Soil organic matter cycle

Composts
can be made from a wide selection of organic materials and can help to improve soil structure and provide a source of slow release nutrients in the soil.

It is important to make sure that the compost is corectly made otherwise it can actually hamper plant 

growth by removing N from the soil leaving less available for plant growth.  A good compost is an important aid to maintaining SOM, especially under cultivation.

is formed from the decaying and decomposed remains of dead plants and animals but also includes the soil fungi and bacteria and small animals living within the soil. Animal excreta is also another important source of SOM.

There are 4 distinct pools of SOM:

Soluble (i.e. in water)

Particulate -partially decomposed material of small size but still recognisably cellular in structure. Can be up to 25% of SOM.

Humus -a generally stable material with recognisable molecules like proteins and cellulose but also other much less recognisable super molecules that we refer to as fulvic and humic acids. These molecules have reactive regions that allow them to be stabilised by reacting with other soil components and thus often help bind soil particles together. Humic materials can be up to 50% of SOM.