Green Manuring: 

Green manuring is an age-old agricultural practice that focuses on improving soil fertility and structure by growing specific crops to be turned into the soil. This process is essential for enhancing soil productivity and conserving essential nutrients for subsequent crops. Let’s break down each aspect of green manuring in detail.

• Definition of Green Manuring

Green manuring refers to the practice of growing crops for the sole purpose of plowing them into the soil while they are still green, or shortly after they have matured. These crops are typically rich in organic material, which improves soil fertility, structure, and moisture retention when decomposed. The goal is to enhance soil health for future crops by replenishing the soil’s nutrients, especially nitrogen, which plays a key role in plant growth.

• Objectives of Green Manuring

Green manuring serves multiple important functions in agriculture:

1. Increasing Organic Matter Content of Soil: Green manure crops are high in organic matter. When plowed into the soil, they decompose and contribute to increasing the organic matter content, which improves the soil’s ability to hold moisture and nutrients.
2. Maintaining and Improving Soil Structure: The incorporation of green manures helps in the formation of aggregates in the soil, leading to improved soil structure. This prevents soil compaction, enhances root penetration, and improves drainage.
3. Reducing Nutrient Loss, Particularly Nitrogen: Green manuring helps to trap and conserve nutrients like nitrogen. It prevents nutrient loss by reducing the leaching of nitrogen, especially in the case of leguminous green manures, which have a high nitrogen-fixing capacity.
4. Providing a Source of Nitrogen for the Following Crop: Legumes used in green manuring, such as sunnhemp or daincha, fix atmospheric nitrogen in the soil through symbiotic relationships with rhizobial bacteria. This nitrogen is made available to subsequent crops, reducing the need for synthetic fertilizers.
5. Reducing Soil Erosion: The dense growth of green manure crops protects the soil surface from water and wind erosion. The root systems bind the soil particles together and help to maintain the integrity of the soil structure, especially on slopes or exposed areas.

• Types of Green Manuring Green manuring is adapted to various soil and climatic conditions. It can be classified into two main types based on the method of incorporation:

1. Green Manuring In Situ This method involves growing and burying the green manure crops in the same field. These crops may be planted as a pure stand or as an intercrop alongside the main crop.

• Common Crops Used:
  • Sunnhemp (Crotalaria juncea): A versatile crop with a high nitrogen content, used under both irrigated and dry conditions.
  • Daincha (Sesbania aculeata): A deep-rooted, fast-growing legume, useful for improving heavy soils and correcting sodic soils.
  • Guar (Cyamopsis tetragonoloba): A drought-resistant legume often used in arid conditions.
  • Indigo (Indigofera tinctoria) and Wild Indigo (Tephrosia purpurea): Suitable for poor, coarse soils and rainfed conditions.

• Sowing & Incorporation: The crops are often broadcasted, especially in systems where no preparatory cultivation is done. Heavy seed rates are used for broad coverage. Green manuring in situ is most effective in irrigated conditions or regions with at least 900 mm annual rainfall.

• Benefits: Adds large quantities of organic matter. Provides a significant nitrogen boost for subsequent crops. Protects against soil erosion.
• Challenges: Requires a minimum of 2–3 months to grow the green manure crop. Additional costs for growing and maintaining the crop. Prone to pest and disease attacks, and may require extra protection. Depending on rainfall or irrigation for successful growth.

2. Green Leaf Manuring This method involves using leaves, twigs, and other green plant material collected from trees and shrubs and incorporating them into the soil.

• Sources of Green Leaf Manure: Trees and shrubs such as Glyricidia, Sesbania, Karanj (Pongamia pinnata), Leucaena leucocephala, and others.
• Advantages:
  • Easier and quicker to implement compared to in-situ green manuring.
  • No need for sowing and cultivating a specific crop.
  • Leaves are readily available from waste lands, bunds, and forest areas.
• Challenges:
  • Requires collection and transportation of materials, which may involve extra costs.
  • Availability can be seasonally dependent, especially in regions without access to forest or waste land.
  • Nutrient content may be lower than that of green manure crops, but still contributes valuable organic matter.

• Advantages and Limitations of Green Manuring

Advantages of Green Manuring (In Situ):

1. Suitability to Soil, Season, and Water Conditions: Green manure crops can be chosen according to the local soil type, water availability, and cropping patterns.
2. Reduces Nitrogen Loss: Green manuring reduces nitrogen leaching, especially when using legumes that fix atmospheric nitrogen.
3. Cost Efficiency: Helps reduce the need for chemical fertilizers and other soil amendments.
4. Incorporation is Timely and Efficient: The crops are generally easy to incorporate, ensuring optimal decomposition and nutrient release.

Limitations of Green Manuring (In Situ):

1. Time-Consuming: Requires 2–3 months for crop growth.
2. Additional Expenditure: Involves extra cost for seed, planting, and management of the green manure crop.
3. Pest and Disease Risk: Some green manure crops may host pests and diseases that could affect the main crop.
4. Moisture Sensitivity: Requires timely rainfall or irrigation, which can be problematic in dry or drought-prone areas.

Advantages of Green Leaf Manuring:

1. Quick and Efficient: No need to grow a separate crop; simply collect and incorporate leaves.
2. No Pest or Disease Spread: The risk of pests and diseases is lower since green leaf manuring doesn’t involve growing a crop that could act as a host for pathogens.
3. Seasonal Flexibility: Can be done throughout the year depending on the availability of green leaf material.

Limitations of Green Leaf Manuring:

1. Availability Issues: Green leaves may not be available in sufficient quantities throughout the year, especially in non-forest areas.
2. Transport Costs: Collecting and transporting green leaves can add to the cost.
3. Nutrient Content: Green leaves may have lower nutrient content compared to specific green manure crops.

Criteria for Choosing Green Manure Crops The ideal green manure crop should meet the following criteria:

• Ability to Fix Nitrogen: Legumes are particularly valuable because they can fix nitrogen from the atmosphere through symbiosis with rhizobial bacteria.
• Heavy Vegetative Growth: The crop should produce a substantial amount of biomass to improve soil fertility.
• Succulent Vegetation: The crop should decompose easily, contributing organic material rapidly.
• Deep Root System: A deep root system helps break up compacted soil and recycle nutrients from deeper layers.
• Short Duration: The crop should have a short life cycle, allowing it to fit into the crop rotation system without disrupting other crops.