Legume cover crops

Benefits and considerations

• Legumes are broad-leaved plants that fix nitrogen, which raises soil fertility
• The amount of nitrogen fixed depends on the species, growth and temperature
• When used as an overwinter cover crop legumes fix only a low level of nitrogen
• Legume roots help improve soil structure; benefits depend on species, field conditions and cover crop duration
• Legumes tend to be slower growing than brassicas; for autumn use, they often need to be sown earlier (late July to August) to aid growth and promote nitrogen fixation
• Sowing and establishment of small-seeded legumes require careful planning
• Many crops have rotational conflicts with legumes, especially where other legumes and pulses are in the rotation

Legume cover crop examples

Biennial and perennial legumes

Alfalfa: See Lucerne.

Crown vetch: Adapted only to well-drained soils. It can be grown under lower-fertility conditions. Although it can provide permanent ground cover, it establishes slowly and should only be used for perennial cover.

Red clover: Vigorous, shade-tolerant, winter-hardy, and can be established relatively quickly. It is commonly inter-seeded with small grains. Because it starts growing slowly, the competition between it and the small grain crop is not usually significant.

Sweet clover: A reasonably winter-hardy and vigorous crop that can help break up compacted subsoils. It can withstand high temperatures and droughty conditions better than many other cover crops. It does poorly in wet, clayey soils. It requires a near-neutral soil pH and a high calcium level. However, if the pH is high, sweet clover can grow well on low-fertility soils. It is sometimes grown for a year or more since it flowers and completes its life cycle in the second year. When used as a green manure crop, it is incorporated into the soil before full bloom.

• Sowing (autumn): August
• Example sowing rates kg/ha (as a single species): 10–15

White clover: Mainly used in fertility building. It does not produce as much growth as many of the other legumes and is less tolerant of droughty situations. Although it does not grow very tall, it can tolerate shading better than many other legumes.

• Sowing (autumn): August
• Example sowing rates kg/ha (as a single species): 10–15

Lucerne (alfalfa): A good choice for well-drained, highly fertile soils that are near pH neutral. It is faster growing than some clovers. It can be grown in a rotation for several years, helping to improve soil structure. Alfalfa is commonly inter-seeded with small grains – such as oats, wheat and barley – and it continues to grow after the grain is harvested.

• Sowing (autumn): August
• Example sowing rates kg/ha (as a single species): 20

Sainfoin: A long-term perennial species which can last more than four years. It helps build nutrients in low-nutrient soils and is often used in grazing mixes. Does well in poor nutrient soils and in drought conditions.

• Sowing (autumn): August
• Example sowing rates kg/ha (as a single species): 70

Winter annual legumes

Peas: Tend to establish quickly and proliferate in a cool, moist climate, producing a significant amount of residue. Mainly used in fertility building, as part of mixtures or single species. Better suited to later sowing than many legumes.

• Sowing (autumn): Late August to mid-September
• Example sowing rates kg/ha (as a single species): 200–400

Beans: Mainly used in fertility building as a single species or part of mixtures. The crop can be sown later than many legumes.

• Sowing (autumn): Late August to September
• Example sowing rates kg/ha (as a single species): 100–200

Hairy vetch: Grows well, even in areas that experience hard frosts. It produces a large amount of vegetation and can fix relatively large amounts of nitrogen. Its residues decompose rapidly and release nitrogen more quickly than most other cover crops. This can be an advantage when a fast-growing, high-nitrogen-demand crop follows hairy vetch. Hairy vetch does better on sandy soils than many other green manures, although it requires good soil potassium levels to optimise its productivity.

Common vetch: Mainly used in fertility building and grazing mixes. It is less well suited to droughty soils than some other legumes.

• Sowing (autumn): August to September
• Example sowing rates kg/ha (as a single species): 80

Annual legumes

Berseem clover: Grown as a summer annual in colder climates. It establishes easily and rapidly and develops a dense cover, making it a good choice for weed suppression. It is also drought-tolerant and rapidly regrows when mowed or grazed.

Crimson clover: It has a relatively rapid establishment compared to some longer-term clovers. However, it needs to be sown relatively early for autumn cover as it does not like cool soils.

• Sowing (autumn): August
• Example sowing rates kg/ha (as a single species): 10–15

Black medic (yellow trefoil): This legume grows faster than some clovers and can improve soil structure. However, good establishment requires a mid- to late-August sowing. Establishment declines when the crop is later.

• Sowing (autumn): August
• Example sowing rates kg/ha (as a single species): 8–10

Soya: Usually grown as a cash crop for its oil- and protein-rich seeds, soya can serve as a summer cover crop if allowed to grow until flowering. It grows best in fertile soil and is easily damaged by frost. If grown to maturity and harvested for seed, it does not add much in the way of lasting residues or nitrogen.

Note: Example sowing dates and rates (where shown) are taken from the AHDB cover crop review.