Green Manure Calculator & Seed & Free Nitrogen
Sows dhaincha
Plan a green-manure cover crop — pick a legume and your area to get the seed to sow, the nitrogen it adds when ploughed in, and the urea it replaces.
Enter your field
Next: sow the cover crop, grow it 6–8 weeks, and plough it in at flowering before your main crop — the decomposing biomass releases nitrogen right where the next crop needs it.
N added is an incorporated-at-flowering estimate; green manure also improves soil structure, water-holding and microbial life.
Green manure — key facts
- Dhaincha seed
- ≈ 25 kg/ha
- Sunhemp seed
- ≈ 35 kg/ha
- N added
- ≈ 40–100 kg/ha
- Incorporate at
- early flowering (6–8 wk)
- Wait before main crop
- 1–2 weeks
- Also adds
- organic matter, structure
- Supplies
- mainly N (not much P/K)
- Privacy
- Runs in your browser; nothing uploaded
Grow your own nitrogen
A green-manure legume captures nitrogen from the air and, when ploughed back in, hands it to your next crop — free fertiliser plus a big dose of organic matter that improves soil structure, water-holding and biology. Grown in a fallow or the gap before the main crop, it turns idle land and moisture into fertility instead of leaving the soil bare.
This tool turns your chosen crop and area into the seed to sow, the nitrogen it adds, and the urea that replaces, with seed cost. Sow the seed rate shown, grow it for 6–8 weeks, and incorporate at early flowering into moist soil for the most nitrogen — then wait a week or two before sowing the main crop. Remember it mainly supplies nitrogen, so top up phosphorus and potassium. Pair it with the Fertilizer (NPK), Biofertilizer and Seed Rate tools.
Free nitrogen
See the N your cover crop adds and the urea it saves.
Seed it right
The exact seed to sow for your area and chosen legume.
Build the soil
Add organic matter and structure, not just nutrients.
Use the fallow
Turn idle land and moisture into next season's fertility.
Frequently Asked Questions
What is green manure?+
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.
How much green-manure seed do I need?+
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.
How much nitrogen does green manure add?+
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.
When should I incorporate green manure?+
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.
Which green-manure crop should I grow?+
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.
Does green manure replace fertiliser?+
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.
How does a legume add nitrogen for free?+
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.
Does green manure improve the soil beyond nitrogen?+
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.
Can I use a green manure between main crops?+
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.
How accurate is the nitrogen estimate?+
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.