'Without Azolla, there is no rice'

Cultivating 60 mau of Azolla

In 2020, Mr Nguyen Duc Du (born in 1975), from Dong Tam village, A Sao commune, Hung Yen province, collected individual duckweed fronds from ponds and fields in his area and released them into fish ponds as fish feed. In a short time, the duckweed covered the entire surface of the pond, and he then spread it in the rice fields after the summer harvest.

The duckweed variety he cultivates is a native strain from An Dong (formerly Quynh Phu), Thai Binh province.

People initially considered Mr. Du because he grew duckweed only to harvest it again afterward. One of his biggest challenges in scaling up the model was that no one was willing to lend him their rice fields. Farmers were worried that duckweed would deplete nutrients and damage their land. It took a long time for them to personally see that the fields Mr. Du had borrowed for duckweed cultivation, after being returned and replanted with rice, still produced good yields.

From the initial 2-3 sao (local measurement, 1 sao = 360 m²), Mr. Du gradually expanded the duckweed-growing area to 60 mau (1 mau = 10 sao) after a pharmaceutical company approached him to purchase dried duckweed as medicinal material. The 60 mau consisted of land borrowed from local farmers after the autumn rice harvest.

After harvesting all the duckweed for drying and selling to the pharmaceutical company, he immediately transplanted rice on the same land. The first crop's results surprised him, as the rice grew significantly better without chemical fertilizers.

According to Mr. Du, the only input needed for duckweed cultivation is Lâm Thao phosphate fertilizer. From fertilization to harvest takes about 22-25 days, which is enough time for biomass to multiply, for the roots to absorb phosphorus and accumulate nitrogen, and for the remaining roots in the mud to continue decomposing into organic matter for the next rice crop.

Mr. Du calculated that if fertilizer is applied for duckweed production, each sao requires only 25 kg of phosphate fertilizer over 3-4 cycles. The total fertilizer cost for one sao of duckweed cultivation is about 400,000 VND. If duckweed is used as organic fertilizer, only two applications are needed, each time about 7-8 kg depending on soil quality, costing only 100,000-200,000 VND per sao. The following rice crop can be planted directly on the duckweed-treated soil without additional chemical fertilizers. For the second rice crop, depending on soil conditions, only about 10-20% of chemical fertilizer may be needed.

However, duckweed farming is not always smooth. Fungal diseases can occur, as well as leaf-eating pests. The most dangerous is a red-bodied worm that attaches to the roots, feeding on them and causing the duckweed to separate and die in large patches, with no specific treatment available.

Based on Mr. Du’s model, Dr. Pham Gia Minh argues that Azolla is not merely an old farming practice but can become a viable avenue for modern green agriculture.

According to Dr. Minh, Azolla is essentially a form of living nitrogen for rice plants. It contains symbiotic bacteria capable of fixing atmospheric nitrogen and converting it into natural nutrients for plants. When Azolla dies, its biomass continues to nourish the soil, softening it and helping restore microbial ecosystems, rather than causing the soil compaction that chemical fertilizers do.

He notes that the strength of Azolla lies in its slow-release nutrient mechanism, allowing crops to absorb nutrients gradually and reducing environmental losses. In contrast, urea fertilizer dissolves too quickly; rice plants can absorb only about 60%, while the remainder volatilizes or runs off into rivers and lakes, contributing to eutrophication.

In addition to providing organic nitrogen, the layer of duckweed covering the entire rice field surface helps retain moisture and suppress weed growth, thereby reducing the need for herbicides.

However, according to Dr. Minh, reintroducing Azolla to rice fields cannot be done using traditional methods. He proposes applying science and technology from seed preservation and propagation to mechanization of cultivation. “Azolla could be compressed into pellets and distributed by drones across thousands of hectares,” Dr. Minh suggested.

The biggest bottleneck for developing Azolla cultivation in rice fields, according to him, is the lack of coherent policies, the absence of enterprises specializing in Azolla, and the lack of stable market outlets for organic agricultural products.

Rice yield reaches 3 quintals 

Regarding rice yields under Azolla-based cultivation, Mr. Du said that one northern Vietnamese sao cultivated using conventional chemical fertilizers typically yields about 2.5–2.8 quintals of paddy rice. In contrast, rice grown on Azolla-treated fields in his model usually reaches 3–3.2 quintals per sao. Moreover, because no chemical fertilizers or pesticides are used, the rice is clean and well-suited for organic production. The grains are fuller, tastier, and richer in flavor.

From the field edge, rice grown on Azolla-based fields appears deep green, upright, and heavy-headed, with plump grains densely packed from the base to the tip of the panicle. The stems show no signs of yellowing, and the leaves remain green even as the rice reaches maturity.

With the same variety, the same soil conditions, and the same season, the adjacent field cultivated using conventional methods looks completely different. The rice panicles are sparser, the stems and leaves turn yellow more quickly, and some panicles are partially empty.

The superiority of rice grown on Azolla-based fields comes from the decomposed Azolla roots left in the soil and the gradually accumulated organic nitrogen over successive crops. Instead of being continuously stressed with chemicals, the soil is nourished by microorganisms. As a result, the rice plants are stronger, less susceptible to pests and diseases, and mature earlier than those in conventionally farmed fields nearby.

The most noticeable difference lies beneath the surface of the mud, which is invisible to the eye. “In fields using chemical fertilizers, the soil becomes compacted, whereas in Azolla fields the soil remains loose and friable, making plowing and tilling much easier. In chemically fertilized fields, tractors and tillers often get stuck in the mud due to its sticky, compact nature,” Mr. Du shared.

Regarding his plans for the coming time, Mr. Du shared: “I can only do this much with my capacity. Wherever there is a rice field, I grow duckweed. Anyone who wants to scale up this model, I am willing to provide seedlings and support them.”

After the rice harvest, Mr. Du incorporates the straw back into the soil using a machine, then spreads a layer of lime to accelerate decomposition and reduce soil acidity before planting duckweed for the next crop.

It is a closed-loop process: rice straw returns to the soil, the soil nourishes the duckweed, the duckweed in turn nourishes the soil again, and the soil supports the rice.

$1 = VND 26,407 - Source: Vietcombank.