Guide to Green Manuring for Soil Health

Modern intensive farming often keeps fields occupied year-round, leaving little time for the soil to recover. Constant cultivation depletes organic carbon, which is the foundation of soil fertility and structure. Without sufficient organic carbon, the soil struggles to hold water, becomes compacted, and shows poor response to chemical inputs. The soil becomes hard to plow, and beneficial earthworms disappear. Green manuring is a traditional, cost-effective way to restore organic matter to your fields. By growing fast-growing crops and plowing them back into the soil, you return vital nutrients and improve soil health. This practice helps prepare the ground for major crops like paddy, wheat, and sugarcane, reducing the need for synthetic nitrogen.

What Is Green Manuring

Green manuring is the practice of growing specific, fast-growing plants and burying them in the soil at a particular stage of growth. Usually, leguminous crops are chosen because they can fix nitrogen from the air through their root nodules. When these green plants are plowed back into the soil, they decompose quickly, releasing nutrients like nitrogen, phosphorus, and potassium. The organic matter also acts as food for beneficial soil bacteria, fungi, and earthworms. This process improves soil aeration, loosens hard clay soils, and increases the water-holding capacity of sandy soils. It is a natural way to rebuild the biological activity of your land, helping the soil retain moisture during hot summer dry spells.

The process works by transferring nutrients from the deeper layers of the soil to the surface. Green manure crops have deep root systems that draw up minerals from layers that shallow-rooted crops cannot reach. When the crop is plowed back into the topsoil, these nutrients are concentrated in the root zone of the next crop. This recycled nutrition helps seedlings establish quickly and grow strongly during the critical early weeks.

Suitable Green Manure Crops

Several crops are suitable for green manuring in India, depending on your soil type, climate, and water availability. The two most popular choices are Dhaincha and Sunnhemp. Other options include cowpea, cluster bean, pillipesara, and wild indigo. Leguminous crops are preferred because they add nitrogen to the soil. Non-leguminous crops like mustard, niger, or millet can also be used to add bulk organic matter, though they do not fix nitrogen. Selecting the right crop for your region and soil type is important to get the best biomass yield and ensure proper decomposition.

For instance, pillipesara is highly suitable for the delta regions of southern India, where it is sown in standing paddy crops just before harvest. Cowpea works well in warm, humid regions and provides a quick ground cover that prevents soil erosion during heavy pre-monsoon rains. Cluster bean, or guar, is a drought-resistant legume that fits well in sandy soils of Rajasthan and Haryana where water is scarce.

Dhaincha for Tough Soils

Dhaincha (Sesbania aculeata) is a hardy crop that grows well under difficult conditions. It is highly tolerant of wet or waterlogged soils, salinity, and alkalinity. Dhaincha has a strong, deep taproot system that can penetrate hard soil layers, helping to open up the soil structure and improve water drainage. In addition, its roots release acidic compounds that help neutralize alkaline soils, making it an excellent choice for reclaiming sodic lands. It is widely grown in states like Punjab, Haryana, Uttar Pradesh, and Bihar before transplanting Kharif rice, helping to prepare the soil for the waterlogged conditions of paddy cultivation.

Sunnhemp for Loamy Soils

Sunnhemp (Crotalaria juncea) is a fast-growing green manure crop best suited for well-drained, sandy loam soils. It does not tolerate waterlogging but grows very quickly under dry conditions with minimal water. Sunnhemp produces a high amount of green biomass in a short period and suppresses weeds by shading the ground with its broad leaves. It also decomposes rapidly after incorporation, making its nutrients quickly available to the next crop, such as sugarcane or maize.

Sowing and Seed Rate Guidelines

To establish a good green manure crop, sow the seeds during the early summer months of May and June, or in spring after harvesting Rabi crops. The seed rate depends on the crop you choose. For Dhaincha, use about 35 to 40 kilograms of seeds per hectare. For Sunnhemp, a seed rate of 25 to 30 kilograms per hectare is recommended. Prepare the field by giving it one or two plowings, then broadcast the seeds evenly. Light irrigation or a pre-monsoon shower is usually enough to start germination. The crop grows rapidly, covering the soil surface and requiring very little maintenance or weeding.

When sowing, ensure that you broadcast the seeds evenly to avoid patchy growth. If the soil is very dry, a pre-sowing irrigation helps ensure uniform germination. Some farmers also practice seed treatment with Rhizobium for green manure crops to maximize nitrogen fixation, though many soils already contain the necessary native bacteria. The rapid growth of these crops prevents weed seeds from germinating, saving the labor costs of weeding.

Pre Flowering Incorporation Timing

Timing is crucial when plowing the green manure back into the soil. You should incorporate the crop when it reaches the pre-flowering stage, which is usually 45 to 50 days after sowing. At this stage, the plants are leafy, tender, and contain the highest amount of nitrogen. The stems are soft and easy to chop and turn over. If you wait until the plants flower and set seeds, the stems will become woody and fibrous. High fiber or lignin content makes decomposition much slower, and the microbes might temporarily lock up soil nitrogen to break down the wood fibers, which can starve your next crop of nutrients.

At 45 to 50 days, the carbon-to-nitrogen (C:N) ratio of the green manure is narrow, around 15:1 to 20:1. This narrow ratio allows soil microbes to decompose the plant tissue quickly without drawing nitrogen away from the surrounding soil. If the plants are allowed to mature to 70 or 80 days, the C:N ratio widens significantly, meaning decomposition will take much longer and may delay the sowing of your main Kharif crop.

Plowing and Decomposition Steps

To incorporate the green manure, first use a disc harrow or a rotavator to chop the standing plants into smaller pieces and mix them with the topsoil. Next, use a moldboard plow to turn the soil over and bury the chopped biomass completely. The soil must have sufficient moisture for proper decomposition. If the soil is dry, apply a light irrigation. Let the buried material decompose for 10 to 15 days before transplanting paddy or sowing other Kharif crops. During this time, soil microbes will break down the green plants, turning them into humus and releasing nutrients. Keeping the soil moist during these two weeks is essential for bacterial activity.

If you are incorporating the crop in a heavy clay soil, ensure that the plants are not buried too deeply. In heavy soils, deep burial can limit oxygen availability, causing the green matter to ferment under anaerobic conditions instead of decomposing cleanly. A burial depth of 10 to 15 centimeters is ideal. In lighter, sandy soils, you can bury the plants slightly deeper to help build organic matter in the lower root zone.

Biomass and Nitrogen Returns

Green manuring returns a significant amount of nutrients to the soil. On average, a 45-day-old Dhaincha or Sunnhemp crop can yield about 15 to 25 tons of green biomass per hectare. When fully decomposed, this biomass adds approximately 60 to 80 kilograms of organic nitrogen per hectare back to the soil. This nitrogen is released slowly as the plant matter rots, matching the growth needs of the succeeding crop. This allows farmers to reduce their chemical nitrogen applications by up to 25 to 30 percent, saving money on urea and protecting groundwater from nitrate pollution.

In addition to nitrogen, green manure crops return significant amounts of organic phosphorus and potassium. While they do not add new phosphorus or potassium to the soil system like they do with nitrogen, they convert unavailable soil minerals into organic forms that are much easier for the following crop to absorb. This improves the overall nutrient use efficiency of the farm.

Reclaiming Alkaline and Saline Soils

In many parts of India, soils have become alkaline or saline due to poor drainage and excessive chemical fertilizer use. Dhaincha is a useful tool for reclaiming these soils. Its root system penetrates the hard soil layers, which improves water infiltration and allows harmful salts to leach down below the root zone. As Dhaincha decomposes, it produces carbon dioxide and organic acids. These acids help convert insoluble calcium carbonate in the soil into soluble calcium sulphate, which helps displace sodium ions on the soil particles. This process gradually lowers the soil pH and improves soil structure, making the land productive again.

For reclamation to be successful, green manuring should be combined with proper drainage. If the displaced sodium salts have nowhere to go, they will rise back to the surface as the soil dries. Sowing Dhaincha at the start of the monsoon season allows the heavy rains to wash the displaced salts deep into the subsoil, leaving the topsoil clean for the following crop.

Seed Subsidies and Government Schemes

To encourage soil conservation, many state departments of agriculture in India offer subsidies on green manure seeds. Under these schemes, farmers can purchase Dhaincha and Sunnhemp seeds at subsidized rates, sometimes ranging from 50 to 80 percent off the market price. These seeds are usually distributed through block agriculture offices, cooperative societies, or state seed centers. Farmers should verify current prices, subsidy availability, and distribution dates on their state's official agriculture portal or consult their local block extension officer. You may need to present your Aadhaar card and land records to claim the subsidy.

In states like Punjab and Haryana, the government promotes green manuring as a way to reduce water usage and urea consumption in the paddy crop. These programs often open for applications in April and May, allowing farmers to procure seeds in time for pre-monsoon sowing. Participating in these schemes helps lower the initial cost of implementing green manuring on your farm.

Precautions for Green Manuring

While green manuring is highly beneficial, a few precautions are necessary to ensure success. First, make sure you have enough water or rain scheduled for the decomposition period. If the soil remains dry after you bury the crop, the biomass will not rot, and it will interfere with the sowing of the next crop. Second, do not let the green manure crop grow too tall or set seeds. If seeds mature and fall to the ground, they will germinate as weeds during your main crop season. Finally, ensure the plants are buried deep enough so they do not regrow from the root crowns, especially if using a crop like wild indigo which can be difficult to kill.

Another precaution is to watch for insect pests that can build up on the green manure. For example, Sunnhemp can sometimes host hairy caterpillars or flea beetles. If you notice a high population of these pests, incorporate the crop slightly earlier to prevent them from moving onto your main crops. Proper crop rotation also helps prevent pest buildup.

Integrating with Soil Health Cards

Before starting a green manuring program, it is helpful to check your Soil Health Card. The card will show your current soil organic carbon (SOC) levels. If your SOC is below 0.5 percent, your soil is deficient and will benefit greatly from green manuring. After incorporating green manure for two or three seasons, test your soil again to see how your organic carbon levels and nutrient availability have changed. For specific crop rotations and advice, consult your regional Krishi Vigyan Kendra (KVK) agricultural experts. They can help you plan your green manure schedule based on your soil test results.

By tracking your soil test results over time, you can see the direct benefits of green manuring. Many farmers report that their soil test values for available nitrogen and phosphorus improve after green manuring, allowing them to adjust their chemical fertilizer purchases. This evidence-based approach helps you manage your soil nutrients more efficiently.

Economic Benefits of the Practice

Green manuring offers long-term economic advantages. By adding organic nitrogen, it reduces the amount of chemical nitrogen you need to buy. It also improves soil structure and water retention, which can reduce the number of irrigations needed during dry spells. Over time, the increased organic matter helps plants build stronger root systems, leading to better nutrient uptake and higher yields. These savings and yield improvements help lower the overall cost of cultivation, making your farming operation more resilient to rising chemical fertilizer prices.

In addition, the physical improvement in soil structure means that tillage operations require less fuel and tractor power. Soil that is rich in organic matter is softer and easier to till, reducing wear and tear on machinery. These hidden savings add to the direct financial benefits of reduced chemical fertilizer purchases, making green manuring a wise choice for Indian farmers.

Frequently asked questions

What is green manuring?

Green manuring is the practice of growing fast-growing plants, usually legumes, and plowing them into the soil at the pre-flowering stage to improve organic matter and nutrient levels.

Which crops are best for green manuring?

Dhaincha and Sunnhemp are the most popular choices. Other crops like cowpea, cluster bean, and pillipesara are also suitable depending on soil and water conditions.

Why is Dhaincha preferred for saline soils?

Dhaincha tolerates high salinity and waterlogging. Its deep roots open up the soil, and its root exudates help neutralize alkalinity.

When should I plow the green manure crop into the soil?

Plow the crop back into the soil during the pre-flowering stage, which is usually 45 to 50 days after sowing.

What happens if I delay plowing past the flowering stage?

The stems will become woody and fibrous, taking much longer to decompose. This delayed breakdown can temporarily lock up soil nitrogen.

How much biomass does a green manure crop produce?

An average 45-day-old Dhaincha or Sunnhemp crop yields about 15 to 25 tons of green biomass per hectare.

How much nitrogen does green manuring add?

It can add about 60 to 80 kilograms of organic nitrogen per hectare, helping to reduce chemical nitrogen inputs for the next crop.

What is the seed rate for Dhaincha and Sunnhemp?

The recommended seed rate is 35 to 40 kilograms per hectare for Dhaincha and 25 to 30 kilograms per hectare for Sunnhemp.

Do green manure crops require chemical fertilizers?

No, they generally do not need chemical fertilizers. They can grow well on residual soil nutrients and biological nitrogen fixation.

How long should I wait after plowing before planting the main crop?

Allow the buried green manure to decompose for 10 to 15 days in moist soil before sowing or transplanting your main crop.

Is there a government subsidy for green manure seeds?

Yes, many state departments of agriculture offer 50 to 80 percent subsidies on green manure seeds through block offices or cooperatives.

Can I do green manuring in dryland farming?

Yes, but you need at least some moisture during sowing and enough water or rain at the time of plowing to ensure the buried plants decompose.

Does green manuring help control weeds?

Yes, fast-growing crops like Sunnhemp quickly cover the ground and shade out weeds, reducing weed populations in the field.

Can non-legume crops be used for green manuring?

Yes, crops like mustard or millet can be used to add bulk organic matter, but they do not fix nitrogen from the air.

Where can I find green manure seeds?

You can buy them from local government seed depots, block agricultural offices, cooperative societies, or authorized seed dealers.