Sustainable agriculture specialist Justin Duncan admits he is more of a legume guy than a grass guy when it comes to cover crops, which explains why his list of the best cover crops for hot and humid climates is heavy with legumes and absent any grasses.
Duncan has spent years sweating it out in South Texas fields to figure out what works for organic farming in areas where the heat and humidity have a lot to do with what grows and what doesn’t. Grasses don’t put anything back in the soil, he explains, but legumes replace both organic matter and nitrates.
“Down here, that’s the most important thing,” Duncan said in a recent webinar on cover crops in hot and humid climates. “The soils are hot, the organic matter is burning up, and when organic matter leaves, it takes all the nitrates and carbon with it. I want to replace everything right back into the soil by using legumes.”
Duncan is putting together a booklet for NCAT — the National Center for Appropriate Technology — on cover crops for organic fields. The July webinar covered most of same topics as the booklet, which will be available later this year, but in less detail
His interest in cover crops for hot and humid climates began when he was a student at Prairie View A&M University, where he received his bachelor’s degree, and Texas A&M University, where he received a master’s degree in agronomy. Duncan noticed that a lot of the recommendations he read about didn’t link up with what he was seeing in his own fields.
“Most of the literature I read while I was doing my own trial-and-error organic farming was geared more toward what was going on up north, where cooler climates help preserve soil organic matter,” he said. “Down here, we’re lucky if we get one-half percent of organic matter when we’re supposed to get closer to 5 percent.”
Legume cover crops are the foundation of Duncan’s organic system. Since he doesn’t believe in replacing conventional weed sprays with ones approved for organic use, Duncan used cover crops and tillage alone to reduce the weed population in his organic fields by 89 to 99 percent.
“The idea is to set the weeds back, prepare the field, let the weed seeds germinate, apply a light tillage to that, and come in with your cover crop seeds and press that down with a roller,” he said. “You’ve knocked down the first set of weeds, planted your cover crops, and now they grow unopposed. That’s important, because some of these crops don’t compete well when they’re small.”
The cover crops also help the soil retain water, which takes organic matter with it when it’s lost, which in turn leads back to a loss in water-holding capacity. Organic material, he said, degrades twice as fast in areas where it’s real hot for a real long time — like South Texas.
When choosing a cover crop, Duncan recommends considering the pH and texture of the soil along with local conditions. For example, is it prone to floods? Is it susceptible to drought?
Among the legumes Duncan recommends are Arachis, a perennial peanut, and Cajanus cajan, or pigeon pea. Arachis fixes about 150 pounds of nitrogen per acre. The pigeon peas are also efficient nitrogen fixers and provide about 2.5 dry tons of organic matter per acre.
Cassia fasciculata, a strain of partridge pea (Cassia rotundifolia) is a Texas native. Some livestock raisers use it as forage, but Duncan warns that it’s toxic in its later stages of development. The hyacinth bean, or lablab (Lablab purpurea) is a popular deer attractant and has showy flowers, making it something of an ornamental as well.
Mucuna pruriens, or velvet bean, used to be more popular than it is today because it’s the prime ingredient in itching powder, a favorite substance of pranksters everywhere. Soybeans supplanted it as a major crop, Duncan said, primarily because of its itching properties. Bean sprouts come from another crop that farmers can use as a cover crop, Vigna radiata, or Mung bean.
Duncan’s cover crop research is part of the Subtropical Organic Agriculture Research (SOAR) project with the ATTRA Sustainable Agriculture Program for NCAT and the University of Texas-Rio Grande. NCAT has six offices nationwide, including the Southwest office in San Antonio.