Mulch Improves Soil Texture
A layer of organic mulch on bare soil around plants improves the soil they grow in several ways. Because it cushions the force of hard rains on the soil, it reduces the potential for soil compaction. Because mulch slows down the rate at which rain enters the soil, it gives it a chance to absorb the water, rather than letting it run off to a storm sewer. By buffering heavy rains it also prevents excess water from leaching nutrients down through the soil out of reach of the roots or washing the rich topsoil away from the surface.
An organic mulch harbors lots of beneficial soil organisms which work nearer the surface of the soil because mulch keeps the soil there moist and fluffy. There they prey on the eggs of pest insects and other undesirables and participate in helping the mulch to decompose. As they do this they are also processing nutrients for the plants growing in the soil.. While they use some nitrogen in the soil to accomplish all these good things, it is an insignificant amount compared to the contribution they make to overall soil health.
Mulch Feeds Soil Critters
Earthworms and beneficial soil microbes are essential to having a healthy soil and thereby healthy plants. Mulch is food for these valuable soil creatures.
Earthworms pull the organic material down into the soil and what they don't eat is consumed by the soil microbes. If you have no mulch, you have few earthworms and beneficial soil microbes. See the section on the Ecology of Soil for more information about this phenonema.
Mulch Conserves Soil Moisture
Soil exposed to the harsh influence of sun, air and wind dries out more rapidly than soil that is protected by a layer of mulch. A good mulch can reduce the rate of moisture evaporation from soil by as much as 40%, depending on the material used.
Mulch may make the difference between plants making it through a drought in the middle of August or burning up. Also, as it decomposes, mulch adds spongy humus to the soil which helps it retain moisture longer during periods of drought.
Regulates Soil Temperature
In parts of the country where summers are quite hot, organic mulch is a valuable asset because it actually keeps the soil below it considerably cooler than bare soil in the same area. Most plants do not grow much when the soil temperature exceeds 85 degrees. This can be a critical factor for production in the backyard vegetable patch. Soil covered by 2 to 3 inches of organic mulch such as chopped leaves or straw is 8 to 15 degrees cooler, reducing the amount of time a plant endures an uncomfortable soil temperature.
In the winter a layer of mulch insulates soil somewhat. Mulch is not intended to prevent soil from freezing, but it can help hold the freeze so that intermittent thawing does not disturb the soil and cause it to heave. Frequent, rapid fluctuations in soil temperature are more harmful to beneficial soil microorganisms than a steady cold period. While desirable soil bacteria and fungi come back each spring, insulating the soil from the cold with a layer of mulch helps speed their return.