The practice of growing and ploughing or mixing the plant’s green, tender and un-decomposed parts like leaves, stem, twigs etc. in soil in order to improve its structure and fertility status is known as green manuring.
Objective of green manuring
The main objective of green manuring is to make the nutrient deficient soil, more fertile and more productive, by adding organic matter and nitrogen into the soil.
Importance of green manuring
Green manuring is very important because it improves the soil structure and fertility by
- Adding nitrogen at the rate of 40 to 80 kg/ha
- Minimizing the leaching and erosion losses of nitrogen
- Binding the soil particles tightly together due to its vigorous root system
- Removing the soil salinity and alkalinity
Beside these, green manuring also serves as fodder in many areas of Pakistan.
Types of green manuring
There are two types of green manuring which are given as following
- Green manuring in-situ
- Green leaf manuring
Green manuring in-situ
Sowing of manure crop either as main crop or inter-crop and burying it in the same field is known as green manuring in-situ. Some crops used for green manuring in-situ are sunhemp, dhaicha, mung, senji, berseem, urd, shervi, cowpea etc.
These crops are sown as;
- Main crop
- Inter row sown crop
- On bare fallow, depending upon the soil and climatic conditions of the region.
Green leaf manuring
Mixing of green leaves and tender green twigs of plants into the soil, collected from shrubs and trees grown on bunds, waste lands and nearby forest area is known as green leaf manuring. Some crops used for green leaf manuring are glyricidia, wild dhaicha, karanj etc.
Characteristics/desirable qualities of a green manuring
- It should have quick growth habit and should produce a large quantity of green material within a short period.
- It should be able to suppress the weeds due to its early and quick growth habit.
- It should be more succulent and have more leafy growth, so that its decomposition will be rapid.
- It should be legume due to its quality of fixing atmospheric nitrogen.
- It should have fibrous and deep root system so that it can absorb nutrients from lower depth of soil and bring them on the upper surface of soil in order to improve the soil structure.
- It should have the ability to grow even on poor and degraded soils.
Crops suitable for green manuring
Crops suitable for green manuring are divided into two groups:
- Non-legumes or non-leguminous crops
Green manuring crops which crops which only add organic matter into the soil are called as non-leguminous crops.
Examples: Mustard, wheat, radish, carrot, sorghum, maize and sunflower etc.
- Legumes or leguminous crops
Green manuring crops which add organic matter as well as nitrogen into the soil are called as leguminous crops. Due to its nodule bacteria (Rhizobium), legumes have the ability to fix the atmospheric nitrogen into the soil.
Examples: Sun hemp, sesbania, mung, cowpea, lentil, pigeon pea, berseem and guar etc.
Why legumes are preferred for green manuring?
Almost all the crops can be used for green manuring, but legumes are preferred for green manuring because they have the ability to fix atmospheric nitrogen due to the nitrogen fixing bacteria (Rhizobium), which lives in the root nodules of crops.
Green manuring with legumes (clovers, lentil, peas etc.) is called legume green manuring. Green manuring with legumes also adds organic matter in the soil.
Time of incorporation
Crops for manuring should be incorporated into the soil before flowering or at flowering stage because at flowering stage, the crops have maximum biomass. Rotavator should be used for incorporation in standing water for the purpose of quick decomposition.
Green manured crops, usually take 1 to 1.5 months for decomposition depending upon the temperature and microbial activity in soil. New crop should be grown just after 7 to 15 days of incorporation.
Advantages of green manuring
- Addition of organic matter into the soil
Green manuring adds organic matter into the soil. The organic residues from green manure crops improve the soil structure which in turn helps the plant for it proper growth and germination.
Humus formed from green manure enhances water holding capacity of soil, improves soil aeration, drainage and granulation. Organic matter also helps to increase the soil microbial activity.
- Fixation of nitrogen
If the crop used for green manuring is legume, then it adds organic matter as well as nitrogen with the help of its nodule bacteria (Rhizobium). The legume crop adds nitrogen for succeeding crop.
So, all the legumes crop leave the soil in better physical condition and richer in nitrogen content. They return the plant nutrients deeper layers to the upper top layers of soil.
- Minimize nutrient and soil loss
Green manured crops also act as cover crops. By taking all the soluble nutrients fro soil, they helps to save the soil and soil nutrients from water and wind erosion. Additional, they also increase the availability of phosphorous and other plant essential micro and macro nutrients for succeeding crop.
- Stimulate the biochemical activity
The presence of organic matter into the soil due to green manuring acts as food for soil microbes. In this way, the microbial activity is increased which in turn promotes the biochemical changes in soil.
- Increases crop yield
Due to organic matter and fixation of atmospheric nitrogen, yield of succeeding crops is increased, especially in legumes.
Table: Green manure crops with their quantity of nitrogen fixation
Green manure crops | Seed rate (kg/ha) | Green biomass (t/ha) | Dry biomass (t/ha) | Nitrogen content (%) | Nitrogen fixation (kg/ha) | Incorporation time (Days after sowing) |
Azola | 50-90 | 8-10 | 1.8-3 | – | 52 | 35 |
Cowpea | 40 | 9-10 | 3-4 | 1.4-1.5 | 140-150 | 40-60 |
Lablab | 7-18 | – | 5-7 | – | 220 | 45-60 |
Mustard | 5-12 | – | – | 3.5 | – | 21-37 |
Pigeon pea | 45-67 | 9-10 | 5-7 | – | – | 45-60 |
Sesbania | 20-90 | – | 1.5-4 | 4.3 | 250-360 | 45-50 |
Soybean | 30-56 | – | 5-7.4 | – | 310 | 45-60 |
Sun hemp | 35-40 | 5-19 | 5.5-6 | 1.7 | 108 | 50-60 |
Sweet clover | 30-40 | 12 | 6 | 2 | – | 40-50 |
Disadvantages of green manuring
- Due to less rainfall or presence of little quantity of water in soil, the germination and growth of succeeding crop is badly affected due to slow decomposition rate.
- Manuring takes about three months, so it means that there is a loss of one crop.
- The chances become more for diseases to attack the green manure crops, if they are not saved from them with proper planning.
Authors: Tanveer Ahmad*, Shoaib Nadeem and Muhammad Hamza Latif
*Agro-Climatology Lab, Department of Agronomy, University of Agriculture, Faisalabad