Mulch provides natures protection similar to our skin. Mulch slows evaporation, allowing the soil surface to remain moist, and inviting for the organisms working 24/7 to improve your garden soil.
Mulch can be straw, wood chips, or a cover crop of legumes such as clover, beans or vetch. One method of mulching which saves work is to pull weeds, and lay them back on the ground.
As mulch is broken down by organisms; the nutrients held within are taken down into your soil. You may occasionally hear that the mulch will rob the soil of nitrogen. This would be true if the mulch were mixed into the soil, but that is not what's happening with mulch. It lays on top and adds nitrogen as it decomposes, and returns to it's basic elements. Some mulches will provide a better carbon to nitrogen ratio than others.
Even materials such as recycled pallets with a C:N ratio of 125:1 and decorative barks are better than nothing, but they provide far less nitrogen and more carbon.
Materials such as composted manure 12:1, and composted yard waste 17:1 are best. Composting will kill weed seeds if left in a hot pile (131-140°F) at least one week. Also keep in mind that some organic materials contain elements which can
inhibit the growth of your garden. For example rhubarb leaves contain
oxalic acid which lowers pH and inhibits microbial activity.
Composted wood chips 40:1, fallen leaves 55:1, pine needles 64:1, and fresh wood chips with foliage 65:1 are not quit as high in nitrogen, but are beneficial because they shade the soil, retain moisture and promote the health of the "soil web". Adding a second source of nitrogen such as blood meal,
manure, or coffee grounds can be beneficial when high C:N mulches are used. But be very careful with fertilizers such as ammonium nitrate, or urea as these are extremely strong, and can kill your helpful microorganisms. For a more complete list visit this site from the University of Illinois .