Growth of Azolla in paddy field as affected by phosphorus fertilizer

Abstract The aquatic fern Azolla can grow without combined nitrogen due to the N2-fixing symbiotic cyanobacteria Anabaena azollae. The biomass production and nitrogen accumulation of Azolla pinnata in Philippine paddy fields were examined. Twenty-two continuous croppings of Azolla in 1 year yielded 8 t dry matter/ha and 465 kg N/ha. The growth rate and N2-fixing activity fluctuated between the seasons. Different methods of applying phosphorus fertilizer were tested to increase the Azolla yield. Split applications of 2.5 kg P2O5/ha as superphosphate applied 6 times (total, 15 kg P2O5/ha) increased the Azolla yield more than basal applications. This yield was equal to that obtained at 30 kg P2O5/ha basal or split applications. The highest apparent phosphorus absorption efficiency was 23%. Another experiment suggested that split applications of about 1 kg P2O5/ha were sufficient if the phosphate fertilizer was applied every 2 to 4 days. From these experiments, we calculated that Azolla will produce 1.9 kg ad...