The Role of Nitrogen Fixing Blue-Green Algae in Rice Soils

The role of nitrogen fixing blue-green algae in rice field soils is particularly important in an expanded programme of utilization of biological nitrogen fixation. In this situation, their ecological significance may be immense. Many of the tropical rice fields receive no or limited artificial nitrogen fertilizers. When one appreciates that over half the world's population lives on rice as a staple diet, and there is a shortage of fertilizers, the importance of these autotrophic biological agencies can readily be appreciated. Where nitrogen fertilizers are not used for various reasons, algal application (1 kg/h) has been found to give a benefit of 20–30 kg N/h. Where nitrogen fertilizers are added, a saving of about 30% in fertilizer is possible by algal supplementation without affecting the yield. Consecutive application of algae for 3–4 seasons sustains the high yield in the subsequent years at reduced levels of nitrogen fertilization. Even high levels of nitrogen fertilization can be advantageously supplemented by algae to get a higher output per unit input. An input of Rs. 5–10/ha (cost of algae) results in an additional output of about Rs. 400–600/ha. Definite proof has been obtained with ¹⁵N for the transfer of fixed nitrogen from algae to rice plants. Besides nitrogen algae also contribute biologically potent substances to the crop. Recommended pest control measures do not interfere with the activity of the algae. Biological nitrogen fixation by algae has been found to proceed even at reduced moisture levels (30%).