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

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A review of leguminous fertility-building crops, Defra project OF0316 1. Introduction 1. INTRODUCTION 1.1. What are ‘fertility-building crops’? Nitrogen can be imported onto organic farms by several routes (Table 1). However, the cornerstone of the organic philosophy is the use of legume based leys to build fertility. Indeed, the organic standards require the inclusion of legumes in the rotation. Fertilitybuilding crops and green manures are therefore key components of organic rotations. However, the regulations contain no specific requirements for ley management and the paucity of research means that their management is often based on hearsay rather than empirical evidence. Management options for these crops are many, including: • choice of species and cultivar; • length of growing period within the rotation; • rotational position and management of foliage (cutting and exporting, cutting and mulching, grazing or combinations of these). Table 1.1. Typical sources and loss pathways for nitrogen in organic farming • Fixation of atmospheric N • Purchased feed stuffs • (Cover crops) • Imported manures/composts • Rainfall Sources Losses • Nitrate leaching • Ammonia volatilisation • N2 and (NOx) emissions • Crop/animal produce • Exported manures Typical crops under UK conditions include red clover, white clover, vetch, lucerne, sanfoin, grass/clover leys (white clover/perennial ryegrass or red clover/Italian ryegrass), peas and beans. Lupins and soya are also being used in southern Britain. Recent work in NE England has indicated that two new legumes may also be suitable: lentil (Lens culinaris Medik.) and pinto bean (Phaseolus vulgaris). 1.2. Role in rotations Organic farming aims to be self-sufficient in nitrogen through fixation of atmospheric N2, recycling of crop residues and application of internally sourced manure. As well as legume based leys, organic rotations also often include a supplementary boost of N during the fertility depleting phase by growing a leguminous crop, such as field beans or peas. However, it is the legume-based ley that is the most important element of fertility-building. A major objective in the application of fertility-building crops (including green manures) is to provide the N required for optimal performance of the subsequent exploitative crops in the rotation (Watson et al., 2002). Nitrogen is usually the limiting nutrient in organic systems, particularly in the later stages of the rotation, as the N supply from fertility-building crops declines. If organic farmers are to optimise the capture and use of this nutrient, they require a reliable estimate of the amount of N available and its pattern of release. Supply of N to the following crops relies on mineralisation of the residues that have been accumulated during the fertility-building phase. Captured N can also be returned to the soil Written by S Cuttle, M Shepherd & G Goodlass Page 14
A review of leguminous fertility-building crops, Defra project OF0316 1. Introduction as manure, either directly by grazing animals or indirectly in manure produced by animals that have been fed on leguminous forage or grains exported from the field. Other aspects of fertility-building crops also influence the design and performance of rotations, particularly the effect of pests and diseases during the fertility-building phase and implications for the following crops. These effects may be detrimental (e.g. clover and lucerne could increase the risks of Sclerotinia trifoliorum for field beans) or beneficial (e.g. green cover may reduce common scab of potatoes) to the rotation. Therefore, these issues also need to be incorporated into planning advice for organic producers. 1.3. What is ‘fertility’? ‘Soil fertility’ can be considered to be a measure of the soil’s ability to sustain satisfactory crop growth, both in the short and longer-term. Organic farming recognises the soil as being central to a sustainable farming system. It could be argued that organically managed soils require a higher level of baseline fertility because shortfalls cannot be made up by applications of soluble fertilisers. Soil fertility is determined by a set of interactions between the physical and chemical environments of the system and by biological activity (Fig. 1.1). Organic matter is linked intrinsically to soil fertility, because it is important in maintaining good soil physical conditions (e.g. soil structure, aeration and water holding capacity) and is an important nutrient reserve. Organic matter contains most of the soil reserve of N and large proportions of other nutrients such as phosphorus and sulphur. Soil fertility is thus markedly affected by not only quantity, but also quality of the organic matter. Soil nutrient status organic matter Soil biological activity Soil physical conditions Figure 1.1. The concept of soil fertility To maintain soil fertility, organic farming relies on sound crop rotations to include fertilitybuilding and fertility depleting stages, returns of crop residues, N-fixation by legumes/Rhizobium, nutrient retention by green manures and effective use of manures/composted wastes. Fertility-building crops clearly impact on the N status of the soil but also have effects on other nutrients. They also provide an important input of organic matter to the soil. Written by S Cuttle, M Shepherd & G Goodlass Page 15
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