Green manure is an agricultural practice that involves cultivating specific crops for the sole purpose of incorporating their biomass into the soil rather than harvesting them for market. These cover crops, which can include legumes, grasses, or other broadleaf species, are grown for a period and then terminated and mixed into the topsoil, often while still green. The following mechanisms explain how this biomass transforms the soil environment, making it more productive for subsequent cash crops.
Boosting Soil Organic Content
The physical incorporation of green manure introduces a substantial volume of fresh organic matter into the soil profile. When this plant material begins to decompose, a complex process mediated by soil microbes, it contributes to the formation of soil organic carbon (SOC). This decomposition releases a steady supply of nutrients, but more importantly, it leads to the creation of humus, which is a highly stable, dark, amorphous form of organic matter.
Humus acts as a long-term reservoir for nutrients and water, fundamentally altering the soil’s chemical properties. A primary function of this stabilized organic matter is the significant increase in the soil’s Cation Exchange Capacity (CEC). CEC measures the soil’s ability to hold onto positively charged nutrient ions, such as calcium, magnesium, and potassium, preventing them from leaching away with water.
The presence of organic matter provides numerous negatively charged sites, which physically bind these positive nutrient ions, ensuring they remain available for future plant uptake. This capacity for nutrient retention is amplified by the increased microbial biomass and activity that green manure encourages, enhancing overall soil health and the cycling of essential elements.
Nitrogen Fixation and Nutrient Scavenging
Leguminous green manures, such as clover or vetch, possess a unique ability to capture atmospheric nitrogen (N₂) through a biological process called nitrogen fixation. This is achieved through a symbiotic relationship with specialized soil bacteria called rhizobia. The bacteria colonize the plant’s root system, inducing the formation of small, specialized structures known as root nodules.
Within these nodules, the rhizobia bacteria use the enzyme nitrogenase to convert inert atmospheric N₂ gas into ammonia (NH₃), a form that plants can readily use. This process provides a natural, renewable source of nitrogen for the green manure crop itself and, subsequently, for the following crops when the plant biomass decomposes.
Beyond nitrogen, deep-rooted green manures perform a service known as nutrient scavenging or recycling. Their extensive root systems can reach and absorb nutrients like phosphorus and potassium that have leached below the root zone of shallow-rooted cash crops. When the plant is later incorporated into the soil, these previously inaccessible nutrients are released through decomposition back into the topsoil, where they become available to the next crop.
Improving Soil Structure and Water Dynamics
The physical presence of the green manure’s root system and its resulting organic matter dramatically modifies the soil’s physical structure. The roots penetrate compacted soil layers, creating numerous channels and fissures known as macropores. These microscopic pathways improve soil aeration, allowing for the necessary exchange of gases and facilitating better drainage during heavy rainfall.
The decomposition of the incorporated biomass provides the organic compounds needed to stabilize soil aggregates. Soil aggregation is the clumping of individual soil particles (sand, silt, and clay) into larger, stable structures. Good aggregation reduces the soil’s bulk density and increases its total porosity.
These structural improvements have a direct and beneficial impact on water dynamics. The network of macropores and the spongy nature of the organic matter enhance the rate of water infiltration, ensuring less runoff and erosion. The increased porosity and organic content also improve the soil’s water-holding capacity, allowing the soil to retain more moisture, which buffers subsequent crops against periods of drought.