5.3 Green Manure

The process of green manure is to grow green manure plants (legume grasses and others) for one or two months and return all the biomass to the soil as organic fertilizer.

5.3.1 Advantages

Organic Matter Supply

Why can farmers not supply enough organic matter to the land? The main reason is the shortage of organic matter. The necessary amount of organic matter per acre is usually about 8 – 10 tons per year. That amount of organic matter is not easily available from outside and collecting it is quite tedious work. Green manure is a very effective method which can easily supply the necessary amount of organic matter to the land and bypasses the need for outside collection.

Improve Soil Quality

Green manure provides a large amount of organic matter to soil at once which other methods cannot do easily. Therefore, this improves soil quality dramatically. First, soil structure is improved so that soil becomes soft and water holding capacity and drainage ability increase. Second, availability of nitrogen in soil increases with leguminous green manure crops and other chemical qualities (CEC, Ph, etc.) are improved. Third, numbers and activity of micro-organisms increase. This minimizes the numbers of nematodes (increase of algae decreases nematodes).

Less Labour and Cost

Green manure practice only requires labour in broadcasting seeds and plowing or cutting afterwards. For one acre only 20 – 30 kg of seed (e.g. Dhaincha) is required which costs only Tk 200 – 300.

5.3.2 Disadvantages

Time Required

Though there are many advantages, farmers are not very interested in practicing green manure. The main reason is the time required to produce it. (1.5 – 2 months fro growing and 2 weeks for decomposition.) The land cannot be used for growing other crops during this time and therefore, this necessitates a cropping pattern to leave time for green manure. A shorter term practice is green manure mulch. As the crop is not mixed in but just cut and left on the soil surface, 2 – 3 weeks of decomposition time can be eliminated.

Risk

If insufficient time is given for decomposition after mixing the green manure crop with soil, it creates harmful gas (Section 5.1) and crop plants cannot grow well. Leaf and fruit crops are particularly susceptible to this gas while cereal crops experience less difficulty with appropriate time is dependent on temperature, soil moisture, kinds of planting crop, etc. High temperatures and optimal moisture reduces the time. A farmer should decide the time from his/her experience, but 2 – 3 weeks should be sufficient.

5.3.3 Green Manure Crops

Favourable characteristics of a green manure crop include:

1. fast growing so that a large amount of biomass is gained in a short time
2. leguminous crop which can fix nitrogen (N) from air into soil by N-fixing bacteria at its roots

Proshika farm has mainly used leguminous crops including Mashikalai (back gram), Mung bean, Dhaincha (Sesbania aculata) and large amounts of organic matter and heal the land when used as green manure.

by Buckman and Brady 1984 (Natural Property of Soil)

5.3.4 Green Manure Mulch

This is a variation of green manure. The difference is you cut the manure plants and put them on the soil as mulch. Advantages are:

1. reduces time of decomposition
2. soil protection
3. reduces the labour for plowing

Cauliflower and cabbage were supplied with Mashikalai (black gram) green mulch material at the Proshika farm in 1989–90. The results were remarkable.