Sustainable Gardening

Add Soil Organic Matter to the Garden

young plant

Before you go to the garden center, you should stop and think about your soil. The most important component in your soil is the Soil Organic Matter (SOM). This is where all the action happens. 

SOM is critical for the soil to function properly and support plant life naturally. It provides structure and a place for water, air, and biological life to exist. A soil with insufficient organic matter may not hold water adequately or supply an environment for beneficial microbes. These soils become quickly dependent on high levels of watering, fertilizer and pesticides to maintain the appearance that our society expects.

Most of the native soils in North Carolina have low percentages of SOM. These soils are called Ultisols. They are strongly leached, acid forest soils with low native fertility and contain less than 1% of SOM. Our nation’s most fertile soils have SOM readings in the 5-7% range.

Increasing soil organic matter

You can increase your SOM with a soil amendment. Aged pine bark, peat and compost are three commonly used amendments high in organic matter. However, aged pine bark is acidic and it will adjust your soil’s pH downward. If the pine bark you purchased has not aged properly it can actually rob nitrogen from the soil as it decomposes. Peat on the other hand does not supply nutrients or beneficial microbes and it is mined as a limited natural resource. Compost is a green sustainable product and is economical to use as a soil amendment because of its price and availability.

Buying certified compost

You can make your own compost from leaves, yard trimmings and food waste, but you may not make enough to meet all of your needs and will need to purchase compost made at a large facility. This compost is produced from the thermophilic (high temperature), aerobic (with air) decomposition of organic residues under controlled conditions. It can be made from yard debris, food waste, biosolids (municipal wastewater treatment plant solids), clean wood waste and other environmentally safe organic materials.

The compost you purchase should meet EPA and state environmental requirements governing chemical contaminants and be processed at high temperatures necessary to destroy weed seeds and potential pathogens. When purchasing compost, ask for compost produced under the US Composting Council’s Seal of Testing Assurance Program (USCC-STA). To be sure that it is certified, look for the USCC-STA compost logo and ask for a copy of the Compost Technical Data Sheet. The data sheet includes directions for use, a list of ingredients and analytical test results.

The fringe benefits of compost

Compost has high organic matter content (50-75%) relative to most fertile soils. By incorporating compost into soil, SOM is increased. The benefits of increasing SOM fall under four categories: biological, physical, chemical, and environmental.

Biological benefits – compost promotes the growth of beneficial micro-organisms. A teaspoon of healthy soil or compost can have millions of bacteria, miles of fungi, hundreds of thousands of protozoa, and hundreds of beneficial nematodes. These living organisms create a diversity of life in a healthy soil.

Physical benefits – compost improves soil structure, lessens compaction and surface crusting, increases infiltration and aeration, and improves water-holding capacity. This results in improved plant growth and soils that absorb water and hold nutrients more efficiently.

Chemical benefits – compost helps make nutrients more available to plants, and binds trace elements so that they can be released slowly as needed for plant uptake. Compost provides a balanced source of slow release nutrients necessary for healthy plants.

Environmental benefits – adding compost helps reduce global warming, adsorption of toxic metals and toxic organic compounds such as pesticides, and decreased soil erosion. The percentage of organic content directly relates to water-holding capacity in soil. Soil scientists report that for every 1% of organic matter content, the soil can hold 16,500 gallons of water per acre of soil to one foot deep.

Quality compost parameters

What do you look for in a quality compost product? Look for a product with a neutral pH (7.0), low in soluble salts, one that provides a slow release of nutrients, is light in weight, low in moisture content, high in organic matter content, screened less than 1 inch in size, meets EPA requirements, and is both mature and stable to be used in the garden.

Using compost helps both the environment and your garden. Here’s your opportunity to think global and act local. You can improve your soil the green and natural way by adding compost.

For additional information on where to find USCC-STA compost go to – and download the free brochure “10 Tips for Healthier Soil & Plants.”

Frank Franciosi is a horticulturalist. He has over 20 years experience in the composting industry in North Carolina. For more information, visit

Copy link