Why organic matter is essential for good soil in SC gardens

03/28/2013 12:00 AM

03/27/2013 5:47 PM

The McNulty family makes one kind of cake and that’s that. No matter what the occasion, we have Gregg Street pound cake. My Aunt Irene, who brought the recipe with her from Montgomery, Ala., gave the butter measurements in ounces. Consequently, the fat content of the cake sometimes varied from cousin to cousin, but the vanilla extract addition was consistent, one and half teaspoons.

We children used to stand at our mother’s side waiting for the Mixmaster to stop so we could have tastes of the batter. It was good at every stage, but the addition of that little bit of vanilla extract at the end made it perfection.

Organic matter is like vanilla extract for our soils. Organic matter is the highly decayed portion of the soil that was once living organisms. The Master Gardener Training Manual describes an ideal soil as containing 5 percent organic matter. Soils in our part of the state average .75 percent, hardly enough to equal two drops of vanilla extract.

Why is organic matter so important? First, the more organic matter, the higher the cation exchange capacity of the soil is. I describe it as like the number of electrical outlets in your soil. The active portions of soils, organic matter and clays, are negatively charged and can hold necessary nutrients (mostly positively charged) and water against the pull of gravity. This electrical connection reduces leaching, water pollution, and holds valuable compounds in the root zone. A sandy soil is like my upstairs bedroom where you have to unplug the iron to use the vacuum cleaner. A clay soil with 2 percent to 3 percent organic matter is like Bill Gate’s media center, it can hold massive amounts of nutrients and water molecules for future use.

Additionally, organic matter feeds microorganisms. Bacteria, protozoa, amoebae, and fungi all digest certain portions of the organic complex. They can actually trade nutrients, such as phosphate molecules, to plant roots in exchange for carbon-rich exudates secreted from the root surface. In the process, these organisms release substances that glue soil particles together, creating stable soil aggregates – little blocks of different shapes and sizes – that allow water and air to easily infiltrate soil and plant roots to freely grow and expand.

Why are our S.C. soils so low in organic matter? The short answer is that it’s hot and humid.

The microorganisms that convert leaves, crop residue, and dead possums into organic matter don’t know when to stop eating. Unlike in those great topsoil places like Iowa, our soils don’t freeze and there’s plenty of water to keep those natural decaying processes working year round. Tilling also speeds up the decay of organic matter as more oxygen is available to aerobic decomposers.

It’s like your son and three of his fraternity brothers came home for the weekend and never went back to college. You have to constantly restock the pantry. We South Carolinians have to constantly restock our soils to keep organic matter levels constant. Using organic mulch with a relatively low carbon to nitrogen ratio around trees and in shrub, flower, and vegetable beds will gradually build up that critical component of your soil. Always recycle your grass clippings onto the lawn. Compost your leaves and kitchen scraps and when you can’t see anything recognizable, top dress, top dress, top dress with an inch a year all over your yard. It’s healthy icing for your soil cake.

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