Legumes are plants in the family Fabaceae, and they have a trick most crops lack: they can pull nitrogen out of the air. Bacteria living in nodules on their roots convert atmospheric nitrogen into a form the plant can use, and when the plant dies, the nitrogen stays in the soil for the next crop.

The family is enormous and includes beans, peas, lentils, chickpeas, soybeans, peanuts, alfalfa, clover, and lupins. Some are grown for food, others for forage or green manure. The soybean is now one of the most traded agricultural commodities in the world, feeding both livestock and people.

Why farmers rotate them

• They leave nitrogen behind for cereals the following season
• Their deep roots improve soil structure
• They break disease cycles that build up under continuous cereals
• They are high in protein, both for humans and for animals

Legumes are not free nitrogen. They still need phosphorus, potassium, and the right rhizobia strain in the soil. But in a well-managed rotation, they cut fertiliser bills and improve the ground for the crops that follow.