As a general rule, mulch should be applied at a depth of 2-3 inches. This does not mean that you need to put down two to three inches of new mulch every year. When too much mulch is applied it prevents air and water from reaching the soil. This is detrimental for the health of your plants. The amount of mulch that is right for your property will vary depending on your current depth of mulch in your plant beds. Use less mulch for heavy, clay soils and more for lighter, sandy soils.