Influence of Drainage on the Growth of Rice Plant in Rice Field : II) Relation between irrigation methods and the growth and yield of rice at various levels of nitrogen fertilization and sowing density

Abstract

The experiment was conducted to evaluate the effect of various irrigation methods on the growth and yield of rice plant in direct sowing cultivation. The field experiment involving three irrigation treatments, two levels of nitrogen fertilization and four levels of sowing density was established in 1962. Because of practical consideration in applying irrigation water, the irrigatin treatments were located in separate block. The nitrogen and sowing treatments were randomized in a split plot design with nitrogen levels as the main plot and sowing density as subplot. The replication was three. The irrigation treatments involed three plots of early irrigation, late irrigation and interval irrigation. In the early irrigation plot, the water had been kept 5 cm deep since early tillering stage of rice plant. The late irrigation plot was kept dry during tillering stage of plant, and irrigation was begun when the plant reached young ear formation stage. The interval irrigation plot was applied 5 cm of water in depth at an interval of 7 days. The nitrogen plots were divided into two, that is 0.6 kg and 1.2 kg-N per are. The levels of sowing density were 10, 25, 50 and 100 hills per m2. The results obtained are as follows. 1) The amounts of NH3-N in soil in the plots of both late irrigation and interval irrigation were smaller than that in the plots of early irrigation. The amounts of NO3-N in soil generally were trace, except the case of late irrigation kept dry through the tillering period of plant. It seems that the decrease of available nitrogen in the late irrigation and interval irrigation plots are caused by the leaching of NO3-N and the denitrification resulting from the alternation of oxidation and reduction of soil by irrigation and rainfall. 2) The treatments of late irrigation and interval irrigation indicated the increased activity of root, as measured by means of α-Naptylamine, over the early irrigation. The root activity in the high density (100 hills/m2) was lower than that in low density (10 hills/m2). The root weights per unit area in the late irrigation and interval irrigation plots were lighter than that in the early irrigation plot. The treatment of high density indicated the increased weight of root. 3) The grain yields in the late irrigation and interval irrigation plots were about 10 % less than that in the early irrigation plot, while there was little difference between the former two. The grain and total weight at harvest generally increased as the sowing density increased. The highest yield of all treatments was obtained in the plot with early irrigation, heavy nitrogen fertilization and high sowing density combined. The yield data obtained were fitted with rectangular hyperbola equation of the general form: y=x/a+bx where y is the estimated grain or total weight at harvest in kg per are, x is sowing density, and a and b is a coefficient, respectively. The values of two coefficients in both the late irrigation anb interval irrigation are higher than that in the early irrigation, and heavy dose of nitrogen decreased the values of coefficients. It is therefore concluded that the early irrigation method with heavier nitrogen fertilization and higher sowing density is effective for getting high yield of rice.