Thursday | August 17, 2017
About Us | Contact | Subscribe

Soil carbon: Precious as jewels, stuff of life

It is commonly stated life on planet Earth is carbon-based, and much is made of this fact. As planetary scientists search the cosmos for signs of extraterrestrial life, it is always carbon-containing organic molecules they seek as a sign that life might exist on other worlds.

Carbon is important because it forms the backbone of many molecules important to living things, from animals to plants, fungi, micro-organisms and all that is derived from them.

If it is alive, or the remains of a living organism, it has a high probability of having carbon as an integral part. Just think of all of the fossil fuel we use today, which is remnants of plants growing millions of years ago. Carbon and life are linked together because it is the stuff all known life forms are dependent on and a common denominator of life as we know it.

Elemental carbon is the product of the stars, where solar furnaces fuse hydrogen and other lighter elements to form carbon, the sixth-lightest element we know (one of 118 known elements). Carbon and carbon compounds can have many forms, with diamonds and graphite being most pure, to oil, coal and natural gas, which have many bonds with hydrogen to be useful as fuels. In a living organism, 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, oil and other products, and breaking these compounds down by digestion and decomposition releases the energy stored within.

Nearly all of us were introduced to the carbon cycle in science classes while studying Earth Science. 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 herbivore animals and insects eat plants, the primary producers; omnivores feed on animals and plants and carnivores feed primarily on other animals. In rare cases, plants are known to feed directly on insects and small animals. The fungi, bacteria and other micro-organisms feed on live and dead plants and animals, returning carbon into the carbon cycle. While most of it returns to the atmosphere as carbon dioxide, some of it is retained in various carbon-rich materials for which we as gardeners are all grateful to have.

The carbon found in soil comes in many forms and serves many functions important to all of us.

One very 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 has been described as the dark, spongy particles that are amorphous, having no regular shape or structure. Humus influences soil properties in that the amount of humus affects soil moisture-holding capacity, bulk density, nutrient retention and serves as a matrix on which micro-organisms can flourish.

Soils with good humus levels tend to have tiny soil particles aggregated into clumps rather than fine dust-like particles that can easily be displaced by wind and water on mismanaged fields.

When soil is left in its natural state, or when we manage agricultural fields well, humus levels are stable and can remain so for centuries or more. Humus is naturally occurring in woodland forest, grass prairies and other locations where organic matter is allowed to decompose, and can be best seen as the dark brown-to-black layer right below the decaying leaves and stems. It can also be found in significant quantities in well-managed agricultural fields and gardens. 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, especially with a power tiller. The use of organic mulches perpetuates soil humus, as they eventually decompose into humus if left long enough in place. The addition of well-made compost is also very 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.

It was reported humus holds onto nutrients and releases them slowly for plant use. As mentioned before, soil high in humus has a higher water-holding capacity. But in addition, because of the resulting soil structure, water percolation is also improved, allowing more moisture to seep into the ground before running off. Many think the microporosity of high-humus soils allows beneficial micro-organism 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 about this and other gardening topics, please visit the CTAHR electronic publication website at or visit any of the local Cooperative Extension Service offices around the island. I can be reached at

This information is supplied by the University of Hawaii College of Tropical Agriculture and Human Resources.


Rules for posting comments