Soil carbon: precious as jewels and the stuff of life


Life on Earth is carbon based, and much is made of this fact. As planetary scientists search the cosmos for signs of extra-terrestrial life, they seek carbon-containing organic molecules to show that life may exist on other worlds.

Carbon forms the backbone of many molecules that are important to living things — animals, plants, fungi, microorganisms and all that is derived from them. If it is alive, or once was, chances are, carbon played an integral role. Even the fossil fuels we use today are remnants of plants growing millions of years ago.

Elemental carbon is the product of stars, where solar furnaces fused hydrogen and other lighter elements to form it. Carbon is also the sixth lightest element we know. Carbon and carbon compounds have many forms, from mostly pure manifestations as diamonds and graphite, to oil, coal and natural gas which has bonded with hydrogen. In living organisms, carbon compounds store the energy derived from the sun through photosynthesis and provide the energy for life. Much of the energy is stored in the form of sugars, fats, oils and other products; breaking these compounds down by digestion and decomposition releases the energy stored within.

In theory, carbon dioxide from the atmosphere was incorporated into plants by way of photosynthesis, forming organic carbon compounds that serve as the energy basis of the food chain. Carbon compounds move up the chain as herbivores and insects eat plants; omnivores feed on animals and plants; and the carnivores feed on other animals. In rare cases, plants are known to feed directly on insects and small animals. The fungi, bacteria and other microorganisms feed on live and dead plants and animals, returning carbon in to the carbon cycle. While most of it returns to the atmosphere as carbon dioxide, some of it is retained in materials for which we gardeners are all grateful to have.

The carbon found in soil is held in many forms and serves many functions. One important constituent of soil is carbon-rich humus. Humus is the organic matter in the soil that has broken down to a condition that it is considered stable within the soil. Humus is dark, spongy particles that have no regular shape or structure. Humus affects soil moisture holding capacity, bulk density, nutrient retention and serves as a matrix on which microorganisms can flourish. Soils with good humus levels tend to have tiny soil particles aggregated into clumps, rather than fine particles that can easily be displaced by wind and water of mismanaged fields. When soil is left in its natural state or when we manage agricultural fields well, soil humus levels are stable and can remain so for centuries. Humus is naturally occurring in woodlands, prairies and other locations where organic matter is allowed to decompose. No-till fields tend to have higher quantities of humus than conventionally tilled fields.

In our gardens, we can contribute to the formation and stability of humus by using practices that maintain organic matter in the soil. Keep tilling to a minimum and don’t overwork your soil. The use of organic mulches perpetuates soil humus as they eventually decompose into humus. The addition of well-made compost is also beneficial as it is high in humus.

The reason humus is prized in the garden is because of its ability to help plants grow better. Humus holds on to nutrients and releases them slowly for plant use. Many believe that the microporosity of high humus soils allow beneficial microorganisms to thrive, thereby suppressing disease causing organisms.

As with precious jewels, safeguard your carbon-rich humus, and it will pay dividends for generations to come.

For more information on this and other gardening topics, visit the CTAHR electronic publication website at ctahr.hawaii.edu/Site/Info.aspx or visit any Cooperative Extension Service office. I can be reached at russelln@hawaii.edu.