1* Recommended – likely to occur in upland and associated lowlands for reasons outlined.
*See Table 2 page 8
In organic grassland farming without N fertilisers, clover is usually regarded as essential for building soil fertility. In grass-clover swards, 25%-30% content (Dry Matter) is considered necessary to produce sufficient nitrogen. Root rhizobia associated with white clover in ryegrass/clover swards can fix nitrogen at rates up to
280 kg N/ha/yr. in lowlands and up to 150 kg N/ha/yr. in uplands. Clover is more difficult to manage on hill and upland farms than on lowland farms; upland soils are slower to warm up in the spring and hence the rhizobia start synthesising N much later; also because of the generally colder and wetter conditions coupled with acidic soils and low phosphorous (P) and potash (K) indices.
Grass-clover swards are less tolerant of acid soil conditions than all-grass swards and clover is likely to be less persistent at soil pH levels below the optimum. This is particularly important in organic systems where clover is the main source of nitrogen. A pH of 5.8 – 6.0 is generally regarded as necessary to maintain good clover levels on mineral soils, with a pH of 5.3 - 5.4 recommended for organic soils. A variety of flowering plants and deep rooting herbs are frequently included in organic seed mixes to aid livestock health by supplying essential minerals and condensed tannins.
6.4.1 Biodiversity Impacts: Flora
Clover/herb silage leys, usually grown on associated lowlands of upland and hill farms are an integral part of organic systems and can meet the forage requirements of an organic farm whilst improving the protein content and overall feeding value of winter forage (RuralNi, 2002). Sown clover is often inoculated with rhizobia to improve its N fixing capabilities. Inoculation involves the introduction of the specific nitrogen-fixing bacteria to the roots or seed of the plant before planting. In terms of its impact on native flora, clover is a fast growing species and when grown with other herbs can quickly smother less vigorous species and encourage coarser, more competitive species. Without added sources of N, clover is a vital component of productive pasture to provide N in its most usable form to plants. In trials on the Pwllpeiran Organic Unit, without inorganic N input, a white clover annual average cover of 25% in swards is necessary to fix N at rates equivalent to 150kg N/ha in upland soils (Frost, 2000). This is a relatively high rate of clover cover which has an impact on floral diversity but must be balanced with the production needs of a farm.
6.4.2 Biodiversity Impacts: Fauna
Clover is favoured by butterfly and bee species including the rapidly declining populations of bumblebees. It can be important as cover for ground nesting birds and mammals. In improved agricultural grasslands, a diverse sward is more productive from a wildlife point of view (see section Error! Reference source not found.); there must be a balance of conservation and production when considering clover as a source of N.
6.4.3 Soil Quality Impacts
When ploughed in as a green manure clover/herb mixes increase N availability while providing high levels of soil organic matter, although its use in an upland situation is limited. Clover produces N in its most soluble and therefore valuable form directly to the plant but is susceptible to leaching. Different rooting habits of different herbs bring trace elements from deeper soil reserves, adding to soil fertility.
6.4.4 Air Quality Impacts
The use of leguminous fertility building crops is likely to increase N2O emissions if cultivation is improperly timed. Emission peaks are usually linked to rainfall events (Shepherd et al., 2003).
6.4.5 Water Quality Impacts
Water sources will be vulnerable to nutrient enrichment resulting from the effects of untimely cultivations of clover leys, especially if this coincides with heavy rainfall. This should be balanced against the problems associated with soluble N fertilisers.
6.4.6 Resource Use Impacts
Clover is valuable as a winter forage, and therefore reduces the demand for purchased feed, although it will not survive all upland situations. Soil depth and warm aspect will enable success. Red clover is winter hardy and relatively pest and disease resistant, reducing the need for herbicide treatment, but will not withstand hard winter grazing by sheep or poaching in wet winters by cattle.
6.4.7 Agri-environment Scheme requirements
The separation of land use for production and conservation should not lead to a farmer being debilitated by conservation objectives. The suppression of weeds is likely to be at conflict with Tir Gofal objectives (see below)
6.4.8 Further Research
Consider other leguminous crops for hills and upland, for example grain lupin which is currently being used in fodder trials by the Scottish Agricultural College. A proposal exists for a project with Farming Connect 2004 on improving the quality of forage for organic ruminants.
6.4.9 Education & Dissemination
Provide clover/herb cultivation guidelines to minimise environmental impacts of N leaching, with specific reference to the limitations of physical conditions of hills and uplands, and technology transfer on research requirements above.