The three Great Groups of the Organic Order found in Saskatchewan are based on the degree of decomposition of the middle tier (i.e., from 40 to 120 cm thickness of the peat). The degree of decomposition can be assessed in the field using the Van Post Scale of Decomposition (see material of horizonation). If the thickness of peat is less than these limits the soil is considered a Peaty Phase of the Gleysolic Order.

Fibrisol Great Group:

The middle tier is dominated by organic material which is recognizable and retains its fibrous material.

Humisol Great Group:

The middle tier is in a more advanced stage of decomposition and does not retain its fibrous material; commonly the material would squeeze through your fingers when squeezed.

Mesisol Great Group:

The material in the middle layer is in an intermediate stage of decomposition between fibric and humic.