Green Manure and Rice: Choose the Right Green Manure Crop

Backgrounder

A green manure crop is a crop grown especially to provide nitrogen for the rice crop and to add organic matter to the soil.  It is planted before or after rice when the land is vacant.  Then it is ploughed into the soil as fertilizer for the rice.

So growing a green manure crop is a good way to provide nitrogen for your rice crop.  But how do you choose the best green manure crop?

To be profitable, the green manure should suit the growing conditions –the climate, soil, and water available in your area.  It should be easy and cheap to grow.  That is, you should easily be able to get good seed that germinates well.  It should not need extra irrigation, fertilizer, or pesticides.  And its growing season must fit the time available between rice crops.  That is, it must not delay planting of the main crop.

Here are some examples of common green manure crops and the conditions in which they grow best.  You may find others that do well in your area.

Irrigated lowlands: Irrigated lowlands are usually planted with two, even three crops a year.  So the time for growing a green manure crop is short – about 60 to 65 days.

In fertile, well-drained soil, fast-growing food legumes such as mung beans or cowpeas can be grown before rice.  Or you can grow black gram varieties that mature in 45 to 70 days.

Crops that take longer to mature can be planted after the rice harvest.  You might think of cool-season food legumes — soybean, faba bean, kidney bean — or forage legumes such as Egyptian clover or sweet clover.  Three to four tons of clover mass per hectare will add about 80 kilograms of nitrogen per hectare to the soil for the next rice crop.

 Sesbania rostrata, a plant that grows as a weed in western Africa, makes excellent green manure.  Sesbania can grow even on waterlogged soils.  Try this crop if you can get seed because it can add as much as 120 kilograms per hectare of nitrogen to the soil!

Rainfed lowlands: In rainfed areas, because the water supply is uncertain, green manures must be able to tolerate drought and, sometimes, waterlogging as well.

Cowpea, pigeonpea, lablab bean, or rice bean can be grown on well-drained soil for 5-7 weeks for green manure and ploughed into the soil before rice seedlings are transplanted.  But if you want to harvest the seed from these crops for good high-protein food for your family, plant them after rice and let them grow to maturity.  After you have harvested the pods, you can use the leftover crop residue for green manure.

Some forage legumes to grow after rice might be siratro, stylo, or hairy vetch.  These can be cut several times for animal feed before they are plowed in for green manure.

Upland areas:  Green manures are especially useful for improving poor upland soils which are often acid or easily eroded. Cluster bean, lima bean, and hyacinth bean are all food legumes that can stand drought and grow on poor soils.  Velvet bean, centro, and puero are fodder crops that can also stand drought and provide animal feed in the dry season, as well as green manure.  Their long, creeping vines cover and protect the soil.

Many perennial legume trees and shrubs, such as gliricidia, leucaena, acacia, and calliandra, can be grown in hedgerows or on field bunds in upland areas.  Leaves and twigs from these are cut and used for green manure.