Why your soil should be like Swiss cheese

09/08/2011 12:00 AM

11/15/2011 11:34 AM

When I started college at Clemson, as a non-traditional (i.e., older) student burdened with a low GPA, I vowed to never skip school again no matter what. A bout with the flu caused me to miss four classes and when my husband returned from a rugby party with a broken arm, he had to wait until after my biology lab for a lift to the infirmary. However, I must’ve had a moment’s distraction in physiology as I missed out on plant respiration.

Everyone knows that plants use carbon dioxide and give off oxygen during that life-sustaining process of photosynthesis, and the waste product – oxygen – comes in mighty handy, too. Somehow I got a horticultural degree without understanding the respiration cycle in which plants use oxygen to fuel their metabolic activity – burning the sugars from photosynthesis (photosynthates) to power their cellular processes.

Ambient air that enters plants through openings on the bottoms of leaves (stomata) is the source of carbon dioxide needed for the sun-powered photosynthesis reaction. Plants then need to metabolize, burn up those compounds to move water up and carbs down, expand leaves, form new tissues, and make delicious tomatoes. They absorb the oxygen they need for this respiration through their root hairs, which grow into the solid and porous spaces in soil. Fortunately, plants give off more oxygen than they use.

Soil, which feels solid most of the time in South Carolina, is actually composed half of mineral components and half pore spaces. Large pore spaces are filled with air and smaller ones with water. After rain or irrigation, the air is displaced and water fills all pores. If the soil doesn’t drain well or rain or irrigation continues too long, plants can literally drown. Different plants can “hold their breath” for varying lengths of time and some plants are adapted to low oxygen conditions. Bald cypress trees live hundreds of years in standing water, but corn succumbs after only two days of water-saturated soils. Many garden plants are lost to over watering.

Your soil type determines the water holding capacity and drainage properties of your yard. Plants growing in sandy soils, with their large pore spaces and minimal water holding capacity, need water frequently but are in little danger of drowning or rotting. If water can penetrate its hardened surface, a heavy clay soil can hold onto that moisture until the cows come home and then some.

The best remedy for either situation is to add well-decomposed organic matter. Organic matter helps heavy soils develop larger pore spaces so both air and water can enter more easily and move downward. In sandy soils, the phenomenal ability of organic matter to hold water and nutrients can transform them into a far superior growing medium. Organic matter attracts earthworms, too, and their up and down tunneling opens the soil as well as causing soil particles to stick together, or aggregate, in a beneficial way. Teeny, tiny dirt clods are good!

Sadly, with our hot, hot, hot, warm, warm, warm, cool, cold, warm, warm, and then hot again South Carolina temperatures, organic matter is quickly broken down by the non-stop munching of those microorganisms who create it for us in the first place. Unlike the Midwest, where soils and microorganisms stay froze half the year and topsoil is measured by the foot, our warm soils allow those decomposers to work around the clock and we must add organic matter each year to make up for what we lose.

Amanda McNulty is an associate extension agent for the Clemson University Cooperative Extension Service and is a co-host of “Making It Grow” broadcast weekly on ETV television stations. Website: www.clemson.edu/extension/hgic/

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