Leguminous Crops – SS2 Horticulture and Crop Production Lesson Note (Third Term)

Leguminous crops (legumes) are plants that produce their seeds in a pod and carry nitrogen-fixing bacteria in root nodules.

• Main Nigerian legumes: cowpea (beans), soybean and groundnut.
• Key feature: bacteria in the root nodules take nitrogen from the air and give it to the plant.

Behavioural objectives

1. Explain what leguminous crops are and the role of root nodules.
2. Describe how cowpea, soybean and groundnut are planted, cared for and harvested.
3. State the major use and the major pest or disease of each crop.
4. List the advantages of legumes on the farm.
5. Describe how legume seeds should be stored to keep out weevils.

What are leguminous crops?

Leguminous crops are plants of the pod-bearing (legume) family that have nodules on their roots.

• Pod: the case in which the seeds grow, for example a bean pod.
• Nodules: tiny bumps on the roots that house friendly bacteria (Rhizobium).
• Nitrogen: the plant nutrient the bacteria take from the air for the plant.
• Big Three in Nigeria: cowpea (beans), soybean and groundnut.

What are the types of legumes grown on the farm?

There are two groups of legumes: grain legumes and forage or cover legumes.

• Grain legumes: grown for their seeds as human food, such as cowpea, soybean, groundnut, bambara groundnut and pigeon pea.
• Forage and cover legumes: grown to feed animals or cover the soil, such as Centrosema and Stylosanthes.

How do root nodules supply nitrogen to legumes?

Root nodules are small swellings on legume roots in which bacteria change nitrogen from the air into a form the plant can use.

1. Rhizobium bacteria in the soil enter the young roots of the legume.
2. The roots swell and form nodules around the bacteria.
3. The bacteria take nitrogen gas from the air spaces in the soil.
4. They change it into nitrogen compounds that the plant can use.
5. The plant gives the bacteria food in return, so both benefit.
6. When the roots and plant remains rot, extra nitrogen is left in the soil.

How are the main legumes grown?

Each of the Big Three legumes needs its own soil, planting method, special care and harvesting sign.

How is cowpea (beans) grown?

Cowpea is the most common legume in Nigerian markets and is called the “meat of the poor” because it gives much protein at a lower price than beef.

• Land preparation: clear the land and make ridges.
• Soil: well-drained soil is best. Cowpea does badly in waterlogged soil (“wet feet”).
• Planting: plant 2 to 3 seeds per hole at a depth of about 3 cm.
• Spacing: 75 cm between rows and 25 cm between plants.
• Special care: cowpea attracts many insects. Spray in time to stop the Maruca pod borer from eating the flowers.
• Harvesting: pick the pods by hand when they turn yellow or brown and feel dry and brittle.

How is soybean grown?

Soybean is the “industrial legume” because it is used to make vegetable oil, soya milk and feed for chickens and pigs.

• Land preparation: same as cowpea.
• Soil: soybean needs a little more phosphorus to grow well.
• Planting: sow in rows. The seeds are small, so do not bury them too deep.
• Weeding: soybean grows slowly at first. Weed early, or grass will grow over it and shade it from the sun.
• Harvesting: harvest as soon as the pods turn brown. If you wait too long, the pods shatter (burst open) and the seeds fall on the ground and are lost.

How is groundnut (peanut) grown?

Groundnut is a legume whose pods grow under the soil, not on the branches.

• Soil: sandy-loam soil is needed. In hard or clayey soil the pegs (the parts that become nuts) cannot push into the ground.
• Planting: shell the seeds carefully so that the thin red skin is not damaged. Plant one seed per hole.
• Pegging stage: after the yellow flowers fall, the plant sends pegs down into the soil. Do not disturb the soil at this stage.
• Harvesting: pull the whole plant out when the leaves turn yellow and the inside of the shells shows dark brown markings.

How do cowpea, soybean and groundnut compare?

The table below compares the three crops by pod position, soil need, use and main pest or disease.

Why are legumes useful on the farm?

Legumes improve the soil, feed people and protect the ground.

1. Soil healing: legumes add nitrogen to the soil through their root nodules. Maize planted where beans grew last year grows green and strong.
2. Human health: they are the best plant source of protein for building muscles and repairing the body.
3. Cover cropping: spreading legumes such as cowpea cover the ground like a carpet. This shades the soil from the sun and reduces erosion.

How should legumes be stored?

Legumes are stored dry in airtight containers to keep out weevils, their biggest enemy.

1. Dry the seeds until they are very hard.
2. Pour them into airtight containers, such as plastic drums with tight lids, or into PICS bags (triple-layered bags).
3. Close the container tightly so that insects cannot enter.

• Warning: never store wet beans, because they grow mould and smell bad.

What classroom demonstration can the teacher do?

This demonstration lets students see root nodules for themselves.

1. Carefully pull a young bean or groundnut plant from the school garden, with the roots whole.
2. Wash the roots gently in a bucket of water.
3. Show students the small round nodules on the roots.
4. Tell them: “These are the plant’s nitrogen factories.”

Evaluation

1. What are nodules, and what do the bacteria inside them do?
2. Name the “Big Three” legumes grown in Nigeria.
3. Why must soybeans be harvested as soon as the pods turn brown?
4. What soil type do groundnuts need, and why?
5. State two ways of keeping weevils out of stored beans.

Assignment

1. Copy the comparison table into your notebook. For each crop, add one sentence on how it is harvested.
2. List the three advantages of legumes on the farm and explain one of them.