Have you ever noticed the car in front of you at the traffic signal that has water dripping out of its tailpipe?  
The result of burning carbon-based fuel in the presence of oxygen is the release of energy plus carbon dioxide and water. That dripping water is a sign that the engine is burning cleanly. Whether it is an internal-combustion engine, a coal-fired power plant, a jet engine or you, the result of breaking a carbon atom off a chain of other carbon atoms is the release of energy and the formation of carbon dioxide and water.
When you eat food (fuel) and breathe in air (21 percent oxygen), your body burns the fuel to release energy your body uses for everything it does. You exhale carbon dioxide and water in your breath. You also perspire water and excrete water in your urine. You use oxygen and burn fuel to release energy and carbon dioxide and water by the same general equation as a bonfire, motorcycle or any air-breathing animal.
Guess what? So does soil.
Soil is alive. For a century, soil scientists have measured soil productivity by the volume of carbon dioxide the soil releases. The more carbon dioxide it releases, the more productive the soil is because the soil supports more life.
What is producing that carbon dioxide? The bacteria, fungi, algae, actinomycetes, insects, earthworms and all the other critters that live there. The more life the soil can sustain, the more plant life the soil can sustain. All that life is based on having chains of carbon atoms in the soil that the various life forms can split apart and “burn” to release the energy stored in those chemical bonds for their use. Putting carbon back into agricultural soil is the key to maintaining soil productivity.  
In the home ornamental-plant landscape, we return carbon by mulching grass clippings when we mow instead of bagging them and carrying them off. Grasses pump 80 percent of their energy into the soil.  Keeping your lawn healthy and deep-rooted probably is the fastest way to increase soil organic matter in the home landscape. We rake the leaves from trees and shrubs, mulch them with the lawn mower, and use them to replenish mulch under trees and in shrub and flower beds. When plants and soil are healthy, the mulch will disappear at a rate of about 2 inches per year. Replenishing mulch to maintain a 2-inch thick minimum depth should be at least an annual event. Don’t scrape away the old mulch — just dress up the top with new mulch.
Any woody-plant-source mulch is OK for woody plants, but don’t put grass clippings on woody plants as grass poisons them.
In addition to returning carbon to the soil, mulch also helps retain soil moisture, lowers soil temperature, reduces erosion and helps reduce disease development.
There is such a thing as too much mulch. Mulch thicker than 4 inches deep can intercept and retain a quarter-inch of rainfall and lead to drought conditions for mulched plants even though they have received light rain.
The most serious abuse of mulch you will recognize as “volcano trees.” They have mulch piled up around their trunks so they look like small volcanoes. If it were not such a serious threat to plant survival, it would be funny. Nature does not abuse plants this way — only people abuse trees with volcano mulch piles.
Mulch piled up against the trunk creates an environment that allows soil fungi, insects and bacteria to assault the aerial bark of the trunk and eat it. Thin-barked trees can be killed in as little as three years, while mature oaks can be killed in 11 years. It also provides cover for mice, rats and voles to hide as they eat the bark off the bottom of the trunk of the tree or shrub, which kills the plant. Mulch should never touch aerial bark. Sunlight and wind must be able to reach the trunk at the ground line.
In field agriculture and vegetable-gardening practices such as low-till or no-till, turning in green manure crops and crop residues all are aimed at protecting soil and increasing soil organic-matter levels. The sandy coastal soils we have are low in organic matter. Those wanting to create a productive soil out of our beach sands know they cannot work too much organic matter into their soils. Like anything else worth doing, you only get out of it what you put into it. No carbon in, no carbon out. Given as much as our sandy soils would benefit from organic matter additions, there should be no green-waste stream from residential properties to coastal landfills. Mulch it, compost it, recycle it, reuse it.
It’s the way nature builds soil.