Blue-Green Algae (BGA) and Azolla

Yesss…. ! absolutely. Both Blue-green algae (BGA) and Azolla are well-recognized and widely accepted biofertilizers, and they have been used in agriculture — particularly in rice cultivation — for decades. However, it is worth understanding what each of them is and why they qualify as biofertilizers.

Nature has always been a better chemist than humans, and nowhere is this more obvious than in the story of Blue-green algae and Azolla. While farmers around the world were busy emptying bags of synthetic nitrogen fertilizer into their rice fields, these two humble organisms had been doing the same job for free, floating quietly on the surface of flooded paddies, asking for nothing but water and sunlight. The fact that it took humans this long to fully appreciate them says more about us than it does about them.

Blue-green algae, despite what their name suggests, are not actually algae at all. Calling them algae is a bit like calling paneer a vegetable just because it sits on the same plate as one — the resemblance is there but the identity is entirely different. They are in fact photosynthetic bacteria called cyanobacteria, which means they possess a truly remarkable double talent. On one hand they capture sunlight and use it to produce their own food exactly the way plants do. On the other hand they can pull nitrogen gas directly from the atmosphere and convert it into forms that plants can absorb, exactly the way nitrogen-fixing bacteria do. This combination of abilities makes BGA almost uniquely self-sufficient among all soil-enriching organisms.

The BGA species most commonly used as biofertilizers include Nostoc, Anabaena, Tolypothrix, and Aulosira. The strains that have been most extensively researched, developed, and recommended for use are Anabaena variabilis, Nostoc muscorum, Tolypothrix tenuis, and Aulosira fertilissima. Among these, Aulosira fertilissima deserves special mention because it is considered one of the most productive nitrogen fixers in the entire BGA group and is found naturally and abundantly in the waterlogged rice fields of India, making it exceptionally well suited to Indian paddy cultivation conditions.

In practice, BGA is applied to flooded rice fields where the organisms grow freely on the surface of the standing water. As they grow, they continuously fix atmospheric nitrogen and accumulate it within their cells. When they eventually die and decompose, this nitrogen is released into the soil water and becomes directly available to the rice crop. Over a single crop season, a well-established BGA population can contribute a meaningful and consistent supply of nitrogen to the field at absolutely no chemical cost.

Azolla is an entirely different organism but ends up in the same conversation because it does a very similar job in a very similar setting. It is a small, fast-growing water fern that floats on the surface of waterlogged fields and looks almost like a green carpet spreading across the water. By itself, Azolla has no ability to fix nitrogen whatsoever. Its secret lies in a tenant it carries inside the tiny cavities of its leaves — a cyanobacterium called Anabaena azollae that lives permanently within the fern’s leaf tissue. This is one of nature’s most charming arrangements. The fern gives Anabaena a safe, sheltered home along with a steady supply of nutrients, and in return Anabaena fixes atmospheric nitrogen and shares it generously with the fern and ultimately with the soil beneath it. It is the kind of mutually beneficial living arrangement that most roommates can only dream about.

Because Azolla grows extremely rapidly under warm and waterlogged conditions, it can be cultivated quickly in rice fields, allowed to multiply across the water surface, and then physically incorporated into the soil just before rice transplanting. As the incorporated Azolla decomposes, it releases its accumulated nitrogen directly into the root zone of the rice crop. Azolla can also be grown simultaneously alongside the standing rice crop as a green mulch on the water surface, continuously fixing nitrogen throughout the growing season.

Together, BGA and Azolla represent one of the most elegant and time-tested natural solutions to nitrogen nutrition in rice farming. They are locally available, inexpensive to produce, easy to apply, and have been enriching Indian rice soils long before the first bag of urea ever arrived on the scene.