Legumes
Senior 2 · Agriculture
Legumes
Lesson 2 of 4
18 min
Let's begin
Dig up a healthy bean or soybean plant carefully and wash its roots. You will see small round lumps clinging to them. Cut one open: if it is pink or red inside, it is busy turning nitrogen from the air into fertiliser for the plant. No other major crop family can do this, and it is one of the main reasons legumes matter so much to farmers who cannot afford much fertiliser.
Air is about 78% nitrogen gas (N₂), but plants cannot use N₂ directly. Nitrogen fixation is the conversion of N₂ into ammonia (NH₃), a form plants can use. In legumes it is done by Rhizobium bacteria living in the root nodules:
N₂ + 8H⁺ + 8e⁻ → 2NH₃ + H₂
The relationship is a symbiosis (both partners benefit): the plant gives the bacteria sugars and a protected home; the bacteria give the plant nitrogen. That is why legume seeds are so rich in protein, since nitrogen is a building block of proteins.
When the crop residues (roots, leaves, stems) rot after harvest, their nitrogen is released into the soil for the next crop.
Signs of effective nodules: many nodules near the top of the root system, and a pink to red colour inside (from a pigment called leghaemoglobin). White, green or grey nodules are not fixing much nitrogen.
Nodulation is poor when: the right Rhizobium strain is absent from the soil (common for soybean where it has never been grown), the soil is very acid or waterlogged, phosphorus is lacking, or large doses of nitrogen fertiliser are applied (the plant then "switches off" fixation).
Question: Suppose a good soybean crop leaves residues that release about 40 kg of nitrogen per hectare to the following maize crop. (a) How much urea (46% N) would supply the same nitrogen? (b) If urea costs 900 Frw per kg, what is that nitrogen worth per hectare?
(a) Urea = 40 ÷ 0.46 ≈ 86.96, so about 87 kg of urea.
(b) 87 kg × 900 Frw/kg = 78,300 Frw per hectare saved, in addition to the value of the soybean harvest itself. (The price is illustrative; use current local prices in real planning.)
Tip
Memory hook: "Pink means it's working." Cut a nodule: pink or red inside means active nitrogen fixation.
Exam technique: when asked how legumes improve soil fertility, mention BOTH nitrogen fixation by Rhizobium in root nodules AND the organic matter added by their residues.
What is nitrogen fixation, and which organism carries it out in legumes?
Solution: The conversion of nitrogen gas from the air into ammonia, a form plants can use. In legumes it is done by Rhizobium bacteria in the root nodules.
Explain why the relationship between a bean plant and Rhizobium is called symbiosis.
Solution: Both partners benefit: the plant supplies sugars and shelter; the bacteria supply fixed nitrogen.
A farmer uproots soybean plants and finds very few nodules, all white inside. Suggest two possible causes.
Solution: Any two: the right Rhizobium strain is absent (the seed was not inoculated); the soil is very acid or waterlogged; phosphorus is lacking; heavy nitrogen fertiliser was applied.
Give four reasons why legumes are important in Rwandan farming.
Solution: Any four: protein-rich food; nitrogen fixation improves soil; useful in rotation and intercropping; cover crops reduce erosion; livestock feed; income and raw materials for industry.
How much urea would supply 23 kg of nitrogen?
Solution: 23 ÷ 0.46 = 50 kg of urea.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
