Azolla, the nitrogen-fixing water fern: grow it in a tub, feed your vegetables
Azolla hosts a cyanobacterium that fixes nitrogen; dried, it is about 4.6% N. Grow it 5-10 cm deep, scoop out half when the tank is full, and mix it into potting soil.
- Azolla is a floating fern whose leaf cavities house the cyanobacterium Anabaena azollae, which fixes nitrogen from the air; the two live in partnership.
- Thailand's Department of Agriculture measured dried azolla at 4.58% nitrogen and 5.08% potassium, and 15 kg of fresh azolla dries down to 1 kg.
- It grows best in water 5-10 cm deep, at 25-32°C, in about half shade, at pH 5.5-6.5; when the tank is full, scoop out half.
- For pots, mix fresh azolla 1:1 with potting medium, or use 20-40 g of dried azolla per kg of soil.
Azolla (แหนแดง) is a small water fern that floats on the surface, and inside its leaves lives a cyanobacterium that fixes nitrogen from the air. The result is a plant rich in nitrogen that never needed nitrogen fertiliser. Thailand’s Department of Agriculture (DOA) measured dried azolla at about 4.6% nitrogen, and a home grower can raise it in a small tank or tub and scoop it out as fertiliser.
This lesson summarises the DOA’s trials: how to grow it in a 1 × 2 m tank, how much to take out each time for the biggest harvest, and the rate that grew the best vegetables in pots.
A fern and a bacterium: a nitrogen team
On the back of each azolla leaf is a small cavity where a blue-green alga, the cyanobacterium Anabaena azollae, lives in partnership with the fern, much like rhizobia in legume root nodules. The bacterium turns nitrogen gas from the air into ammonium for the fern; the fern gives it shelter and food.
The DOA says the strain it recommends yields up to 10 times more than local azolla and, in a rice field, covers the water at about 3 tonnes fresh per rai (1,600 m²) within 30 days.
What is in it
| Nutrient (dried azolla) | DOA analysis |
|---|---|
| Nitrogen | 4.58% |
| Phosphorus | 0.64% |
| Potassium | 5.08% |
| Calcium | 2.59% |
| Magnesium | 0.39% |
| Carbon to nitrogen ratio (C/N) | 9.95 |
A DOA leaflet gives about 5% nitrogen, and a Land Development Station table gives 3.30%, so the figure sits somewhere around 3.3-5% of dry weight.
A C/N ratio this low means it breaks down fast. In soil with fresh azolla, nitrogen came out mostly as ammonium during the first 1-35 days. Dried azolla released more phosphorus and potassium than fresh, and kept releasing them for about 150 days.
Growing it in a tank or tub
Good conditions, from DOA documents:
- Water 5-10 cm deep, never more than 1 m
- 25-32°C
- About 50% light, that is, dappled shade
- Water pH 5.5-6.5, not above 8
- On sandy soil, add a little phosphorus and organic fertiliser
The DOA’s starter tank in a concrete ring: put in about 10 cm of paddy soil, add 1 kg of manure, fill with water to about 10 cm above the soil, and scatter 50 g of starter azolla over it. When it covers the surface, scoop some out to use or to start another tank.
Watch for caterpillars and beetles that eat azolla. Its tissue is delicate, so the DOA advises always keeping a separate starter tank. Standing water in small tanks also needs checking for mosquito larvae.
How much to scoop out for the biggest harvest
The DOA tested a 1 × 2 m tank, scooping out azolla every time it filled the surface, over three periods of the year of about 3 months each.
| Removed each time | Harvests in ~3 months | Average per harvest (g fresh) | Average total (g fresh per 2 m²) |
|---|---|---|---|
| 25% of the tank | 13 | about 1,100-1,230 | 15,317 |
| 50% of the tank | 9 | about 2,160-2,520 | 21,573 |
| 75% of the tank | 5 | about 3,060-3,310 | 15,997 |
Taking half gave the most, about 41% more than taking a quarter. Leave too little and it regrows slowly, because azolla likes to grow in a crowd; leave too much and it gets so packed that growth slows.
When the tank is full, scoop out half and leave half to regrow. That gives the largest total harvest.
Worked example: what a 1 × 2 m tank gives your vegetables
- Fresh harvest: scooping half each time, about 21.6 kg in about 3 months.
- Dry weight: 15 kg fresh = 1 kg dry, so 21.6 ÷ 15 = about 1.44 kg
- Nitrogen: 1.44 kg × 4.58% = about 66 g
- Potting soil: kale grew best at 30-40 g per kg of soil, so 1,440 ÷ 40 to 1,440 ÷ 30 = about 36-48 kg of soil, several vegetable pots’ worth
- In a bed: the field trial rate was 0.5 kg dry per m², so 1.44 ÷ 0.5 = about 2.9 m²
How to use it on vegetables
DOA documents give these rates:
| Form | Rate |
|---|---|
| Fresh azolla | mixed 1:1 with potting medium |
| Azolla sun-dried for 2 days | 20-30 g per kg of potting medium (the kale pot trial did best at 30-40 g) |
| Vegetable bed | 0.5 kg dried per m² |
In a kale field trial in Chaiyaphum on low-organic-matter soil, dried azolla alone gave 1,236 kg per rai, about 39% more than the full recommended rate of chemical fertiliser (888 kg), and azolla raised soil organic matter more than chemical fertiliser alone.
It pairs well with compost; see composting kitchen scraps. If you have a bed sitting empty between crops, sunn hemp green manure is another option.
Common mistakes
- Water too deep, or full sun all day. Azolla likes shallow water, 5-10 cm, and dappled shade.
- Taking too little and letting it pack the tank. Growth slows. Scoop out half whenever it is full.
- Too much in a pot. Above 40 g dry per kg of soil the plants did not grow better, and the soil tended to turn slightly more acidic.
- No backup tank. One attack of caterpillars or beetles can clear a tank. Always keep starter azolla in a second one.
Sources
- Using azolla to raise soil fertility in crop production (Thai) — Department of Agriculture
- Azolla, the wonder water plant, leaflet (Thai) — Lampang Agricultural Research and Development Centre, Department of Agriculture
- Building a fertiliser factory in the field, nutrient table for green manures and azolla (Thai) — Suphan Buri Land Development Station
FAQ
How is azolla different from ordinary green duckweed?
Azolla is a tiny water fern with overlapping scale-like leaves, and its leaf cavities hold a cyanobacterium that fixes nitrogen. Ordinary duckweed is a different group, tiny flowering plants, with no nitrogen-fixing partner of this kind.
Where do I get azolla to start with?
Thailand's Department of Agriculture has selected and recommends higher-yielding strains than local ones. Contact the department's agricultural research and development centre in a province near you. A small starter tank needs only about 50 g.
Is more azolla always better?
No. In the department's pot trial with Chinese kale, plants grew best at 30-40 g of dried azolla per kg of soil. Less or more gave smaller plants, and high rates tended to make the soil slightly more acidic.
General information, not medical advice. If you are pregnant, take regular medication, or have a liver or kidney condition, ask a doctor or pharmacist before taking herbs beyond normal food amounts.