Insoluble factor of Organic Matter is

Soil organic matter is the organic component of soil, consisting of three primary parts including small fresh plant residues and small living soil organisms, decomposing (active) organic matter and stable organic matter (humus).
Humin is the insoluble component of soil organic matter. It is a carbon-based macromolecular substance. Humins make up about 50% of the organic matter in the soil.