Green Manure Calculator & Seed & Free Nitrogen

Green manure is a fast-growing crop — usually a legume like dhaincha, sunhemp or cowpea — grown specifically to be ploughed back into the soil while still green. It adds nitrogen (fixed from the air by the legume), organic matter and improves soil structure, reducing the fertiliser the following crop needs.

It depends on the crop: dhaincha ~25 kg/ha, sunhemp ~35 kg/ha, cowpea ~30 kg/ha. This tool multiplies the recommended seed rate by your area to give the seed to sow, and also estimates the nitrogen the crop will add and the urea that replaces.

A well-grown legume green manure incorporated at flowering can add roughly 40–100 kg N per hectare depending on the species and biomass — dhaincha and sunhemp are among the highest. The tool estimates the N added and converts it to a urea equivalent so you can cut your fertiliser accordingly.

At early flowering (about 6–8 weeks after sowing), when biomass and nitrogen content are highest and the material is still soft and decomposes quickly. Incorporate into moist soil and wait 1–2 weeks before sowing the main crop so decomposition doesn't compete for nitrogen.

Choose by season, soil and water: dhaincha tolerates waterlogging and salinity and gives high N; sunhemp suits well-drained soils and also suppresses nematodes; cowpea and green gram double as short food/fodder legumes. The tool covers the common options with their seed rates and N contributions.

It substantially reduces nitrogen needs (often 40–100 kg N/ha) and builds long-term fertility, but it doesn't supply much phosphorus or potassium, so you'll still need those. Treat green manure as a major nitrogen and organic-matter source that you top up with a soil-test-based dose.

Legumes host Rhizobium bacteria in root nodules that fix atmospheric nitrogen into a plant-available form. When the green crop is ploughed in and decomposes, that nitrogen (plus what's in the leaves and stems) is released into the soil for the next crop — nitrogen captured from the air at no cost.

Yes — it adds organic matter that improves structure, water-holding and microbial life, protects the soil from erosion while growing, suppresses weeds, and can break pest and disease cycles. The nitrogen is the headline benefit, but the soil-health gains compound over seasons.

Absolutely — that's the classic use: grow a quick green manure in the gap before the main crop or during a fallow, then incorporate it. It turns idle land and moisture into free fertility instead of leaving the soil bare. The tool helps you seed the gap correctly.

It's a planning estimate based on typical incorporated-at-flowering values; actual N depends on the biomass grown, the species, soil and how well it's incorporated. Treat it as a reliable guide for cutting fertiliser, and confirm with a soil test where precision matters.