The diminutive mosquito fern Azolla | Sunday Observer

The diminutive mosquito fern Azolla

14 October, 2018

The adverse effects of chemical fertilisers on the environment and its related impacts on human health has geared and steered public concern towards safe and healthy food consumption. This phenomenon has resulted in increasing attention towards organic farming. Recently, organic farmers in the Northern Province- from Mullaitivu, Mannar and Jaffna – have shown a great interest in growing a fern known as Azolla which belongs to the Saliviniaceae family. This tiny aquatic plant can be a great weapon in fighting climate change as it cuts down the need for and the use of chemical fertilisers. The fern in scattered throughout the world, especially in tropical and temperate countries. In countries like China, India, Vietnam, and Sri Lanka it is grown as a source of nitrogen in the paddy fields and also as animal feed. As this minuscule plant thrives naturally in wetlands, ponds and reservoirs, it is very easy to cultivate it in farms. Even though there are a number of Azolla species, a few selected species like Azollapinnata, Azollacaroliniana and Azollafiliculoides are the popular ones in agriculture. Azolla has survived for about 800,000 years on earth. According to evolutionary history, around fifty million years ago when the earth’s temperature rapidly rose, these ferns played a vital role in bringing down the temperature and cooling the earth by capturing atmospheric carbon dioxide.

These aquatic ferns live in a symbiotic relationship with the blue-green algae Anabaena azollae. The algae have the ability to fix the atmospheric nitrogen which facilitates the availability of nitrogen for the fern in the form of Ammonium.In return, the ferns provide a safe environmentfor the algae. Azolla can fix about 25kg of nitrogen per hectare:for this extreme level of nitrogen fixation it is often called ‘¢Super Plant’.

There are numerous ecological and commercial benefits from Azolla. In agriculture, the fern is used as green manure. When inter - cropped in paddy fields, it helps to enhance the soil fertility and cut down the cost of chemical nitrogen fertilisers. Moreover, it increases the amount of available potassium and ultimately contributes towards a better crop yield.

The fern is a rich source of protein for cattle, goats, poultry, swine, rabbits and fish. Azolla feed contains approximately 25-35% of protein, 67mg/100g calcium, and 7.31mg/100g iron. It also contains high amounts of essential amino acids minerals like Calcium, Phosphorous, Potassium, Iron, Copper, and Magnesium which are necessary in the diet of lactating cows. Incorporating Azolla into animal feed gives a significant increase of about 15-20% in the milk production. In poultry industry, Azolla fed fowls show a considerable gain in weight and an increased egg production.

The name ‘¢mosquito fern’ comes from the fact that when these minute plants float on the surface of a water body, they block the water-air interface and prevent water-air interaction which is essential for mosquito breeding. A floating layer of Azolla can control up to 50% of aquatic weed growth in tanks, reservoirs and canals. Azolla concentrates heavy metals like Copper, Cadmium, Chromium and Lead in the water body and purifies the water. In water treatment plants, polluted water is flushed through Azolla tanks to remove toxic substances.

In Asia, there are two common ways of cultivating Azolla.One is growing the fern as a sole crop in pits for mass production targeting the green manure and animal feed markets. The second method is inter-cropping with paddy in the early stages of the crop. A temperature range of 20oC to 32oC and an optimum pH of 5.5-7 are ideal for the growth of the fern. Partial shade is required in the pit method.In the paddy-fern inter-cropping, however, the shade given by the rice crop is sufficient for the fern’s survival.

Pit cultivation of Azolla

Pit farming of Azolla is easy and cheap. A pit size of 2m x 2m x 0.2m preferably under the shade of a tree is required for cultivation. After digging and leveling the pit, a Silpaulin sheet is laid covering the pit to avoid water leakage. Sieved fertile soil is spread over the sheet and forms the base for the cultivation.

A mixture of cow dung slurry, Super Phospate, and water is then poured into the pit. Water is filled up as the final layer and about 500 grams of healthy pure Azolla culture is inoculated in the water. The ferns show rapid multiplication day by day. Azolla can be harvested within 15 days of cultivation. With adequate nutrients approximately 500-600g of Azolla can be harvested daily.

To improve the mineral content of the ferns a micro - nutrient mixture of Magnesium, Iron, Copper and Sulphur is added in the water. Periodical inspections and maintenance are needed to remove overcrowded Azolla biomass, identify nutrition deficiency, judge the need of re - inoculation of ferns and check on pest and disease attacks in order to sustain the production.

This year, the genome of Azolla has been sequenced by scientists and published in the prestigious science journal Nature Plants.

One of the authors of the paper Fay-Wei Li, a plant evolutionary biologist, says “We can now research its properties as a sustainable fertiliser and perhaps gather carbon dioxide from the atmosphere.” Sri Lanka can consider these scopes and focus on research related to Azolla as a potential fertiliser in organic farming.


The author is a researcher in Biology and Agriculture based in Jaffna.