Optimisation of nitrogen from winter cover crops and use by subsequent crops
Aim
Tto assess the fate of nitrogen from winter cover crops. Optimisation of the use of this nitrogen by subsequent cash crops in the organic rotation will minimise nitrogen leaching.
Background
Cover crops (green manures) are integral to organic cropping systems. They are grown to conserve, accumulate and recycle organic matter and plant nutrients, to improve soil fertility and structure and to assist in the control of weeds, pests and diseases. The use of green manures is particularly important in horticultural systems; vegetable growing is demanding on the soil in terms of both nutrient depletion and deterioration of soil structure. Grass leys, which are an important part of the fertility-building phase in organic agriculture, are frequently inappropriate for horticultural regimes so the correct use of green manures is vital.
The autumn and winter is often the best time for a green manure to be grown since it may be fitted in between cash crops. Research at HDRA and elsewhere has shown that nitrate leaching can be effectively reduced by the use of winter cover crops (many conventional farmers are making increasing use of them for this purpose alone, especially in Nitrate Sensitive Areas). Although they accumulate considerable amounts of nitrogen there is a serious lack of knowledge concerning its release after incorporation; this presents a major barrier for the efficient use of cover crops since their contribution is not fully accounted for when designing rotations. This leads to both inefficient use of N and loss from the system by leaching. More quantitative data concerning the pattern of nitrogen mineralisation from different species under a range of conditions is required to improve our understanding of the factors controlling this release.
Approach
A broad range of species have been grown during the many years of green manure research at HDRA but the present work concentrates on two contrasting crops: grazing rye (Secale cereale) and winter vetch (Vicia sativa). Fully replicated trials have been set up at HDRA. Long term experiments of up to three years in duration have the aim of assessing the utilisation of nitrogen by three contrasting sequences of cash crops: agricultural (including winter cereals), intensive horticultural (land cropped throughout the year), horticultural (summer crops only), bare soil control. Short-term experiments have been conducted to investigate how the pattern of mineralisation can be manipulated by management techniques (e.g. incorporation date and method). Ceramic cups are used to quantify nitrate leaching while plant-available N is monitored by regular soil sampling. The yields of all crops are measured - including debris returned to the field. Some use has also been made of 15N labelled rye. A number of simple on-farm trials have also been carried out, supporting the more detailed monitoring at HDRA.
Results gathered from the experiments are being used to evaluate computer models of crop nitrogen requirements that have been developed at HRI, Wellesbourne. This type of model could make general predictions about optimum practices for managing cover crop residues in the field but before this project began there was insufficient data for the necessary refinements and tests under organic conditions.
Conclusions to date
- Of all the species investigated grazing rye is by far the most effective for preventing nitrate leaching since it becomes established so rapidly and continues to grow throughout the winter period.
- Legumes, particularly vetch, significantly increase the mineral N available to plants in the first season after their incorporation. The timing and quantity of nitrogen released can be manipulated by altering the incorporation date.
- Crops grown after the incorporation of green manures tend to be higher yielding than those following bare soil; this effect is most evident in soil where available nitrogen levels are low.
- The incorporation of both rye and vetch can cause inhibition of seed germination and this can have serious implications for the establishment of drilled crops (such as spinach) if they are sown too soon.
- Mineralisation of the nitrogen in rye continues throughout the first autumn after its incorporation. If the land is kept bare this can result in considerable leaching (effectively postponed from the previous winter) but this excess nitrogen can be effectively utilised by winter agricultural and horticultural crops, e.g. winter cereals and winter cabbage. By the second winter after incorporation this extra leaching has declined to background levels. Green manures must therefore be carefully integrated into the rotation to maximise their benefits and to reduce the risk of loosing their nitrogen by leaching.
Dissemination of the results
The findings from this work will be used to provide advice and recommendations to farmers concerning the maintenance of soil fertility. As well as technical articles in the farming press and publications in scientific journals regular open days and workshops are held.
For further details please contact: Dr Francis Rayns, International Research Department, The Henry Doubleday Research Association, Ryton Organic Gardens, Coventry CV8 3LG. Telephone: 024 76303517 Email: research@hdra.org.uk
Top of pageGarden Organic is the working name of the Henry Doubleday Research Association (HDRA).
We are not responsible for the content of external web sites.
Bookmark this page on: