I have written before on Salle Farms in North Norfolk and their experience of introducing a seven year rotation around the turn of the century. It replaced a variety of short intensive rotations that were employed in different parts of the estate. The crop that seems, in particular, to benefit in terms of yield comparisons with both national and regional performance is winter wheat. This is now grown either two times in the seven years or three times depending on the date of the sugar beet harvest.
Some of the increase in yield may be due to reduced levels of black-grass but I do not think this explains everything. Hence, the magnitude of the relative improvement in wheat yield compared to other crops was a bit of a mystery to me until someone sent me a paper from the Journal of Ecology. This may start to provide a basis for an explanation.
The paper reports on an investigation into how soil biota may over time influence changes in plant species on uncropped land. I found the paper challenging to read but there was some clear guidance as to how soil fungi and bacteria may influence the relative growth rates of different plant groups. One of the main conclusions was that wild grass species are particularly reduced in growth when grown in soil that is inoculated with the biota extracted from soil that has previously just grown the same species. Hence, winter wheat may not grow so well in soil where wheat has been recently grown due to the soil biota that it encourages or the loss of organisms that are inhibited. In addition, the paper also reports that grass growth (and so perhaps wheat growth) is better when the soil contains a greater variety of soil biota left after growing a range of crops.
I have always been led to believe that the reduction in yield of wheat in wheat intensive rotations is by no means all due to take-all and it seems that the rest of the reduction could be due to other unidentified soil biota.
The overall conclusion of the study is that all the plant groups studied grow better in soil that has just previously grown other species because of less net negative effects of soil biota (in grasses) or because of more net positive soil biota effects. This may provide a more scientific basis to crop rotations and indicates that increasing the number of species grown in a rotation, perhaps including cover crops, may be better for the growth of all crop plant types.
Another conclusion in the paper is that the growth of plant species that tend to first infest bare soil, which have high root length and low colonisation from arbuscular mycorrhizal fungi (AMF), particularly suffers when they are grown in soil inoculated with the biota that they encourage. Oilseed rape fits this description and this may explain the NIAB TAG results that show that yields decrease as the intensity of its cropping increases. This latter research does identify two species of soil biota that are associated with intensive rape cultivation and may be the cause of the yield reduction. It is not clear whether this is because of direct effects via the crop roots or because they out compete other more favourable species.
A recent review of the role of AMF and the impacts of agricultural management highlights the very negative effects of intensive soil disturbance and fertilisation on AMF. The review states that the fundamental principle of crop rotations is to exert a control function that prevents particular AMF from dominating the soil matrix. The review explains that continuous wheat favours the selection and proliferation of less co-operative and more aggressive AMF species. These are likely to enact behaviour similar to parasitism. Hence, at least in some circumstances, there can be ‘bad’ AMF species as well as ‘good’ AMF species. It goes on to say that this effect can be toned down by ‘break crops’, such as Brassicae (e.g. oilseed rape) or legumes. Brassicae are non-mycorrhizal crops which act as inhibitors of the dominant AMF species proliferation and legumes are AMF dependent crops which favour the overall propagation of AMF communities.
Complicated isn’t it?! New laboratory techniques are revolutionising research into soil biota and so I hope that we are now on the cusp of a better understanding of how crop rotations tick.