Green manure is an agricultural practice that involves growing specific plants solely for the purpose of improving the soil rather than harvesting them for food or sale. This technique transforms living plant material into a natural soil amendment, enhancing fertility and overall soil health. The practice dates back at least to the ancient Greeks around 300 B.C., who utilized crops like broad beans. The goal is to incorporate the crop’s biomass into the soil while it is still green and actively growing.
How Green Manure Improves Soil Chemistry
Green manure crops enhance the chemical composition of the soil through two primary mechanisms: nitrogen fixation and nutrient scavenging. Leguminous plants, such as clover and vetch, form a symbiotic relationship with Rhizobia bacteria found in specialized root nodules. These microorganisms capture atmospheric nitrogen gas and convert it into a plant-usable form, specifically ammonia, through biological nitrogen fixation.
When the green manure is incorporated into the soil, this fixed nitrogen is released as the plant matter decomposes, significantly enriching the soil’s nitrogen content for the subsequent cash crop. A successful leguminous crop can contribute a substantial amount of nitrogen, reducing the reliance on synthetic fertilizers.
Deep-rooted green manure crops, often non-legumes like rye or chicory, perform a function known as nutrient scavenging. These plants extend their root systems deep into the soil profile to absorb leached nutrients, such as nitrates and potassium, that have moved below the topsoil layer. By storing these mobile nutrients in their above-ground biomass, the plants prevent them from being washed away.
When the plant is terminated and incorporated, these previously inaccessible nutrients are released through decomposition back into the upper soil layers, making them available to the shallower-rooted cash crop. As the organic matter breaks down, soil microorganisms produce organic acids that increase the solubility of mineral nutrients, such as phosphate and various trace elements, making them more readily available for plant uptake.
Essential Steps for Using Green Manure
Effective implementation of green manuring begins with careful selection of the crop, matching the plant to the specific climate and soil condition. The choice should align with the desired benefit, such as nitrogen addition, breaking up compacted soil, or suppressing pest cycles. Once selected, the seeds are typically broadcast at a dense rate across the prepared soil surface, often followed by a light raking to ensure good seed-to-soil contact.
Timing the planting is critical; green manure crops are usually sown during fallow periods, either between main crops or over the winter. The soil must be kept adequately moist until the seeds germinate and the young plants establish themselves. This strategic planting ensures the soil is covered and protected when it would otherwise be bare.
The most important step is the termination and incorporation of the plant material, which must be executed precisely to maximize benefits. For optimal nutrient density, the plant should be terminated when it reaches the flowering or bud stage, before it begins to set seed. Waiting too long results in tougher, woodier material that decomposes slowly, and allowing the plant to set seed creates a future weed problem.
Termination methods vary and include mowing, crimping, or cutting the plants at the base. The biomass is then either tilled into the soil for rapid decomposition or left on the surface as a nutrient-rich mulch. After incorporation, a decomposition window of three to four weeks is necessary before the next main crop is planted. This waiting period allows the material to break down sufficiently, preventing it from suppressing the growth of the newly planted cash crop.
Major Categories of Green Manure Crops
Green manure crops are broadly classified into distinct categories based on their botanical characteristics. Legumes form the first major group and are prized for their ability to fix atmospheric nitrogen. Common examples include various clovers, winter tares (vetch), and field beans, which are especially beneficial for crops with high nitrogen requirements. These plants provide a natural fertility boost, reducing the need for external nitrogen inputs.
Grasses and cereals constitute a second category, including species such as winter rye, oats, and Italian ryegrass. These crops produce large amounts of fibrous biomass, which significantly increases soil organic matter content and stabilizes the soil surface. Their dense root systems are effective at controlling soil erosion and scavenging leached nutrients.
A third group comprises broadleaf crops, often used for their specific structural or pest-controlling properties. Buckwheat is a popular example, known for its rapid growth and ability to smother weeds and break up minor soil compaction. Certain Brassica family members, such as white mustard, are sometimes used for biofumigation, where compounds released during decomposition can help suppress soil-borne pests and diseases.