Today I met with a couple of researchers from the Agriculture Research Service. One of them was Kris Nichols, a well known soil microbiologist who specialises in mycorrhizal fungi. The other was Mark Liebig, who is also a soil scientist, but more in the physical rather than biological sphere. I can’t be bothered to structure this as an article, so I will go through the areas that I thought were interesting, point by point.
Mark showed me around the research farm, which has been operating since 1916, and currently has a few trials going on rotations and cultivations. It is not located in a terribly suitable place, as the soils are incredibly resilient, fertile, 2m deep, silt loams. This means that both positive and negative changes to the soil structure and makeup take a long time to manifest. Some of the trials have not yet shown significant results, even after many years, or sometimes decades. Here are some of the things I learnt.
- Tillage vs no-till trials showed that in most years there was no yield difference. However, in drought years, no-till performed better. I would take from this that there is no point in performing tillage in this location. Mark pointed out that another interpretation was that tillage was generally not harmful, and if it was needed, say in an organic system, then it would not be a big problem.
- One trial compared continuous wheat with the straw being chopped against continuous wheat with straw removed. Over a period of time (I think 10 years), the field with the straw removed had higher SOM than when the residue was kept. This is obviously counterintuitive. Mark’s explanation was that when the straw was removed, weeds would germinate and grow, before being killed in the spring. This is effectively a free cover crop, and shows the potential benefits that covers can bring.
- They use a tool called SMAF to measure soil quality. It is “not farmer friendly” and is really a research tool that measures physical characteristics. Often, but not always, the results correlate to SOM levels. The same can be said for humus testing when compared to traditional SOM tests.
- The fields were last cultivated in 1983, with a chisel plough. Even now, after 31 years of no-till, there is still a compaction pan in some plots. The more diverse the rotation, the less obvious the pan. However, Mark does not think that this compaction is enough to significantly effect rooting, or yield.
- In a set-stocking grazing system, lower animal density generally results in more native species being lost from the pastures.
- As in the rest of the world, researchers in the US are uncomfortable giving direct recommendations to farmers!
Now on to Kris Nichols.
- SOM testing is a useful, but basic metric. It is probably just as good as testing for humus, but neither method actually measures the most useful forms of organic matter. The Haney test is probably the best currently available.
- Mycorrhizae can link the roots from different species of plants together. For example, a legume and grass growing next to each other can share nitrogen fixed by the nodules on the roots of the legume. This has been scientifically proven using labelled nitrogen, and some 50-70% of the nitrogen present in these grass plants can originate from the legume. She does not know if this means that the grass has any more nitrogen in it that it would have done without the legume present. This lends support to the idea of growing leguminous cover crops in with a cash crop, and then reducing nitrogen applications. I am still skeptical.
- Crops with higher levels of mycorrhizal associations are more tolerant of drought. This is because nutrients are available in the fungal hyphae, so the plant roots do not have to exude water to extract them from the soil.
- Plants which are not under stress do not form as many mycorrhizal associations, as there is a cost to doing so, and little benefit. Also, microbes that unlock a particular nutrient are unlikely to thrive if that nutrient is freely available in high concentrations, such as when it is applied as fertiliser. This means that in order to build up the soil life, and therefore increase nutrient efficiency of the plants, it may be necessary to go through a period of plant stress that results in lower yields. This may be 2-3 years in duration. It could be possible to mitigate this either through the use of cover crops, or by weaning off fertiliser rather than going cold turkey.
- There seems to be a benefit through using livestock to graze plants cover crops, rather than either mowing or drilling directly into them. Experiments have shown that this is not due to microbes coming from the saliva, urine or dung. Kris believes it is due to the grazing action of cattle (and perhaps sheep too?) which rips the leaves in a way what stimulates the plant to produce a burst of root exudates. These in turn kickstart microbial activity in the soil. As long as soil temperatures are above freezing then the effect can be seen. This theory could explain why mob grazing can provide such quick results in soil improvement.
- Plants take 3 weeks to start forming mycorrhizal associations. Do not skimp on starter fertilisers if reducing total fertiliser applications, so that establishment is not compromised. She also thinks that for this reason it is always valuable to plant a cover crop; even if it does not create a lot of biomass then it will still be feeding the microbes.
- It is possible that varieties are being bred which make less use of microbial assistance, as they are designed to achieve maximum yields in situations where nutrients are easily available. This may reduce their efficiency of nutrient use.
- She is ambivalent about glyphosate. Although it becomes inactive in the soil as far as the plants are concerned, she thinks it feasible that there may be an effect on microbial life. The same is true of fungicides, which do not harm mycorrhizae directly, but may do so to the other organisms that are associated with them.
So, once again, no real concrete pieces of research that make anything definitive, but perhaps some more pieces to the puzzle?
One quick fact to end on: a field of oilseed rape in flower produces 100l/ha of nectar. That’s a lot of pollinator food.