Most feeder roots for our plants live in the top two or three inches of soil. When the soil beneath our plants appears battleship gray and equally hard, you can assume those plants aren't finding moisture and nutrients to perform at their best. Adding three to four inches of mulch softens the soil structure almost immediately. For clay soil mulch allows for better water and air movement through the soil and in sandy soil it improvise its ability to hold water. Suddenly, water from your sprinklers doesn't skitter off the soil's surface. The mulch shows the percolation of water down and allows the soil to absorb it fully and acts as an insulating later keeping it cooler in the summer - the roots like that!