By Sue Edmonds, Waikato-based science and farming writer
For many of us, especially gardeners and farmers, it seems as though science changes its mind about long-held “facts” every five minutes, and the latest to be found inaccurate concerns humus.
For around 200 years we’ve understood that all those dead leaves and other decaying plant matter that fall to the soil are fragmented and broken up by insects and other larger soil organisms before being absorbed into the soil as small organic fragments. These fragments are then further decomposed by microbes to create particulate organic matter, ending up as humus – which lasts a very long time.
But now we need to look at the living plants on the surface, which are photosynthesising with their leaves and passing a large proportion of this as exudates of sugars and proteins down through their roots to feed the soil microbes and mycorrhizae, which return the compliment by supplying the minerals the plant needs.
Different plants produce different exudates, which feed different microbes, which supports the concept of plant variety being needed for healthy soil. Symbiosis is always a two-way street.
In the last decade, with new equipment, a number of the science fraternity have been looking more closely at POM and now consider that, on its way down the soil, microbes have devoured most of the carbon and broken down the remainder and put them inside soil particles, especially clay, where they are protected to form mineral associated organic matte, and it is this that can persist for centuries.
Thus it’s the roots of living plants, coupled with their microbial rhizosphere populations, that are making use of what comes down from above, to enrich the soil sufficiently to produce more above-ground crops. Maybe we should start thinking “keep soil rooted” rather than focusing on what we add on the surface.
Other scientists are carrying this idea further in relation to long-term underground storage of carbon. While many, including the Intergovernmental Panel on Climate Change, have built their climate models on the idea of soil sequestration, and stocks of long-term carbon have been found, computer models that predict the greenhouse gas impacts of farming practices, which are being used in carbon markets, are probably overly optimistic about soil’s ability to trap and hold onto carbon.
It had been thought since around 1786 that humus contained large molecules of long carbon chains that were too large to be eaten by microbes. The new equipment can be used to look at soil in place in the ground, rather than having it removed to a laboratory. This new method has disproved the existence of long carbon chains, and found everything to be small and devourable. Thus the existence of humus has now been debunked; it is microbes, minerals and microscopic soil particles.
This puts the need for a variety of plants on top, and a goodly population of soil life and microbes underneath as the main focus for both farmers and gardeners. If the soil is too disturbed, or what has been applied on top, both helpful and toxic, has killed off our underground friends, then the next lot of plants may find that one half of the symbiosis isn’t around to do what they are expecting.