Abstract: Processes for production of a plant growth substrate or a casing soil from sugar cane bagasse, and plant growth substrates produced from these processes are disclosed. The processes generally include biological removal of residual sugars, suppression of the development of cellulose degrading microorganisms so that the resulting mass loss is less than 10% of the original material dry weight, mechanical disintegration to form a disintegrated material with a defined density and porosity, reduction of pH and extraction of potassium to provide a defined electrical conductivity, and blending of the disintegrated material with one or more mineral components and plant nutrients. Additional steps in the process may include fermentation after the mechanical disintegration, and optionally addition of live organisms to add beneficial functions to the substrates. The substrate may find use as potting soil or blending components for soil mixes, or as casing soil for the production of mushroom.