Sunn hemp green manure in a home bed: when to dig it in and how long to wait
Sunn hemp is a nitrogen-fixing legume. Grow it 45-60 days, chop it in at flowering, wait 10-15 days, then plant. Seed and nitrogen worked out for a small bed.
- Green manure means growing a plant and chopping it into the soil while still green; sunn hemp is popular because it is a legume whose root nodules hold nitrogen-fixing rhizobia.
- Sunn hemp starts flowering at about 45-50 days. Chop it in from first flowers to about half the flowers open; later than that the stems turn woody and break down slowly.
- After digging it in, wait about 10-15 days before planting so the material can break down and release nutrients.
- By Land Development Department figures one rai of sunn hemp gives nitrogen equal to about 30 kg of urea, so a 4 m² bed gets roughly 35 g of nitrogen.
Sunn hemp (ปอเทือง, Crotalaria juncea) is a legume grown to be chopped back into the soil rather than eaten, which is what “green manure” means. Grow it for about 45-60 days, chop it in as it starts to flower, wait 10-15 days, then plant your vegetables. The soil gains both nitrogen and organic matter.
Thailand’s Land Development Department (LDD) recommends this for rice fields and farms, but its per-rai figures scale down to a backyard bed. This lesson works out, step by step, how much seed a small bed needs, what you get back, and how long to wait.
What green manure is, and why sunn hemp
Green manure is organic fertiliser made by ploughing or chopping plants into the soil while they are still green, letting them break down, and then planting the main crop. LDD favours legumes because they are easy to grow, fast, and carry rhizobia in their root nodules: bacteria that fix nitrogen from the air into a form plants can use.
Sunn hemp stands out for fast growth and a heavy fresh yield, and it suits upland ground or low ground that does not stay flooded. The benefits LDD lists are more nutrients, more organic matter, better water holding, looser soil and less erosion.
Sunn hemp by the numbers (LDD)
| Item | Value |
|---|---|
| Seed rate | 5 kg per rai (some documents say 3-5 kg) |
| Days to flowering | 45-50 |
| Fresh weight | about 2,500 kg per rai (another document gives 2-6 tonnes) |
| Dry weight | about 500 kg per rai (dry ≈ fresh ÷ 5) |
| Nitrogen | equal to about 30 kg of urea per rai |
| Organic matter | up about 0.15 percentage points per rai turned in |
One rai is 1,600 m². Nutrient figures for the dry plant differ between documents: one gives 2.76% nitrogen and 2.40% potassium, another about 1.98/0.30/2.41% N/P/K. The US Department of Agriculture’s Agricultural Research Service puts sunn hemp at up to about 120 lb of nitrogen per acre in 2-3 months, roughly 21 kg per rai. Real numbers depend on soil, season and how old the crop is when you turn it in.
Timing: sow, chop in, wait
- Day 0. Sow into moist soil. If the soil is very acid, LDD advises liming before sowing; see how to test and adjust in soil pH at home.
- Around day 45-50. Flowering begins, and this is the time to chop it in. LDD says to turn it in at about 50% bloom, when nutrients and fresh weight peak and the stems are not yet hard.
- Chop and dig in. Cut the stems, chop them short, and mix them into the topsoil.
- Wait 10-15 days for it to break down and release nutrients. LDD documents give anything from 7 to 15 days; in a home bed, lean towards the longer end.
- Around day 60-65. Plant your next vegetables.
Chop it in as it starts to flower, wait 10-15 days, then plant. Left too long, the stems turn hard, are hard to dig in, and break down more slowly.
Worked example: a 1 × 4 m bed
A 1 × 4 m bed is 4 m², and 1 rai is 1,600 m².
- Seed: 5 kg ÷ 1,600 = 3.1 g per m² × 4 = about 12.5 g
- Fresh weight you can expect: 2,500 kg ÷ 1,600 = 1.56 kg per m² × 4 = about 6.3 kg
- Dry weight: 6.3 ÷ 5 = about 1.25 kg
- Nitrogen: 1.25 kg × 2.76% = about 35 g
- Cross-check: 30 kg urea per rai ÷ 1,600 × 4 = 75 g of urea; urea is 46% nitrogen, so 75 × 0.46 = about 35 g. The two agree.
Using the USDA figure (about 21 kg of nitrogen per rai) instead gives about 54 g. So a reasonable expectation for a 4 m² bed is roughly 35-55 g of nitrogen, plus several kilograms of fresh material that becomes organic matter.
Compared with other green manures
| Crop | Seed (kg/rai) | Flowering (days) | Fresh weight (kg/rai) | Nitrogen as urea (kg/rai) | Suits |
|---|---|---|---|---|---|
| Sunn hemp | 5 | 45-50 | 2,500 | 30 | upland, low ground without standing water |
| Jack bean | 10 | 50 | 2,500 | 29.6 | upland, low ground without standing water |
| Cowpea | 8 | 40 | 1,500 | 17.5 | upland, low ground without standing water |
| Sesbania rostrata | 5 | 50 | 2,000 | 25 | tolerates flooding and moderate salinity |
| Pigeon pea | 5 | 60 | 2,000 | 20.4 | upland |
Figures from LDD’s soil improvement research group. For a bed that floods often in the rainy season, Sesbania rostrata suits better than sunn hemp.
Common mistakes
- Letting it get too old. The stems turn hard, are tough to chop and dig in, and need longer to break down.
- Planting straight after digging in. The material has not broken down yet; wait at least about 10 days.
- Sowing where water stands. Sunn hemp wants ground that does not flood. Raise the bed or choose Sesbania rostrata.
- Expecting one round to fix the soil for good. Organic matter rises about 0.15 percentage points per round, so repeat it or pair it with compost; see composting kitchen scraps.
Sources
- Improving soil simply with green manure (Thai) — Soil Improvement Research Group, Land Development Department
- Building a fertiliser factory in the field: green manure (Thai) — Suphan Buri Land Development Station
- Using LDD biotechnology products, green manure section (Thai) — Suphan Buri Land Development Station
- Sunn Hemp fact sheet — USDA Agricultural Research Service
FAQ
I have no tractor. Does this work in a small bed?
Yes. The Land Development Department describes cutting and chopping the plants and then working them into the soil. In a home bed, cut the stems with shears or a knife, chop them short, and mix them into the top layer of soil with a hoe or fork.
Can I grow sunn hemp in pots?
You can, but it is hardly worth it. It grows tall and needs room, and suits an empty bed between crops. For pots, compost or worm castings are easier.
When is the best time to sow it?
LDD documents suggest the start of the rainy season, right after a harvest while the soil is still moist, or about 3 months before the main crop. Sunn hemp suits upland ground or low ground that does not stay flooded.
General information, not medical advice. If you are pregnant, take regular medication, or have a liver or kidney condition, ask a doctor or pharmacist before taking herbs beyond normal food amounts.