There are many factors that need to be taken into account to determine how long it takes for a 100 per cent cotton t-shirt to decompose in a landfill site. First of all, depending on the environment it is found in, a swatch of cotton fibre can decompose in a matter of two weeks if given the proper conditions. As well, the temperature of the soil and the air above-ground, as well as the depth at which the shirt is buried in the landfill site will also have an impact on the decomposition rate of the cloth. Cotton fibre after ginning is composed of approximately 95 per cent cellulose, with the remaining 5 per cent being mainly composed of wax, pectins and proteins. These are produced by various microorganisms, such as bacteria or fungi.
Cellulosic material is considered biodegradable.
However, dyes and other treatments to the t-shirt must be taken into account when estimating the decomposition rate of the garment, as they may contribute to a slowing of the process. We are often told to compost in order to save energy. Garbage has always been an issue with me. The answer to your question may be posted in an upcoming issue.