Green manure

A green manure crop has no single fixed composition; its contribution depends on the species grown, the biomass accumulated before incorporation, and, for legumes, how much atmospheric nitrogen has been fixed through their symbiosis with rhizobia bacteria.

Common species groups

Legumes (for nitrogen fixation) and non-legume cover crops (for organic matter)

Nitrogen contribution

Variable; can be substantial from well-nodulated legumes

Incorporation timing

Typically before or at flowering, before the crop matures

Nutrient release

Gradual, as incorporated biomass decomposes

When the green manure crop is a nitrogen-fixing legume, it can contribute meaningfully to the nitrogen supply of a following crop as its residues decompose. Non-legume green manures contribute organic matter and recycle nutrients already present in the soil rather than adding new nitrogen from the atmosphere.

• Legume green manures can add biologically fixed nitrogen to the system
• All green manures add fresh organic matter as biomass decomposes
• Nutrient release timing depends on the carbon-to-nitrogen ratio of the incorporated material

Green manuring is used within crop rotations to build soil organic matter, contribute nitrogen ahead of a following crop when a legume is used, suppress weeds through competition, and, in some systems, reduce erosion during the period the green manure crop occupies the ground.

• Adds organic matter and, from legumes, biologically fixed nitrogen
• Can suppress weeds through competition during its growth period
• Provides ground cover that can reduce erosion before incorporation

General practice involves selecting a species suited to the rotation and climate, timing incorporation to balance biomass accumulation against the needs of the following crop, and allowing sufficient time for decomposition before nutrients become available.

Green manuring can reduce reliance on manufactured nitrogen fertilizer when legumes are used effectively, and ground cover during the green manure period can reduce erosion and nutrient runoff compared with bare fallow. Poorly timed incorporation, however, can lead to a temporary nitrogen tie-up (immobilisation) if residues are high in carbon relative to nitrogen.