Soil management Soil husbandry specialist, Steve Townsend of Soil First Farming turns his attention to organic manuring in the second of our series trying to sort out the wheat from the chaff in modern soil management.
‘Organic manuring is the best way to improve your soils’ Building organic matter is vital to improving the productivity, workability and resilience of our soils. We keep being told this. And it’s true. So, it’s easy to believe the key to soil improvement is to add as much organic matter as we can from wherever we can get it. If only biology were that simple! Unfortunately, like living things the world over, soils are far more complicated. This makes it easy to get things wrong. Especially so, if we apply modern ‘quick fix’ agronomic thinking. The sort of thinking that suggests metal at depth is the answer to soil compaction. Or that extra nitrogen fertiliser is a panacea for higher yields. Or, indeed, as we discussed last month, that cover cropping will solve just about every soil problem. In looking to improve soil organic matter, the first thing we need to appreciate is that it’s not a single thing. As well as dead and decaying material, it includes a vast array of different microbes and other living things, together with organic compounds like humic acids, polysaccharides and the glomalin – or soil glue – produced by mycorrhizal fungi. All of which act and interact together to govern the medium on which our cropping so
fundamentally depends. I also find it helpful to understand that organic matter is essentially of two main types; long- term – which is very stable and takes a long time to influence; and short-term – which is as unstable as a revolving door and acutely sensitive to change. This and the sheer complexity of organic matter makes current testing next to useless. Not least because both the methods available really only measure one thing – organic carbon. What’s more, they only tell us how much of it we have. Vital though this nutrient is, the tests make no distinction between long-term and short-term organic matter. Nor between the earthworms, plant roots, straw, humus, fungi and other components that all play different roles in its value.
Biological quality So let’s focus on soil biological quality rather than organic matter content in our improvement efforts. And let’s think about how best we can nurture and feed that biology in much the same way we do with the complex microbial systems we call cows and sheep. If we think like this several things are immediately obvious. We have to know exactly what we are
feeding. We need to balance this carefully and, wherever possible, avoid rapid change. And we can disrupt performance badly if we get other elements of our stockmanship wrong. Feeding our soils with organic manures can certainly be valuable. The key challenge, though, is to get that feeding right. In particular, to make sure we maintain the soil carbon:nitrogen ratio as near to the optimum of around 10:1 as we can. Adding too much carbon without sufficient nitrogen in manures like paper waste, sawdust, wood chippings and some composts (depending on what they’re made from) will mean carbon choking: nitrogen, phosphate, sulphur and other nutrients being tied-up by the micro-organisms involved in breaking them down and unavailable for crops. Some manures – like FYM – are clearly better for earthworm populations than others too. Chicken layers muck can cause problems as a result of high calcium levels. And large inputs of single manures can be very disruptive to the biological balance. So, if we don’t want organic manuring to do more harm than good to crop performance and soil biology it’s important to know exactly what we are applying; apply it in the right quantity; and, wherever necessary, provide extra nitrogen, in particular, to balance the additional carbon input. We also have to be wary of doing more harm than good in the application process itself; applying manures to dry soils that will get wet, for instance, rather than wet soils vulnerable to the heavy trafficking involved.
Another major drawback of organic manures, of course, is that they have to be incorporated. This can physically destroy important links in the biological network as well as forcing large amounts of air into the ground to actively promote organic matter loss through oxidation. So, do we actually have to add organic manures to improve soil biology? Not in my experience, at least. Indeed, I’ve found it amazing how much improvement can be made without any manuring whatsoever.
Quality This may not show-up in organic matter testing because it’s more a matter of quality than quantity. However, I’ve seen a combination of chopping and spreading straw, minimal soil disturbance and the use of well-managed cover cropping alone transform the health and biological activity of soils. So my preference is to disturb soils as little as possible; leave crop residues on the surface to provide a mulch before being incorporated naturally; and include cover crops in the rotation to keep the ground in the best condition over-winter. It will take time. But I’m convinced it will lead to a far more sustainable improvement in what really matters and with far less risk of getting things wrong than just relying on organic manuring. Between exploring more of what he sees as today’s most damaging soil management myths with us, Steve Townsend is more than happy to discuss any tillage or soil management issues, interests or concerns with Farmers Guide readers by email: steve. firstname.lastname@example.org; or phone 01452 862696. ■
Feeding our soils with organic manures can certainly be valuable. The key challenge, though, is to get that feeding right, says Steve Townsend.
14 www.farmersguide.co.uk May 2018
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Farmers Guide Magazine May 2018 Issue