Legumes

Lessons

Senior 2 · Agriculture

Legumes

4 lessons

Lesson 3 of 4

Growing Legumes: Propagation and Field Practices

24 min

Let's begin

Rwanda has two main growing seasons: Season A, planted with the rains from about September, and Season B, planted from about February–March. Beans are planted in both, often in the same fields as maize, bananas or cassava. Whether a family harvests one sack or three depends largely on a handful of field practices done at the right time.

Propagation

Legume crops are propagated by seed. Good seed is the cheapest way to raise yield:

Use certified seed of an improved variety suited to the area (bush or climbing, high-iron, disease-resistant), or select your own seed carefully.
Seed should be mature, clean, of one variety, plump and undamaged, with no weevil holes, mould or wrinkles, and with good germination (test 100 seeds in moist cloth first).
Seed dressing with an approved fungicide or insecticide protects seedlings from soil-borne diseases and bean fly.
Inoculation: coating seed with the right Rhizobium culture just before planting ensures good nodulation, especially for soybean in fields where it has not been grown before. Inoculated seed must be kept out of the sun and planted the same day.

Field practices

Land preparation: dig or plough early, remove perennial weeds, and prepare a fine, firm seedbed. On slopes, prepare along the contour.
Time of planting: plant at the onset of the rains so the crop grows while moisture is available and matures in drier weather. Late planting raises disease and pest attack.
Spacing and seed rate: typical examples are bush beans 40 cm × 10 cm, soybean 50 cm × 5 cm, groundnut 45 cm × 15 cm, all one seed per hole. Follow local recommendations.
Manure and fertiliser: apply well-rotted manure or compost and a phosphorus fertiliser such as DAP at planting. Phosphorus helps nodules to form; legumes need little extra nitrogen. Groundnuts need calcium for filling pods, so gypsum or lime may be added on poor soils.
Staking climbing beans: put in stakes about 2 m long (or strings on a trellis) soon after emergence so the vines can climb. Without support they tangle, rot and yield poorly.
Weeding: weed two or three times, starting early, when the crop is most sensitive to competition. Avoid working in bean fields when leaves are wet, since this spreads diseases from plant to plant.
Earthing up groundnuts: at the start of flowering, loosen and gently heap soil at the base so the pegs (the stalks that push the young pods into the ground) can enter easily; avoid disturbing plants after pegging.
Pest and disease control (see below), using resistant varieties, clean seed, rotation and timely planting first.
Crop rotation and intercropping: rotate legumes with cereals; do not plant beans after beans on the same plot season after season.
Harvesting at the right stage: beans when most pods are dry and yellow-brown; soybean when most leaves have dropped and pods are brown, before pods shatter; groundnuts when leaves yellow and the inside of the shell has darkened.

A farmer in Nyagatare, Eastern Province, pushes stakes into the soil to support his young climbing beans. The stakes lean together at the top in small groups, which makes them sturdier. Photo: Neil Palmer (CIAT), CC BY-SA 2.0, via Wikimedia Commons

Common pests and diseases

Problem	Crop	Signs	Control
Bean fly (bean stem maggot)	Beans	Young plants wilt and die; swollen, cracked stem base with maggots	Seed dressing; early planting; earthing up; tolerant varieties
Aphids	Beans, groundnut	Clusters of small insects on shoots; spread viruses	Early, close planting of groundnuts; natural enemies; sprays if severe
Anthracnose, angular leaf spot	Beans	Dark sunken spots on pods; angular brown spots on leaves	Clean certified seed; resistant varieties; rotation; burn infected residues
Bean rust	Beans	Small reddish-brown powdery spots on leaves	Resistant varieties; rotation; fungicides if severe
Root rots	Beans	Yellowing, wilting, rotten dark roots	Well-drained soil; manure; rotation; tolerant varieties (climbing beans are often more tolerant)
Rosette virus	Groundnut	Stunted, bushy plants with yellow or mottled leaves	Resistant varieties; early and close planting to deter aphids
Bruchids (weevils)	Stored grain	Round holes in seeds; powder	Dry grain well; hermetic bags; clean stores (lesson 4)

Worked example: seed needed

Question: Gatete plants bush beans at 40 cm × 10 cm with one seed per hole. (a) What is the plant population per hectare? (b) If 100 seeds of his variety weigh 40 g, what mass of seed does he need per hectare? (c) His seed germinates at 90%. How much should he buy?

(a) Area per plant = 0.4 m × 0.1 m = 0.04 m². Population = 10,000 ÷ 0.04 = 250,000 plants/ha.

(b) One seed weighs 40 ÷ 100 = 0.4 g. Seed mass = 250,000 × 0.4 g = 100,000 g = 100 kg/ha.

(c) Only 90% will grow, so he needs 100 ÷ 0.9 ≈ 111 kg/ha.

Tip

Memory hook: for legumes, "P before N": they need phosphorus to build nodules, and the nodules then supply most of the nitrogen.

Exam technique: in population questions convert centimetres to metres FIRST (40 cm = 0.4 m), then divide 10,000 m² by the area per plant.

✦

Practice

Give four qualities of good bean seed for planting.

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Solution: Any four: certified or of a known improved variety; mature and plump; clean and of one variety; free of damage, weevil holes and mould; high germination.

Why is soybean seed often inoculated before planting in a new field?

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Solution: The specific Rhizobium strain that nodulates soybean may be absent from soils where soybean has never grown; inoculation supplies it so the crop can fix nitrogen.

Calculate the plant population of soybean per hectare at 50 cm × 5 cm, one seed per hole.

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Solution: 0.5 × 0.05 = 0.025 m² per plant. 10,000 ÷ 0.025 = 400,000 plants/ha.

Why should farmers avoid working in bean fields when the leaves are wet?

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Solution: Bacteria and fungal spores of diseases such as blight and anthracnose spread easily in water films on leaves and on clothes and hands moving between plants.

Explain why soybean must be harvested promptly once mature.

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Solution: Dry soybean pods shatter (split open) and scatter their seeds on the ground, so a late harvest loses yield.

Summary

Legumes are propagated by seed: use certified, clean, healthy seed; dress it; inoculate soybean with Rhizobium.
Key practices: early land preparation, planting at the onset of rains, correct spacing, manure and phosphorus at planting, staking climbing beans, timely weeding, earthing up groundnuts, rotation, and harvest at the right stage.
Main problems include bean fly, aphids, anthracnose, rust, root rots, groundnut rosette and storage bruchids.
Population per hectare = 10,000 ÷ (row spacing × plant spacing, in metres).

Sources and credits

Legume (section: Pests and diseases), Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Peanut (section: Cultivation), Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Phaseolus vulgaris, Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Climbing beans1 lo (4108187001).jpg, Neil Palmer (CIAT), Wikimedia Commons, licensed under CC BY-SA 2.0.
The field-practice guidance, the pest and disease table, the seed calculation and the practice questions are written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Propagation

2Field practices

3Common pests and diseases

Worked example: seed needed

Practice

Propagation

Field practices

Common pests and diseases