Legume Group in Crop Rotation: Peas, Beans and Nitrogen

Legumes are the most generous members of any vegetable garden rotation. While other crop groups take nutrients from the soil, legumes put them back. Understanding this group — which plants belong to it, what they need, and what they leave behind — will help you use them strategically to reduce fertiliser inputs and improve the fertility of your entire growing space.

Which Crops Belong in the Legume Group

The legume group in a kitchen garden rotation includes all peas (garden peas, mangetout, sugar snap, petit pois), runner beans, French beans (climbing and dwarf), broad beans, and borlotti or haricot beans. Green manure legumes such as crimson clover, winter tares, or field beans also belong in this group if you use them in the rotation bed between crops. All of these plants form symbiotic relationships with Rhizobium bacteria in their root nodules, fixing atmospheric nitrogen into a form that plants can use.

How Nitrogen Fixation Works in Practice

When you pull up spent bean or pea plants at the end of the season, leave the roots in the ground. The nodules on the roots release their stored nitrogen directly into the soil as they decompose, feeding the following crop. The top growth — stems and leaves — can be chopped and dug in or composted. A well-grown legume crop can fix between 50 and 200 kilograms of nitrogen per hectare, and even a small garden plot will receive a meaningful boost. The nitrogen becomes available gradually over the following months, providing a slow feed for the next group in the rotation.

What Legumes Need to Grow Well

Legumes are relatively undemanding compared to brassicas or potatoes. They prefer a well-drained, moisture-retentive soil with a pH between 6.0 and 7.0. They do not need heavy nitrogen fertilisation — applying excess nitrogen before planting can actually discourage nodule formation as the plants have less incentive to form the bacterial partnership. A light dressing of compost or a balanced organic fertiliser at planting time is usually sufficient. Phosphorus and potassium are more important for legumes than nitrogen; bone meal or wood ash can supplement these if your soil is poor.

Support is the main practical requirement for climbing peas and beans. Install canes, netting, or a frame before the plants need it, as disturbing the soil around established plants damages the shallow root system and can break the tender nodule-bearing roots that the nitrogen-fixing bacteria depend on.

Where Legumes Sit in the Four-Year Cycle

In the standard four-year rotation, legumes typically follow brassicas, which are heavy nitrogen consumers that leave the soil depleted. Legumes restore this nitrogen over the growing season, and the residues they leave in the ground continue to release nitrogen through the winter. The group that follows legumes in the rotation — often roots and alliums — inherits this improved fertility without needing extra fertiliser. This nutrient handoff is one of the most elegant and economical aspects of a well-structured rotation: each group feeds the next with minimal outside input.