Reducing the productivity gap in organic farming – balancing nutrient supply and demand

What does the research on comparative yields and the long term trials in the UK and Switzerland tell us? How have farmers measured and managed the fertility gap to maintain yields over 10 and 20 years? This session was organised by the Nafferton Ecological Farming Group (NEFG)
Julia Cooper (NEFG): chair.

Jochen Mayer

Session summary

This session pointed out that nitrogen is often the limiting factor for crop yields in organic systems, although other factors such as disease and weather can also play a major role. It was also highlighted that in many cases conventional yields could be considered to be artificial, as they are dependent on manufactured fertiliser. The results presented from Newcastle University also emphasised that in many cases the challenge is not the amount of N but the distribution throughout the rotation (i.e. there is a flush of N following incorporation of leys, whereas crops toward the end of the cropping sequence can face deficits). In this context, the question of whether organic farms should house livestock for longer periods and/or make more use of slurry was raised. The presentations also highlighted the importance of closing the phosphorus (P) loop in organic systems, to prevent P ending up in landfill. The potential for using green manures such as buckwheat and bagged fertiliser from waste-water treatment plants was also highlighted.

Recent results from the DOK trial in Switzerland were presented, which showed that conventional systems with half fertilisation were able to achieve higher yields than organic systems with full fertilisation, suggesting that conventional systems were better able to use the available nitrogen. The DOK trial has also revealed a strong relationship between available P and K with yields of organic clover and grass, with K in particular affecting the yield for clover crops. It was also pointed out that P balances are often negative in organic nutrient balances, and therefore these rotations could be said to be mining reserves.

Despite this, organic farmer Daniel Seaborne presented results from his nutrient balance, showing a small surplus for P and K. Daniel also reported that measured P and K levels and the soil structure had stayed relatively stable over 10 years of cropping at Pound Farm. Daniel also stated that he had considered increasing the amount of temporary ley on the far, but wondered if this would create pollution problems through an excess of N in the system, in view of the surplus reported through a farm-gate nutrient budget.

The discussion that followed the presentations brought up the issue of P supply in organic systems, and the fact that treated sewage is still not allowed on organic land, despite major advances in treatment processes over the last 10 years. It was also pointed out that the focus on promoting yield through feeding the plant available sources of N does not sit well with the organic principles of developing long-term soil health and human nutrition.

Key conclusions

The discussion that followed the presentations brought out the following points:

  • Not only N can be limiting factor for organic yields, P and K also can play a vital role in ensuring sufficient outputs and crop health.
  • Should consider alternative sources for major nutrients (e.g. sewage, green manures for extracting reserves such as buckwheat).
  • Yield is only one aspect, need to consider longer-term health and product quality when comparing systems.

Action points

  • EC Should reconsider organic rules that prohibit use of human-sewage on organically managed land, to help reduce P deficits.

Individual speaker presentations and abstracts

Julia Cooper (NEFG): Comparative yields under organic and low-input systems.

Since 2001 the Nafferton Factorial Systems Comparison trial has provided a unique facility to investigate factors limiting yields in organic cropping systems. The trial compares two different crop rotations: one cereal-intensive ‘conventional’ rotation and one more diverse ‘organic’ rotation, as well as organic and conventional crop protection and fertility management practices. Analysis of annual yield data allows us to determine whether yield reductions in organic systems are due to rotational history, weed, insect and disease pressure, nutrient limitations, or a combination of these factors. Experience to date indicates that some crops (particularly potato) have yield limitations when grown with only compost as an N source. Recently the trial has been modified to include applications of slurry as well as compost, to try to address the problem of quantity and timing of N supply to the growing crop under organic fertility management. Results will be presented from the 2013 season for winter wheat, potatoes and cabbages grown using various nutrient sources in the background of an organic and conventional crop rotation.

Jochen Mayer (Agroscope): Nutrient limitations in organic farming? Results from the DOK trial at Agroscope

The DOK experiment started in 1978 near Basle, Switzerland and provides valuable insights into the long-term effects of different organic management strategies on crop nutrition and yields. This presentation will focus on trends in wheat yields throughout the trial. Substantial differences in yield between ‘organic’ and ‘conventional’ farming systems and different fertilisation intensities (50% and 100% of standard fertilisation) were primarily attributed to the delivery of nutrients – in particular, nitrogen – to the plants. In the two organic systems, bio-organic and bio-dynamic, the doubling of manure application only slightly improved wheat grain yields but did not improve grain baking quality. However in the same systems, a pre-crop of potatoes significantly increased yields of wheat compared with a silage maize pre-crop; this effect was not apparent in the conven-tional systems. Analysis of wheat straw and grains showed sufficient phosphorous supply, but potassium and nitrogen were co-limiting factors in the organic systems at the low fertilisation intensity. In contrast, both the bio-dynamic and bio-organic system exhibited a balanced potassium supply at the high fertilisation intensity. Analysis of clover in the two years of clover-grass ley support these findings and also indicate a limitation of clover growth by phosphorous at low fertilisation intensity.


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