Improve Rice Yields Without Buying Fertilizer

Backgrounder

What is a green manure crop?
A green manure is a crop grown especially to add nitrogen to the soil. It is planted before or after rice, when the land is vacant. Then it is ploughed into the soil, while still green, as fertilizer. Green manure crops can:

• Provide free nitrogen
• Reduce fertilizer costs
• Reduce pest problems
• Increase crop diversity

How to choose the right green manure crop

If you want to grow a green manure crop to provide nitrogen for your rice crop, you will have to put some thought into selecting the right one. To be profitable, the green manure should meet the following conditions:

• Adds nitrogen to the soil — legumes (Leguminosae family) are best.
• Fast growing (it must fit the time available between rice crops so it doesn’t delay planting of rice).
• Suits the local growing conditions — the climate, soil, and water available.
• Seed is inexpensive and easy to get.
• Doesn’t need extra irrigation, fertilizer or pesticides.
• Resistant to insects and diseases.
• Has many uses (adds nitrogen, useful as food or fodder).

Before choosing a green manure crop, you will have to know what green manures are available to you and what characteristics they have. A farmer should be able to grow a green manure crop easily and without spending extra money on irrigation, fertilizers, or pesticides. Again, remember that a green manure crop should suit the soil, rainfall and climate in the local area.

Legumes are the best green manure crops. This is because legumes can fix nitrogen from the air and make it available to the next crop if the legume is mixed into the soil.

How to grow a green manure crop

• Plant the green manure before or after rice, when the land is vacant.
• Let it grow until it is time to prepare the land for the next rice crop.
• Plough the green manure crop into the soil before sowing the next rice crop.
• If you do this regularly, you will get free nitrogen for your rice crop, and save money on fertilizer.

Once you have chosen a green manure crop you should be able to grow it with the least possible labour and cost. Here are some further points to remember.

Use zero tillage. That is, do not plough or till the field before planting. This saves time and labour. However if your soil lacks phosphorus, you may have to add some before planting. Legumes do not need added nitrogen.

When the field is ready, just broadcast the seed. Use enough seed for a dense, close planting. This will help the green manure build up nitrogen quickly. If the field is thinly planted, the crop will have to grow for a longer period to add enough nitrogen to help the next crop.

Ordinarily, the green manure should need no extra irrigation but be able to grow on moisture already in the soil. But if the soil is very
dry, irrigate once after sowing to help the seed sprout well.

Do not use any pesticides on the green manure crop as this adds to its cost. If you have chosen the crop carefully, it will not be attacked by insects and diseases in your area. If you do find a problem, try a different crop next time.

Before planting the next rice crop, chop the green manure and mix it into the soil. Do this before the plants become woody and hard.
You can use an animal-drawn plough or harrow to mix in the green manure.

Benefits of green manure crops
Some green manure crops can add about 55 to 60 kilograms of nitrogen per hectare to the soil in 6 to 7 weeks. This is like getting 55 to 60 kilograms of nitrogen fertilizer almost free of cost. Some legumes add much more. Because legume roots grow deep, they also bring up other nutrients from deep down in the soil where rice roots cannot reach. These nutrients will feed the next rice crop.

Insect pests and diseases that attack legumes usually do not attack rice, so you will get a healthier rice crop, with fewer diseases and pests, when you grow it after a legume crop.

There are no transportation problems  with green manure fertilizers, in contrast to either compost or chemical fertilizers, because the green manure grows in the place where it is needed. And once you’ve bought the seed you don’t have any extra costs.

Potential problems with green manure crops
Sometimes green manure crops are difficult to establish. They are perceived as requiring extra work by the farmer. And if the crop is not carefully chosen it may become aggressive and difficult to eliminate from the farm.

Availability of seeds can also be a problem.  This can easily be solved by growing green manure crops on waste lands to produce seeds.

However, many farmers have learned to overcome these challenges. Although it is true that it can be a lot of work, it is usually worth the effort. Talk to other farmers about how they have managed their green manure crops.

Green manures for irrigated lowlands, rainfed lowlands and upland areas
Following are some examples of common green manure plants for irrigated lowlands, rainfed lowlands and upland areas. Keep in mind they are only examples — you will find many 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. Consider 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.  Food legumes do not provide a lot of nitrogen because they are harvested.

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.

You can plant cowpea, pigeonpea, lablab bean, or rice bean 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 are 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.