Cover crops to improve soil fertility

Goal: nitrogen (N)

Nitrogen uptake by cover crops sown in late summer and autumn ranges from 30 to 120 kg N/ha before spring.

The type and growth of cover crops will influence how much nitrogen is captured from the soil and when it is released.

Most legumes also fix atmospheric nitrogen into a plant-useable form through rhizobia bacteria that associate with their roots.

Many legume species, including trefoils, clovers, vetch and lucerne, are suitable for use as a cover crop. However, to maximise the opportunity to fix nitrogen, you should sow crops in early autumn.

Fixed or captured nitrogen must be broken down to become available for the following cash crop.

Goal: phosphate (P)

Some plants, particularly lupins and buckwheat, can increase phosphate mobility in the soil. Such plants exude organic acids from their roots that solubilise phosphorus and increase its availability to other crops.

Note: lupins typically need a soil pH of less than 7, and buckwheat is not frost tolerant.

Many cover crop species support soil mycorrhiza populations, which are important in phosphorus nutrition. However, brassicas, sugar beet and lupins do not support such populations (non-mycorrhizal).

Goal: other nutrients

Consider species with extensive root systems or mixes with complementary rooting (e.g. deep and shallow) to help mine and cycle nutrients and trace elements.

Some elements, such as sulphur, can leach out of sandy soils and become deficient in low-input systems.

Cruciferous crops, such as winter rape or fodder radish, can prevent sulphur leaching into lower soil horizons.

Other cover crops, such as chicory with its deep taproot, may accumulate large amounts of sulphur, boron, manganese, molybdenum and zinc.

This ‘mining’ effect is particularly useful when a subsoil is rich in nutrient elements, but the topsoil is relatively poor.

Goal: organic material (green manure)

Species that give good autumn growth and ground cover are best; for example, oats, phacelia and brassicas such as mustard or radish.

Relatively fast-growing legumes may also be suitable, especially if they are given the opportunity to fix nitrogen.

Note: large amounts of biomass need careful management.

Nutrient return from grazing livestock

Grazed cover crops provide extra nutrient return from the grazing animals and livestock production from forage.