Effect of nitrogen, phosphorus, and cropping method on azolla productivity

The effect of N, P, and method of azolla culture on azolla productivity was studied in irrigated lowlands during 1983 wet season (Jun-Oct). Soil was sandy clay with pH 6.7, 286, 27, 341 kg available NPK/ha. The experiment was in a split-plot design with three replications. Main plots were combinations of 0, 50 and 100 kg N/ha and 0, 11, and 22 kg P/ha; subplots were three methods of growing azolla with transplanted rice: monocropping, intercropping and dual cropping. Jaya rice was transplanted at 20-x 10-cm spacing 18 Jul. Fresh Azolla pinnata R. Br. at 3 t/ha was inoculated to monocrop and dual crop plots 20 d before transplanting rice and to intercrop plots 1 d after transplanting. Azolla was manually incorporated two times before transplanting in monocrop plots, two times after tranplanting in intercrop plots, and two times each before and after transplanting in dual-crop plots. N as urea was apllied in two equal splits, at transplanting and 30 d after transplanting; P as single superphosphate was applied basally at 42 kg K/ha to all plots. Application of 50 kg N/ha had no adverse effect on yield of fresh azolla, but application of 100 kg N/ha significantly reduced azolla yield in the dual-crop plots. In dual cropping, remnant azolla that had been mixed with fertilizer N during soil incorporation was retained as seed for further multiplication. In monocrop and intercrop plots, fresh azolla was inoculated. Adverse effects of more than 50 kg N/ha would be reduced by minimizing mixing of fertilizer N and azolla or by seeding fresh azolla for further multiplication. The poor response of azolla to applied P may be due to the high available P in the soil. Monocropping for 20 d and dual cropping for 50 d resulted in higher yields of fresh azolla, contributing more N/day, dry matter content, and K content of dry matter. Monocrop azolla did not experience any competition with rice. Intercrop azolla suffered considerable competition with rice for all growth factors, in particular, light and nutrients.