There are various forms of organic matter, each differing in their biodegradability or resistance to decomposition.
Soil organic matter is a dynamic mixture, changing constantly due to new additions of organic matter to the soil and losses from the soil.
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.
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