Soil Organic Matter (SOM)

‘Organic’, in science, means nothing more than ‘contains carbon’. It doesn’t imply ‘healthy’ or ‘good for you’ — atrazine is as much ‘organic’ as is glucose.

Organic compounds are any molecules which contain carbon, whether man-made or natural. Well, that’s what we learnt in high school and university, an eternity ago! It has since been updated to specifically refer to any compounds which contain carbon-hydrogen bonds. [This is a much cleaner definition, and automatically excludes the very simple carbon oxides (CO, CO₂) and carbonates (containing the CO₃^2- ion) which were always regarded, even by chemists, as ‘inorganic’.]

Organic matter refers specifically to the organic compounds from once-living organisms. Organic matter includes plant and animal remains as well as their waste products (faeces and urine from animals; resins, gums, and root secretions from plants).

You’d be forgiven for thinking that soil organic matter is simply the organic matter found in soil. But soil organic matter, or SOM, is actually made of three components:

1. the organic matter which is plant and animal remains and their waste products, in various stages of decomposition
2. the living soil microbial biomass
3. the end product of that microbial biomass acting on that organic matter: humus

1. Organic Matter Composition

Both animal and plant remains contribute to SOM, but SOM is mostly vegetative in origin, whether directly from decomposed leaf litter and branches, or indirectly from the waste products of herbivores, earthworms and arthropods.

Anywhere from 60 – 90% of these remains are water. The bulk of the dry matter contains carbon, hydrogen and oxygen as carbohydrates, lignins, fats, and proteins. Proteins are also a source of nitrogen and sulfur.

Plant and animal tissues also contain other macro- and micronutrients: phosphorus from DNA, magnesium from chlorophyll, iron from haemoglobin, and calcium from cell membranes are just some examples of important elements and their potential sources.

2. The Soil Microbial Biomass

Whether by treading on leaves and twigs, or by digestion and excretion, it is animals, earthworms and arthropods which break plant and animal matter into smaller pieces with increased surface areas. These pieces are still complex however, and not plant-available. It is the soil microbial biomass acting on these increased surface areas which decompose these remains into plant-available nutrients.

Animals ingest complex compounds for their nutrition, but plants cannot. Plants instead rely on their nutrients supplied in elemental form (eg zinc, iron, magnesium) or as very simple molecules (phosphate, nitrate, sulfate).

Only microbes are capable of decomposing organic matter into elements and very simple molecules, and this conversion of organic compounds (such as proteins) into inorganic ones (such as sulfates) is known as mineralisation.

Mineralisation involves any number of different enzymatic actions by various microbial species. These reactions contribute to the heat generated by ‘hot’ composting. Decomposition occurs at different rates for different compounds and their different chemical bonds — sugars break down much more quickly than cellulose and lignin, and proteins break down faster than fats.

Some organic matter is not mineralised into inorganic matter, but rather is converted into humus, a very stable form of organic matter resistant to decomposition.

3. Humus

The past two weeks covered how to make compost on a large scale and a micro scale, and last week’s post ended on the note that ‘[compost] will continue to age and improve during [storage].’

This is because compost is not the finished material it appears to be, but is rather organic matter in a very late stage of decomposition still underway. It has not yet been mineralised or further converted into humus.

Compost isn’t sterile, and microbes are always present. Though rates of decomposition will slow significantly compared to the original composting activity, compost simply left alone and away from soil will still both mineralise and form humus with time — ’time’ being years. It is important to keep stored, unused compost covered and away from rain so as to prevent those ‘minerals’ from leaching away.

Compost makes a great soil conditioner and fertiliser as it is the means by which ‘boosters’, if you like, are added to soil — the resident soil microbes feed on this new input of energy and nutrients, growing and reproducing and producing, as their waste products, the simpler elements and molecules plants require as their nutrient supply.

‘Humus’ is the organic matter that wasn’t mineralised and uptaken by plants, but was instead further converted by microbial and other chemical activity — perhaps as a by-product of the mineralisation process — into another, more stable form, of organic matter resistant to further decomposition.

Compost isn’t humus, but both are organic matter, and two of the three parts that make up SOM. Adding one part, highly decomposed compost, to soil stimulates another part, the microbial biomass, which in turn builds up the remaining part, humus levels. More on humus next week!