A mulch layer on the soil surface allows the soil to soak up more water. Mulch also reduces the rate of water loss from the soil. A 3-inch layer of mulch on the soil surface dries much faster than the soil below it. Thus it prevents water from moving into the air. See figure 2.
Mulches modify soil temperature in home gardens.
Applied in late fall, winter mulch insulates plant roots, crowns and stems of winter crops from extremely low temperatures. Hypertext markup and graphics colorization by Gretchen Eagle and Dan Lineberger.