A review of leguminous fertility-building crops, with particular ...

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A review of leguminous fertility-building crops, Defra project OF0316 Executive Summary • N content of grass-only reference crop • % legume N derived from fixation • Correction for stubble/root N • Sward management Several studies have shown N-fixation and legume yield to be strongly correlated. Van der Werff et al. (1995) estimated N-fixation in grass/clover pastures on mixed, organic farms on sandy soils in the Netherlands by assuming values of: • 40 kg N fixed per tonne of dry matter for red clover • 54 kg N fixed per tonne of dry matter for white clover. Vinther & Jensen (2000), for grazed grass/clover pasture in a grass/arable rotation on a sandy loam in Denmark, assumed that the quantity of N fixed per tonne of clover shoot dry weight was: • 38.6 kg N fixed per tonne of dry matter for first and second year mixtures • 45.0 kg N fixed per tonne of dry matter for undersown grass-clover. These were similar to other values from the literature, cited in the paper: i.e. • 30-46 kg N fixed per tonne of dry matter for white clover • 24-36 N fixed per tonne of dry matter for red clover. Wheeler et al. (1997), in New Zealand, estimated the amount of N fixed as: • 40 kg N fixed per tonne of dry matter for white clover on a high rainfall site • 46 kg N fixed per tonne of dry matter for subterranean clover on a low rainfall site It has been estimated that lucerne and subterranean clover fixed 22-25 kg N for every tonne of legume dry matter produced. There have also been more complex, modelling approaches to estimating fixation. Utilisation of N Nitrogen fixation by the fertility-building crop is only the first part of the process by which atmospheric N is made available within the rotation. It then has to be released and made available to the following crops. It is necessary to consider the following pools of N in the soil-crop system at the end of the fertility-building stage: • Soil mineral N in the soil profile, the amount depending on history and management of the fertility-building crop. Much of this may well have been derived from mineralisation of organic residues during the fertility-building stage (or from recent manure applications). • N in the microbial biomass. • N in roots and nodules and from rhizodeposition. • N in leaf litter (e.g., at various stages of decay), either from natutral leaf drop or from cutting and mulching the fertility-building crop. This may also be from straw residues if the crop has been harvestsed and removed from the field (e.g. peas/beans). • N in above ground components, such as stubble and foliage. • N in manure residues, if manure has been applied recently or the crop has been grazed. • N in native soil organic matter. Written by S Cuttle, M Shepherd & G Goodlass Page 8
A review of leguminous fertility-building crops, Defra project OF0316 Executive Summary Therefore, the N that can be potentially be used by the subsequent crops may be in mineral or organic forms (most is likely to be in organic forms), and in a range of organic fractions that will vary in recalcitrance. Generally, the organic forms of N associated with the fertility-building crop could be termed ‘residue N’. It should also be noted that not all of the residue N will necessarily be fixed N – some will have derived from uptake of (a) N released from the native soil organic matter and (b) mineral N in the soil at the time of establishment of the fertility-building crop. The proportion of non-fixed N will depend on many factors, as described in the previous Sections. How effectively the residue N is used by the subsequent crops in the rotation will depend on many factors, including: • the rate of net mineralisation • efficiency of uptake by crops • N removal in harvested products • N (and C) return in plant residues and • losses of N The rate of N depletion will be reduced if manures are applied or if the rotation includes further legumes during this phase. Rotational Aspects Lack of technical information and advice on management of fertility-building crops is felt to be a major problem by many farmers. Long sequences of legumes are most common in livestock systems, usually grown in a mix with grass for forage. Conflicts can occur with respect to the management of the sward to optimise grazing potential or forage dry matter production, whilst enhancing the residue of soil N sufficiently to support the rest of the rotation. The return of animal manures contributes to the long-term fertility of these rotations. As well as clover, beans are also commonly grown as a legume that can also contribute to animal feedstock. The stockless systems are characterised by shorter (1-2 year) legume sequences, in which the legume is often grown for seed. The legume may also be grown as a ‘set-aside’ crop. A wider range of legumes seems to be grown and there is greater evidence of over-winter cover crops. The optimum ratio of fertility crops to cash crops will depend on the nutrient retention capacity of the soil. In mixed arable/livestock systems, there are generally longer runs of grass/clover leys in the rotation, which may be to the detriment of N fixation in later years. The calculation of nutrient balances is well recognised as technique for studying the sustainability of organic rotations and are recommended as an essential precursor to conversion. However estimating the nitrogen fixation component can be difficult. Some crops are believed to be deep rooting and able to extract nutrients from depth, thereby enhancing the fertility of the topsoil when incorporated, but there is little documented evidence to support this claim. As indicated above, practical considerations are also important in determining crop rotations. Crops that are harvested late do not lead well into a following crop, which requires early Written by S Cuttle, M Shepherd & G Goodlass Page 9
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