Which is the suitable stage of green manure crop for incorporation in field?

Green manuring means growing a fast-growing crop (often a legume such as sunhemp or dhaincha) purely to plough it back into the soil while it is still green, adding organic matter and nitrogen. The right time to incorporate it balances two things: the plant should have accumulated close to its peak green biomass and nitrogen content, while its tissue is still soft and low in lignin (a low carbon-to-nitrogen ratio) so it breaks down quickly once buried.

For this question, the pre-flowering stage is exactly that balance point: by pre-flowering the crop has built up most of its vegetative biomass and nitrogen, but the stem and leaves are still succulent and low in fibre, so the material decomposes rapidly in the soil and releases its nutrients in time for the next crop's sowing.

• Incorporating too early, at the initial stage, means the crop has not yet built up enough biomass or fixed enough nitrogen to be useful.

• Waiting until after flowering lets the stem and leaves start hardening with lignin and fibre, raising the plant material's carbon-to-nitrogen ratio, so decomposition slows and it takes longer for the nitrogen to become available to the next crop.

• At full maturity the stem has turned woody and fibrous with a much higher carbon-to-nitrogen ratio, so it decomposes far more slowly, and soil microbes breaking down this fibrous material can even tie up (immobilise) some of the soil's available nitrogen instead of releasing it, delaying rather than adding to the nutrient supply for the next crop.

The pre-flowering stage is therefore the standard recommended point to plough green manure crops into the field.