Which Companion Plants Fix Nitrogen in the Soil?

Nitrogen-fixing plants are one of the most genuinely useful categories in companion planting — they actively add fertility to the soil rather than simply deterring pests or attracting insects. They do this through a symbiotic relationship with Rhizobium bacteria in their root nodules: the bacteria convert atmospheric nitrogen (which plants cannot use directly) into ammonium (which they can), in exchange for sugars from the plant. Growing nitrogen-fixing plants in or around the vegetable garden reduces the amount of fertiliser needed and improves long-term soil health.

Legumes as nitrogen fixers

All members of the legume family (Fabaceae) have the potential to fix nitrogen — this includes garden beans (French and runner beans), peas, broad beans, clovers, vetches, and lupins. The amount fixed varies by species and by growing conditions: clovers and vetches fix more nitrogen per season than most annual vegetable legumes. After a bean or pea crop, leaving the roots in the soil and digging in the stems provides a meaningful nitrogen boost to the next crop grown in that bed.

Cover crops and green manures

Red clover, white clover, trefoil, and winter tares (Vicia sativa) are commonly grown as nitrogen-fixing green manures or cover crops between vegetable crop cycles. Sow them in an empty bed in late summer or after a main crop finishes, allow them to grow through autumn, then dig them into the soil in spring before the next crop goes in. The decomposing foliage and roots release their nitrogen slowly over the following weeks, providing a steady supply for the next crop.

Alder and other nitrogen-fixing trees

Several trees and shrubs fix nitrogen — notably alders (Alnus species), elaeagnus, and Siberian pea shrub (Caragana arborescens). These are most relevant in forest garden designs or large permaculture systems where nitrogen-fixing trees are planted as support species among fruit trees. In a smaller garden, growing clovers as living mulches under fruit trees provides a similar benefit at a smaller scale.