The development of improved guidance on the use of fertility building crops in organic farming (CTE0204)

Summary

Fertility building crops are a key component of organic rotations where they help to provide nitrogen required for optimal crop performance. It is important that rotations and managements are planned which optimise the capture and use of this nitrogen. Some estimates are available of the nitrogen production from fertility building crops, but there was a need to produce a more comprehensive assessment of likely nitrogen fixation, release and availability under different circumstances. Thus the overall aim of the study was to provide guidelines to enable organic farmers to better estimate the nitrogen supply to a rotation following fertility building crops. This was done by a mix of literature review, empirical measurements, model development and farmer participation.
The detailed objectives were:
1. To produce a literature review summarising the current knowledge on N capture and supply and secondary effects (e.g.pest/disease implications) following fertility building crops.
2. To engage the organic farming community to ensure that the most relevant issues were addressed within the project.
3. To produce an advisory leaflet summarising practical advice arising from the literature review.
4. To conduct field experiments to supply supplementary information that was required to develop a model for quantifying N supply from fertility building crops.
5. To develop a model for quantifying N supply from fertility building crops in organic systems.
6. To make on-farm measurements to validate this model.
7. To synthesise this information into practical advice for the industry in the form of a booklet on fertility building crops.
The study was split into a number of interlinked work-strands: Literature review, Field experiments, On-farm monitoring, Industry engagement, Fertility building crops (FBC) model, Advisory leaflet and Guidelines booklet.
The literature Review (Objective 1) was an essential first activity as it helped to guide the experimental programme (in terms of focus, techniques and approaches), avoided unnecessary replication of experimental work and provided an immediate synthesis of available information for the Industry (see Objective 3). The review also formed the basis of Objectives 5 and 7. Over 1000 references were assessed to produce a summary of the current state of knowledge in the following topic areas:
• Nitrogen build-up, release and availability
• Crops, pests, diseases and rotational issues The structure of the review exemplifies how the project tried to quantify N supply from fertility building crops, i.e. by separating the management process into N capture and N release. Many factors affect both of these processes. The conclusions that we were able to draw from the large body of work were able to serve three puposes:
• Form the basis of advisory literature
• Inform the research programme
• Inform the FBC model development
Industry engagement (Objective 2) was a key component of the study and there was good liaison with the organic farming industry at the beginning and throughout the duration of the project. This liaison was co-ordinated by The Organic Studies Centre at Duchy College in the South West, and by Abacus Organic Services Ltd in the East and North East. Feedback from producers was elicited throughout the project to help ensure that the industry’s requirements were met, notably with respect to trials (Objective 4) and advisory material (Objectives 3 and 7). Some of the farmers were also involved in on-farm monitoring, supplying fields which were used to take measurements of N supply from selected rotations (Objective 6). In all, the project findings were promoted at a total of 20 farmer meetings, and 7 articles were published in the popular press.
A key early deliverable from the project was a summary advisory leaflet (Objective 3) based on the literature review. The draft leaflet addressed the main practical issues of fertility building crops and was presented at farmer meetings. Feedback on the leaflet was invited at these meetings and issues raised were addressed in the final version which is available on the project website and as an attachment to this report.
The main field experiment (Objective 4) was designed to examine the effect of soil-N status on the quantity of N fixed and the impact on N-fixation of returning plant residues to the growing crop in cutting/mulching managements. The original project was extended with approval from Defra during 2004 to include measurements on the amount of N released following destruction of these legume management plots. As a result of industry feedback (see Objective 2), demonstration plots were also set up to look at novel legumes.
The legume management data showed that in terms of fixation, maximum N yield (as measured by N offtake) occurred in the mulched grass/clover sward. But, using the measured data, separate estimates for the effect of mulching on N fixation showed it caused a reduction of between 9 and 61 kg N ha-1. In the presence of FYM there appears to have been an interaction with mulching which was sufficient to depress N fixation by an amount roughly equal to the amount of N gained from the amendment. Availability of fixed N is also important and the amount of N released following destruction of the experimental swards was assessed by measuring the uptake of nitrogen in the following ryegrass crop. Irrespective of management history, N offtake was higher at the first cut where there had been a history of FYM applications. There was positive relationship between N capture (as measured by N offtake) and total N supply (as measured by SMN and N in crop residues) but the impact of the individual N supply components on this relationship varied between sites and between cuts.
Recommendations to organic farmers from this part of the work, are therefore to cut and remove herbage during the fertility building phase. This may be difficult for non-livestock enterprises and care needs to be taken that the mulch does not kill out the receiving crop. Application of organic manures to legumes should also be avoided if the maximum amount of atmospheric N is to be fixed.
On-farm monitoring (Objective 6) included measurements of N accumulation (as soil and crop N) and subsequent N supply (as soil N) in 12 commercially farmed fields covering a range of soil types and rotations. The data collected were used in the FBC model (Objective 5) and highlighted the high proportion of nitrogen held within the root component of the fertility building crops.
The fertility-building crops model (Objective 5) is a spreadsheet-based calculation system for estimating available N in organic rotations based on N accumulation under fertility building crops, its subsequent release and associated losses. It is based on information from the literature review (Objective 1), other soil nitrogen models, results from the field trials (Objective 4) and on-farm monitoring (Objective 6). It was recognised in the original proposal that it would not be possible within the timescale of the present project to deliver a fully functional and validated model that was suitable for release to the industry. Accordingly, a much-simplified version of the model is presented as a flow diagram in the final deliverable.