Azolla: a simple and sustainable biofertiliser

It is the ultimate fertiliser for your garden. It is free, only needs a bucket of water, removes greenhouse gases, fixes nitrogen, and enriches your soil. Sheryn Dean describes a simple and sustainable biofertiliser.
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Azolla can accumulate up to 2–4 kilograms of nitrogen per hectare per day or 1.1 tonnes of nitrogen per hectare per year. That’s almost three times the performance of legumes such as clover at around 400 kg of nitrogen per hectare per year.

It is like a seaweed, but it is not. Azolla is an aquatic fern that grows fast. Really fast. It is the world’s fastest growing plant and able to double its biomass in as little as 1.9 days. As it grows, it absorbs carbon and fixes nitrogen from the air and removes phosphorus from the water – turning pollution into a nitrogen-rich biofertiliser or mulch for your garden. Something the Chinese have known for thousands of years as they use azolla extensively in rice fields.

Azolla will grow in any still or slow-flowing water. I have seen it growing in buckets and baths in urban gardens, irrigation dams on permaculture blocks, in cattle troughs, and had it in my aquaponics experiment (which, incidentally, failed, but it will grow in aquariums). Azolla prefers full sun and the limiting factor to growth is usually the amount of phosphorous in the water until overcrowding limits the light. It prefers temperatures over 20oC, doesn’t like extended freezing but will withstand a light frost, and won’t tolerate salinity.

To start, all you need is some azolla. Simply grab a handful next time you see some, keep it moist (preferably with the water it grew in as it has symbiotic bacteria), and transfer to your container of phosphorous-rich or dirty water. You can even buy a starter bag of azolla online if you need to.

As it multiplies, scoop handfuls out to use as mulch or add to the compost so there is always space for it to keep growing. Studies have shown the use of azolla can increase production by up to 30 per cent, suppresses weed growth, reduce evaporation, and solubilise zinc, magnesium and iron in soils.

Azolla filiculoides and A. pinata are both introduced species common in the upper North Island (no it is not a pest plant but it can be invasive in wetlands). New Zealand has an indigenous kārearea red azolla (A. rubra) also known as duckweed.

Azolla is an important food source for tadpoles of the Australian frogs Litoria raniformis and L. aurea. (Fun fact: New Zealand frogs don’t croak.) It also provides food for various insects, birds and aquatic life but conversely a thick mat can prevent mosquito larvae from reaching the surface, effectively drowning them.

Rich in protein (25-35%), essential amino acids, vitamins, and minerals (high calcium and iron values), studies describe feeding azolla to dairy cattle, pigs, ducks, chickens, and fish, but no research has been done on its benefit to humans.

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