Azolla as a Nitrogen Fertilizer for Increasing Crop Yields :
Azolla plants are often described by the Chinese and Vietnamese as miniature nitrogen fertilizer factories. The Vietnamese use Azolla as a nitrogen source because it continuously provides nitrogen for their rice paddies. The nitrogen fixed by Azolla becomes available to the rice once the Azolla mat is incorporated into the soil, where its nitrogen is released through decomposition. Typically, Azolla takes 25 to 35 days to supply sufficient nitrogen for a 4 to 6 ton/ha rice crop during the rainy season, or a 5 to 8 ton/ha crop under irrigation during the dry season.
Maintaining Soil Fertility:
As a green manure, Azolla significantly impacts soil fertility through its organic matter and nitrogen content. When incorporated into the soil, Azolla decomposes to form humus. This humus enhances the soil's water-holding capacity and promotes better aeration, drainage, and aggregation, which are crucial for maintaining highly productive soils. Organic matter from Azolla helps bind soil particles together and improves the friability of clayey soils.
In addition to its effects on soil physical properties, Azolla plays a key role in nutrient cycling. While growing in paddy fields, Azolla fixes nitrogen and absorbs nutrients from the water that might otherwise be lost. As Azolla decomposes and forms humus, these nutrients are gradually released into the soil, enriching it over time.
Controlling the Growth of Aquatic Weeds:
Agricultural economists have found that Asian farmers, especially women, spend more time on weeding than on any other rice production activity. Although research on this topic is limited, it is widely believed that Azolla helps suppress the growth of certain aquatic weeds. Azolla achieves this by forming a thick, light-proof mat that inhibits weed growth.
There are two primary mechanisms for this suppression. The most effective mechanism is the light-starvation of young weed seedlings due to the blockage of sunlight by the dense Azolla mat. The second mechanism involves the physical resistance provided by the heavy, interlocking Azolla mat, which prevents weed seedlings from establishing. In weed-infested rice fields, the benefits of Azolla in suppressing weeds may even exceed its advantages as a nitrogen source. Importantly, rice seedlings are not affected by Azolla’s weed suppression because they grow above the Azolla mat after transplantation.
How Azolla Is Used as a Green Manure:
Azolla can be used as green manure in several ways: by growing it as a monocrop and incorporating it as basal manure before rice transplanting, or by growing it as an intercrop and applying it as a top dressing after rice is transplanted. Additionally, Azolla can be used both as a monocrop and an intercrop. In China and Vietnam, Azolla monocropping in winter and spring provides nitrogen for rice crops, though it is less common in summer due to high temperatures and pests. Intercropped Azolla is introduced manually or with a rotary weeder, and its decomposition releases nitrogen, boosting grain production.
