I have written at considerable length on this subject in my book, and as previously posted, will not be repeating the book here, but I am prompted to post a few of the basics because in the last three months or so I have read some misleading and confusing articles related to organic material (shortened to OM for the rest of the blog) soil organic matter (SOM) and humus. Like most gardeners and farmers I am not a soil scientist, but it is necessary for us to understand a little of the subject and some scientific terms if we are to make the best use of our land and keep it fed and healthy. Note that even some soil scientists use OM and SOM interchangeably.
I will begin at the end – with humus. Unfortunately, as with numerous things in life, the word is frequently wrongly used in general terms, and in the case of humus to mean several different things relating to soil. This is where the misinformation and confusion begins. I am assuming that readers are aware of at least some of the benefits of humus in the soil. If not, please accept that it is vital in improving and maintaining soil fertility and structure.
Humus is a dark brown or black colloidal and amorphous substance that is the result of the breakdown of OM to the point where it will break down no further. Colloidal means that it is a mixture of particles of different components and is neither a solution nor a suspension. In the case of humus it is gelatinous and sponge like. Amorphous means that it is has no definite shape. Imagine a dark sticky blob that can absorb water. The fact that it is an extremely stable substance means it will remain in the soil for a very long time – some soil scientists suggesting up to several thousand years.
Now back to the beginning. Under natural circumstances, and often with our help, viable seeds will sooner or later develop into plants - if something does not previously consume the seeds. The plant grows, is harvested or dies, and leaves a residue either in or on the soil, or in the faeces of living organisms that consume the plant, anything from small creatures in the soil to elephants. For simplicity we will ignore the loss of material through burning or waste that is not returned to the soil, and there is a prodigious amount lost in these ways, including plant material that passes through humans and goes via drains to a sewage works.
As gardeners and farmers we are concerned with returning as much of the plant as possible back to the soil in order to create more humus. We have already left some residue in and on the soil, but we may also have waste, which, if we have any sense, we turn into compost at every available opportunity, and we may have used some to feed to, and provide bedding for, our animals, producing good old FYM. At this stage it is all OM – material that can be incorporated into our soils. The underground plant residues, together with all the underground living organisms and the remains of dead ones are SOM. They are all potential humus and once our OM becomes incorporated into the soil then it becomes part of the SOM too. Humus is also part of the SOM.
Now for a couple of misconceptions. I have seen it stated that inorganic fertilisers will never produce any humus. One million tons of artificials will not add one ounce of humus according to an old author Friend Sykes in Humus and the Farmer and many present-day people think and write the same. But if you use inorganic fertilisers to grow a crop, that crop is OM and SOM. I should think it is impossible for at least some of that crop not to end up as humus.
I have also read articles by proponents of biochar (a type of charcoal produced at low temperatures) that biochar should be incorporated into the soil instead of organic matter on the basis that organic matter breaks down quickly. Biochar is organic and when it is incorporated into the soil it is SOM. It should be noted that the black soils of South American Indian origin (usually referred to by the Portuguese phrase Terra preta do Indio, which means the same thing) were only partially composed of charcoal, and finely ground charcoal at that, not lumps. It is now accepted that they were purposely formed by the incorporation, over a very long period of time, of a mixed compost of this charcoal, household refuse (including human waste) and broken pottery. The time for the improvement of the original soil through the use of this compost is quoted in a very wide range, from several thousand years ago to pre-Columbian times, or as little as from less than 2,000 years ago onwards. For our purposes the time frame is not desperately important.
Some of this compost is now in the form of humus, and is apparently extremely stable. I am not knocking biochar, I think it is worthy of much research, but do not expect to transform your land in a short space of time through the use of biochar. It is also extremely difficult to produce large quantities. It is much easier to produce the same amount of FYM or compost, and these will give you humus much quicker than biochar that takes a long time to be broken down – the reason it is being suggested as a means of sequestering carbon.
The fastest and best method I know to increase SOM and therefore humus is to use a grazed grass ley. The last paragraph of my earlier blog “2012 and beyond” gives the result of my experience on my present property with the use of such a ley and green manure crops.
