A mulch layer can be used for many different reasons. The prime factors in why many people decide to use a mulch layer are; conserving moisture in the soil, suppressing weeds, introducing organic matter into the soil, giving the soil microbes, arthropods, microarthropods, and worms a place to live, and many more.
When you couple mulching with the application of soil amendments and fertilizers you can actually help to speed up the rate at which the amendments and fertilizers become bio-available by adding a mulch layer. Adding a mulch layer after the application of organic fertilizers will increase the microbial activity at the soil surface, thus the microbes and soil critters, will digest the fertilizer faster, making the nutrients available to the plants quicker than if you hadn't applied a mulch layer.
Conserving water is a huge factor that the agriculture industry is going to have to deal with when it comes to growing crops. In organic production, adding a layer of mulch can help slow the evaporation of moisture from the soil. By doing this it will also keep the top layer of soil from drying out, and becoming hard or compacted. This will give the plants more healthy soil to grow in and less stress to deal with. A mulch layer will not only conserve water, but it can help slow down rains so that they soak up into the soil instead of running off the side of the field, or container. This means leaching will be slowed, and or stopped and less nutrients will be needed for each seasons crops.
Suppressing weeds in agriculture is typically done with machinery scraping through rows, or via chemical sprays like round-up. By using thick seed free mulch layers you can naturally suppress the growth of weeds without the use of any chemicals or extra labor. Weeds can compete for nutrients in the soil, take much needed water, and be vectors for pests and disease. By mulching you suppress the weeds, and also extend the amount of soil is available for the plants roots to grow into, as well as increasing the available biology in the soil.
Adding in organic matter to natural soil can help the texture, and increase performance of said soil. The mulch layer will begin to be invaded by micro-organisms, and insects and start to be broken down on the soil layer. This increased activity will naturally till your soil via insects and soil biology and improve the overall health of your soil.
A mulch layer can be made up of many different inputs. Commonly used are hays, and straws made from grasses. Bermuda hay, alfalfa hay, barley, etc, are all very good options to use as a mulch layer. They are readily available, and the cost is minimal. Using locally sourced waste products can help to clean up the environment, save you money, and help your garden. Things like rice hulls, pecan shells, cotton hull waste, etc.. can be sourced from local manufacturers and purchased or even procured free.
Adding a mulch layer to any organic garden can bring many benefits, which will easily out way the cost. Just make sure that the mulch layer is seed free, and also won't throw off the balance of the soil nutrients or biology. Grasses and straws are always a safe pace to start.