It is possible to find biodegradable fabric if a fashion enthusiast desires completely natural threads. Technically, any type of clothing that is based on organic fiber is biodegradable and can be broken down by bacteria or fire without toxic effects. That said, cotton is sometimes treated and mixed with nylon or polyester fibers. Hemp is very durable. Finding something that breaks down quickly might mean shopping in a natural fiber store or buying something expensive.
The first thing to remember that the only reason why biodegradable fabric is a concern is because synthetic fabric is so common. Plastic-based fabrics do not easily mildew or breed bacteria. They can be saturated in water without much ill effect, and then tend to carry brighter colors. While these attributes make polyester attractive to retailers, it means that a shirt put in the dump might never break down.
To prevent fiber from becoming part of the fossil record, simply buy fabrics that contain organic threads. It is not hard to find these because many of the best options have been around for thousands of years. Hemp fibers are the most durable and are frequently used as work clothes. They are very environmentally friendly because of the ease of growing the plant.
Below that, wool and linen combine durability and versatility. Wool is warm even when wet and is frequently used to make winter wear and long socks. Wool is a bit expensive as it requires tending to the needs of sheep. Linen is less common than cotton, but it is grown in areas where cotton is not suitable. Linen fibers can be made thick or relatively fine. Before cotton, much of the western world used linen for comfort wear.
They say that cotton is king, and it is famous for being a very fine natural fiber. Made from the fluff that plants use to disperse their seed, cotton thread is very fine and can only be matched by silk and synthetic fabric. Unlike synthetic, cotton holds moisture very well and so it is used as undergarments for comfort and to transfer perspiration.
Because cotton is very fine, it biodegrades quickly. On the flip side, cotton tends to consume nutrients from the soil quickly and so it is not considered the most environmentally friendly material. Other options include viscose fiber, which is made from waste wood pulp. Odder plants are used to make fiber, and it pays to find a website that lists all the available options.