical water is repeatedly irrigated by a pump and which has long been practised economically in Toyama Prefecture, the authors devised a method by which one can easily take off the nitrogen from a paddy field at any time as follows. Putting much nitrate nitrogen into water in a paddy field and making water circulate by pumping up ditch-water which comes out through underdrains of the paddy field, the authors could always supply much nitrate nitrogen to rice plants for encouraging their tillering with little leaching of nitrogen (some amount of nitrate nitrogen is, however, lost by denitrification). By changing the irrigation water for new water at the time of 69 leaf-number index (approximately 43 days before heading), they succeeded in stopping the supply of nitrogen to the plant and thereafter they irrigated the plant with nitrogen-free water until the time of 93 leaf-number index (approximately 20 days before heading). At the time of 93 leaf-number index and again at full heading time they top-dressed with ammonium nitrate to encourage the carbon assimilation of the plant. By this method they succeeded in, (1) obtaining the necessary number of tillers per unit area corresponding to a target yield by the time of 69 leaf-number index, and (2) decreasing the nitrogen content of the plant, paling of leaf-colour, decreasing the dry weight increment, of the plant at the critical period, decreasing the length of leaves, culms and internodes, and as a result, (3) raising the grain-yield by 5-27% as compared with the control plot respectively and obtaining 7.1 tons of brown rice per hectare as the highest yield which has never been obtained by any other means so far in their experimental paddy fields. By the way, the amount of water consumed for evapo-transpiration in the circulating irrigation method was less by 14-23% than the ordinary irrigation (flooded) method, and a tentative proposition was made on the application of nitrogen for ensuring the number of spikelets per unit area to maximize the yield of rice.