NO problems of soil science have proved more difficult than those concerning the nature and functions of soil humus. The early work of Sprengel and Berzelius yielded a long list of humic and other ...
Humus is highly recalcitrant, and this resistance to decomposition leads to a long residence time in soil. Plant debris is less recalcitrant, resulting in a much shorter residence time in soil.
For clay, it can be a game-changer. Organic matter is basically anything that decomposes and adds humus to your soil. Compost, animal manures, peat, lawn clippings, straw, wood chips, cardboard, leaf ...
Soil with not much compost or humus in it will blow away in the wind very easily. Putting compost in your soil feeds the soil and makes it strong and healthy.