The biggest difference between soil and rock is the presence of organic matter and the associated biological activity that takes place in the former. Soil organic matter is at the heart of healthy and productive soils. Although organic matter typically makes up a small percentage of most mineral soils (less than 10% by weight), it is very important because it positively influences or modifies the effects of almost all soil properties.
Understanding the role of organic matter in maintaining healthy soil is essential to ensure a high level of productivity and to minimise the negative environmental impacts of farming activities.
What’s in organic matter?
Soil organic matter is the fraction of the soil that includes plant and animal residues at various stages of decomposition, plant roots, cells and tissues of soil organisms, and substances synthesised by the soil population. Fresh or undecomposed plant materials such as litter, straw and animal dung lying on the soil surface are not included in this definition although, upon decomposition, these will eventually become part of the soil organic matter.
Organic matter is largely made up of the elements carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur. It is expressed as a percentage of the soil mass less than 2 mm in diameter.
The different forms of organic matter
Soil organic matter is a dynamic mixture, changing constantly due to new additions of organic matter to the soil and losses from the soil. There are various forms of organic matter, each differing in their biodegradability or resistance to decomposition.
They are generally divided into three pools: active, intermediate or slow, and recalcitrant or resistant. The active pool includes microbial biomass and labile organic compounds that make up less than 5% of the soil’s organic carbon. In the slow pool they make up 20% to 40%. These three pools have different rates of turnover . Time for turnover in the active pool ranges from months to years, decades for the slow pool, and the recalcitrant pool takes hundreds of thousands of years.
The active pool includes microbial biomass and labile organic compounds that make up less than 5% of the soil’s organic carbon. The slow pool usually makes up 20% to 40%. These three pools have different rates of turnover. the active pool ranges from months to years, the slow pool in decades and the recalcitrant pool in hundreds of thousands of years.
The active parts of the intermediate pools are involved in nutrient supply and in the binding of small soil particles together to form larger structural units called aggregates. Aggregation is important for water infiltration, aeration and drainage. It also reduces the soil’s susceptibility to erosion.
On the other hand, the recalcitrant pool or humus possesses a large number of negative charges and contributes largely to the nutrient-holding capacity (cation exchange capacity) of the soil. It also imparts a dark colour to topsoil.
Source: Bay of Plenty Regional Council: Land management – Soil organic matter