Trust but verify

19 Jan 2018

The heading for this blog is a quote from President Reagan made during the nuclear disarmament talks with the then USSR. It is something that should always be borne in mind when reading articles or listening to talks, even those by industry experts. There may be an ulterior motive in their slant on ‘the truth’ or they may not be thinking holistically.

I know that you want some examples. Let us start with The Soil Association and their campaign for healthier soils. They have fallen into the same trap as Michael Gove in quoting the loss of organic matter on the fen peats as typical. It is clear that either they have not seriously looked at the scientific literature or, having done so, have ignored it. This is because that over the last few decades soil organic matter may have actually increased on the very same long-term arable soils over which they express particular concern. The map below demonstrates this increase. The colour green indicates increasing soil organic matter between1978 and various time periods up to 2003 and the traditional arable areas are almost solidly green. The yellow colour shows static or slightly falling levels of organic matter and the red colouration represents a more significant fall in organic matter. You need to read the paper* to understand fully the basis on which this map is compiled. The fen peats of north Cambridgeshire are coloured red but their extent indicates how representative they are of the main areas of arable production in England and Wales. Interestingly, some of the areas losing organic matter most rapidly in both England and Wales are not farmed. The authors of the paper attribute this to climate change.


Why has organic matter most probably increased in long-term arable soils since 1978? There are two possible explanations. One is that straw burning generally stopped in the late 1980s and the other is that, because of the application of new technology and scientific understanding, we have been harvesting heavier crops since the mid-1980s. Heavier crops mean the return of more crop residues to the soil. In the long-term straw incorporation experiment at Morley, now funded by The Morley Agricultural Foundation, there has been a significant increase in soil organic matter since 1983 just from incorporating rather than removing straw. Mind you the soil organic matter is still between 1.7-1.8% where the straw has been incorporated but in some measurements, particularly soil aggregate stability, the benefit has been dramatic when compared to where the straw has been removed. It seems that the regular introduction of crop residues and a little bit of extra organic matter can go a long way.

I can hear you saying that The Soil Association is perhaps not an unbiased source of information. Their ‘peak oil’ and ‘peak phosphate’ campaigns, in which they predicted the fall of conventional systems, now seem an amusing aside. Also they are not thinking holistically about the wider environment because the extra manure use they promulgate has got to come from somewhere. I assume largely from extra cattle and sheep. This comes at a time when society is switching off eating meat and when there is a realisation of the profound effect of cattle and sheep on raising greenhouse gas production throughout the world.

Hence, I have another example. It is one that has been a ‘bit of a thing’ for me over the last few years. This is nitrogen advice for winter wheat. New ‘national advice’ has been all over the place for the last ten years or so. In my opinion this is because the advice has not been truly based on the evidence from field trials. Instead, the advice has been based on simplistic models that have been used to try to explain the results of field trials but when applied have not reliably reflected the nitrogen requirements of wheat, particularly at soil nitrogen indices of 2 and above. To some extent this has been overcome in the latest edition of RB 209 by introducing a yield correction.

However the yield correction is also a matter for debate. It was approximately 33 - 38 kg of applied N per extra tonne of wheat/ha in the AHDB Guidelines published in 2009 and then fell dramatically to an extra 20 kg/tonne in the new RB 209. This is still too high according to trials data.

Trying to make any sense of collated nitrogen trials can be soul destroying because of the lack of consistency, but there are exceptions. It has long been known that there is a robust relationship between yield at the optimum nitrogen dose and yield of the untreated control (also see my blog posted 7 January 2017). When a huge database of trials is analysed, including over seventy NIAB TAG trials from close to 60 locations, it shows that the optimum nitrogen dose increases yields by 3 t/ha when the treated plot yields are around 8 t/ha but by 5 t/ha when the treated plot yields are 15 t/ha. This means that the additional yield from nitrogen only increases by 2 t/ha when the optimum plot yield increases from 8 t/ha to 15 t/ha. This provides some verification of the NIAB TAG estimate that around 10 kg of applied N/ha is required for each extra tonne of plot yield above 8 t/ha. However, I have to point out that the differences between the NIAB TAG nitrogen recommendations for winter wheat and those in the new RB 209 are now far less than previously because of the other factors included in the calculations.

What really concerns me now is that some research organisations are gathering ‘facts’ via farmer forums on the internet. It has to be recognised that such ‘facts’ are a result of an inevitably biased survey and as such, the results should be treated as ‘interesting’ rather than true facts.

I have banged on for years in my blogs about seeking reliable practical guidelines from the correct interpretation of good scientific data. The message that should ring in our ears whenever we read or hear advice is “trust but verify”.


* If you have issues reading the paper, copy and paste the following link into your web broswer: