When something in gardening is agreed by absolutely everyone to be a good thing it seems perverse to ask the question “why?”. I tend to the view that that is when it most needs to be asked.
Composting and the multiple benefits of compost are among of the great untouchable sacred cows of gardening, especially among the organic fraternity. However, ask a bunch of gardeners to explain why and I bet you’d get a very mixed bag of responses. Ken Thompson, author of a book on compost, called “Compost”, says “Few things are better for your plants and for the environment than home made compost”. He then goes on to explain how decaying plant material ends up as humus. He points out though that only a small proportion of the original material ends up as humus, the rest being broken down to carbon dioxide and water.
The breakdown process is effected by microbes which secrete mucilages that bind soil particles together into crumbs, creating what is known as “soil structure”. However, a protein called glomalin, produced by mycorrhizal fungi, has been found to be present in soils in far greater amounts than humus, and it is largely glomalin that aggregates soil into crumbs, not humus. Since the mycorrhizal fungus is getting its nutrition from its symbiotic partner, the growing plant, not from organic matter added to the soil, the humus derived from compost is somewhat sidelined in the narrative. It is growing plants (including weeds) and their associated mycorrhiza, that are doing the heavy lifting when it comes to soil improvement.
Wikipedia has a bullet point list of the benefits of soil organic matter and humus, the first of which is that organic matter feeds microorganisms, maintaining high and healthy levels of soil life. There is an implication in much of what you read that there is a direct correlation between lots of life in a soil and how well plants will grow in it. To which I would ask why, in commercial horticulture, do nurserymen almost all grow nursery stock in the all but sterile medium of peat and tomato growers in equally sterile variations of hydroponics.
Plants have evolved to grow in an extraordinary range of environments. The ferns growing on my neighbour’s roof, or the sedum around my other neighbour’s chimney, do not have their roots in a lovely friable living soil. Yet they survive perfectly well. Plants that need constant moisture or higher levels of nutrients would struggle or even die, but could all be grown in a peat nursery compost or by hydroponics.
The role played by the living and once living fraction of soil then is to provide the simple things that plants require, water, air and simple inorganic nutrients. A soil in good condition is open enough to allow the free passage of excess water whilst retaining good quantities in the porous organic matter. It holds nutrients sufficiently tightly for them not to be leached by rain, but loosely enough to be readily available to plants.
Humus, organic matter derived from but no longer recognisable as decomposing plant material, is beneficial to the soil in many ways. It adds to a soil’s cation exchange capacity and its water holding capacity. It acts as a buffer against excessively acidic or alkaline conditions and can absorb toxic materials such as heavy metals and excess nutrients.
Another claim about composting that is not true is that all the nutrients in the plant material added to the heap are retained within the heap. One study found that nearly a quarter of the nitrogen in mixed organic refuse was lost in the first twenty weeks of composting, probably mainly as gaseous ammonia. Adding soil to the mix stopped the loss, presumably because the ammonia was absorbed by the clay fraction of the soil. Additionally, any liquid draining from the heap will be taking dissolved potassium with it.
Composting can generate high temperatures, but only if there is sufficient volume of material for heat loss to be reduced enough. The small volume of a typical domestic compost heap has far too high a surface area to volume ratio to heat sufficiently to destroy weed seeds and plant pathogens.
It turns out that simply mixing plant wastes into the soil, or spreading it on the surface and letting worms do the mixing, adds more humus and more nutrients to the soil than if it is composted first and has also been shown to produce higher yields. The relatively coarse nature of uncomposted material means that adding it to light sandy soils might make them even more open to the detriment of the crop, whereas compost holds much more moisture and will reduce the openness of the soil. On heavy soils there will be a benefit from the opening up of the soil.
What I have been unable to find any reference to is the benefits arising from the early stages of breakdown of organic matter if that takes place in the soil rather than on the compost heap. It seems to me that the level of activity in the early stages is far greater than in the later stages. The very rapid build up of heat, the product of respiration by bacteria, within hours of a pile of suitable material being stacked up, followed by a peak in worm activity in the weeks following, is then followed by a tailing off of activity and a finished product which is not generating any heat or supporting hardly any fauna visible to the naked eye.
It is worth pointing out that nature does not build compost heaps. Plants die down in autumn, or leaves fall from trees, and breakdown happens at ambient temperature. A significant amount of vegetation may be eaten by herbivores and the breakdown process is then well under way when it is deposited on the soil surface.
So it seems to be the case that we put stuff onto a compost heap for convenience. During the growing season there may be no bare ground on which to spread organic material. It is sometimes suggested that fresh material encourages slugs, but if they eat it, rather than the crop, they become part of the solution and less of a problem.
Almost all of what goes on my allotment compost heaps goes through my shredder first, so it breaks down to a material I can use for mulching in my no-dig regime quite quickly. Most of what I accumulate over the growing season is used for mulching bare ground ahead of the winter. There isn’t too much opportunity for nutrients to be lost to the heap in that time, rainfall would likely never be sufficient to run through the heap removing soluble nutrients. Soft material such as peas and beans gets mixed with woodier material like hedge trimmings.
Pea haulms, shredded and spread directly on the soil.
I believe it is very beneficial to have any bare ground covered in the winter to protect its structure and I have used compost for that purpose. What I am now planning to do is to spread shredded material directly onto the plot where bare ground appears that I am not planning to crop again until spring. I would anticipate the softer material disappearing quickly, the tougher stuff remaining to provide protection to the soil. If a suitable area presents itself, I shall do half with composted material, the other half with uncomposted material. Then if I grow the same crop in both areas I can look for any visible differences.
Roger Brook – The no dig gardener – http://www.nodiggardener.co.uk/search/label/Glomalin
USDA AgResearch Magazine – https://agresearchmag.ars.usda.gov/2002/sep/soil
E. W. Russell – Soil Conditions and Plant Growth 10th edition. Composting – pp 271-3