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 cultivation of crop As a part of the wider project, workshops were held with organic growers to discuss the challenges of managing fertility-building crops. The main areas of concern are summarised below: • Optimum crop sequence (including length of build up and break between legumes and/or green manures) • Management of cover crops - options/timing • Rate of release of N from green manures/covers and crop residues • Management of swards (i.e. cutting/mulching/removal/grazing requirements) to optimise N accumulation and minimise losses • N fixation ability of different legume species in UK conditions (including optimum mixes for mixed swards) • Assessment of % clover in mixed swards • N build-up/retention - Interactions with cultivations/seed bed preparation • N build-up/retention - Interactions with pest and disease issues • Bi-cropping • N build-up/retention - Interactions with other nutrients • Management of organic manures • Cost implications • Grain legumes for cash crops Soil fertility-building crops in the rotation Fertility-building crops can be split into three types: • ‘Nitrogen-fixers’ • ‘Nitrogen-holders’ • Those that affect other general fertility aspects such as soil structure. As the fertility-building part of the organic rotation often occupies a large proportion of the rotation (grass/clover leys and green manures can occupy as much as 66% of the organic farming acreage), it is important to gain as much benefit from it as possible. In mixed livestock systems the grass/clover leys clearly have a role to play in forage production. However, there is some evidence that the livestock manures that are often applied to the grass would be better used on the arable crops, especially where higher protein levels are required in cereals. For non-livestock systems it is important to have sufficient fertility-building crops to support the cash crops, as there is no opportunity to supplement the soil N with organic manure (unless imported). It is also beneficial from an economic point of view if the fertilitybuilding crops can also provide income as they can occupy 20-40% of the rotation. Thus, clover grown for seed may be preferred over a cut/mulch system and peas/beans, which can be sold for human consumption preferred over those that are used for animal feed. The choice of species is also important. European studies have recommended that climate is considered when choosing legumes. For example, they recommend red clover or lucerne (both of which are deep rooted) for dry conditions but point out that white clover has a very good ability to recover after drought. Written by S Cuttle, M Shepherd & G Goodlass Page 2
A review of leguminous fertility-building crops, Defra project OF0316 Executive Summary Clearly, there are pest and disease implications with all farming systems. Consideration of disease interactions and implications with fertility-building crops needs to include: 1. Diseases affecting the fertility-building crops directly. 2. Diseases affecting fertility-building crops which will also affect other crops in rotation. 3. Consequences of incorporation of the fertility-building crop into soil: suppressive or antagonistic effects on soil- or air-borne pathogens. 4. Poor performance of the fertility-building crop has implications on the subsequent cash crop The legacy of disease problems left by fertility-building crops is of major concern to growers. The extent of problems will be influenced strongly by previous cropping and its legacy of the soil pathogen population on the field or farm. Thus, farms with a history of legume cropping are likely to carry high populations of pathogens affecting legumes, whilst few legume pathogens may be present where legumes have not been grown for many years. Very intensive legume cropping may leave high soil populations of fungal pathogens, which will only decline slowly. There are two main ways in which fertility-building crops or catch crops grown to maintain fertility can interact with invertebrate pests. Firstly, the pests may attack the fertilitybuilding /catch crop and secondly they may carry over into the succeeding crop, resulting in poor establishment and loss of yield. How much N is fixed? Few studies have measured the maximum quantity of N that can be fixed under ideal conditions. The best that can be done is to estimate potential fixation from the upper range of values observed in different studies and take account of any limitations that may be present. In this respect, measurements from countries where conditions are more favourable than in the UK may be more indicative of the maximum fixation that can be achieved. Methodological difficulties make it almost impossible to get an accurate measure of fixed N. All methods have advantages and disadvantages. The literature has used a number of methods, so that estimates are not always directly comparable. Simple measurements of N yield are an unreliable guide to the quantities of N fixed. Measurements of N-fixation must differentiate between crop N obtained from the soil and from the atmosphere. Also, much of the below-ground N can be present as fine roots or transferred to the rhizosphere, and it was concluded that standard root recovery procedures are likely to underestimate the total N accretion and turnover. Also, the majority of estimates of N-fixation do not include fixed N in unharvested parts of the plant. Jørgensen & Ledgard (1997) suggested a correction factor of 1.65 to account for N in roots and stubble when fixation in white clover is estimated by harvesting the leaves only. This is greater than the value employed by Høgh-Jensen & Kristensen (1995), who considered that biologically fixed N was underestimated by 19-25% when only the harvested material was measured. Where transfer of fixed N occurs, this should normally be included in any assessment of the net input from fixation, though care is required to avoid ‘double-counting’, particularly in systems that are not at equilibrium and where fixation in one year contributes to transfer in Written by S Cuttle, M Shepherd & G Goodlass Page 3
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