Can Green Manure Reduce Fertiliser Needs? What Should Farmers Know Before Growing It?

Farmers can grow certain crops mainly to improve the soil rather than harvest them for sale. This practice, known as green manuring, uses crops such as dhaincha and sunhemp to add plant material and nutrients to the soil.

The crop is grown for a limited period and incorporated into the field before the main crop is planted. ICAR describes green manuring as a way to add organic matter and improve soil fertility.  Green manure is a crop grown specifically to be incorporated into the soil while it is still relatively young and green. Unlike a crop grown for grain, the main purpose is to return biomass and nutrients to the field.

Leguminous green manure crops can also fix atmospheric nitrogen through their association with beneficial soil bacteria. This can increase the nitrogen available to the following crop after the green material decomposes. Dhaincha and sunhemp are among the commonly used green manure crops in India. Other crops can also be used depending on the region, soil and farming system.

Farmers should select a species that can grow during the available fallow period and produce sufficient biomass. Local agricultural universities and KVKs can advise farmers on suitable green manure crops for their area.

Legume green manure crops form associations with nitrogen-fixing bacteria in root nodules. These bacteria convert atmospheric nitrogen into forms that plants can use. When the green crop is incorporated into the soil, its plant material decomposes and releases nutrients. The amount of nitrogen returned depends on the crop species, biomass produced, growth stage and soil conditions.

Can green manure replace fertiliser?

Farmers should not assume that green manure can completely replace fertiliser. It can contribute nitrogen and organic matter, but the amount supplied depends on how much biomass is produced and incorporated. Crop nutrient requirements also vary. Soil testing can help farmers understand existing nutrient levels and decide whether additional fertiliser is required for the following crop.

Adding fresh plant material increases the amount of organic matter entering the soil. As the material decomposes, nutrients are released and soil biological activity can increase. The effect depends on the amount and quality of biomass, soil type, moisture and decomposition conditions. One green-manure crop should not be expected to change soil properties permanently.

Green manure is generally incorporated while the crop is still young and has produced substantial green biomass. Waiting too long can result in tougher plant material that decomposes more slowly. The exact timing depends on the crop and the following crop’s sowing schedule. Farmers need enough time between incorporation and planting for decomposition to begin.

Green manure crops need water to establish and produce biomass. Farmers should consider rainfall and irrigation availability before planting one. In rainfed areas, farmers can choose species and sowing periods suited to local rainfall. Growing a green manure crop during a water shortage can create competition for moisture with the following crop.

Does green manure need fertiliser?

The requirement depends on the green manure crop and soil. Leguminous crops can obtain nitrogen through biological fixation, but they still require other nutrients and suitable growing conditions.

Farmers should avoid assuming that green manure crops can grow without any nutrient management. Soil fertility and the crop’s nutrient needs should guide the choice of inputs.

Green manure returns plant biomass directly to the soil. This adds organic material that can contribute to soil structure and biological activity after decomposition. The effect is greater when farmers regularly return sufficient organic material to the soil. Green manuring works best as part of a wider soil-management system rather than as a one-time treatment.

Green manure can be incorporated before several field crops, depending on the local cropping system. The choice should account for the time required to grow and decompose the green crop. Farmers should also avoid selecting a green manure species that creates pest, disease or weed problems for the following crop. Local recommendations can help avoid unsuitable combinations.

Dhaincha is widely used as a green manure crop in India. It can produce substantial biomass and is adapted to several agricultural conditions. Farmers should still select seed suited to their region and sow at an appropriate time. ICAR and state agricultural departments regularly promote dhaincha and other green manure crops through farmer training programmes.

What about sunhemp?

Sunhemp is another leguminous crop used for green manuring. It can produce significant biomass during a relatively short growing period when suitable moisture and temperature are available.

Farmers should compare its growing period with their main crop calendar. The green manure should not delay the sowing of the main crop or compete with another profitable crop unless the soil benefit justifies the land and water use.

A fast-growing green manure crop can cover soil and compete with weeds during its growth. But its primary purpose is soil improvement, not weed control. Farmers should also ensure that the green manure crop itself does not set seed and become a weed problem. Timely incorporation is important for both soil management and field cleanliness.

Farmers should avoid planting a green manure crop without considering the next crop’s sowing date, water availability and decomposition period. They should also avoid allowing the crop to mature too much before incorporation. Another mistake is treating green manure as a guaranteed substitute for all fertilisers. Nutrient requirements should be based on soil conditions and the needs of the following crop.

The main costs can include seed, land preparation, sowing, irrigation where required and incorporation. Farmers also give up the opportunity to grow another crop during that period. The benefit comes through biomass, nutrient recycling and possible improvements in soil management. Farmers should compare those benefits with the cost of growing the green manure under their own cropping system.

Is it useful for small farmers?

Small farmers can use green manure where a suitable short window exists between crops. The practice can be especially relevant when farmers have access to locally adapted seed and need to improve soil organic matter.

But land is also a source of income. Farmers should calculate whether growing a green manure crop fits their rotation and whether its soil benefits justify the time, water and labour required.

Green manure can return plant biomass and some nitrogen to the soil, but it should not be treated as a complete replacement for fertilisers. Its value depends on the crop, biomass produced, soil and management.

Before planting, farmers should check the suitable species, sowing period, water requirement and incorporation time for their location. A green manure crop works best when it fits naturally into the existing crop rotation.