When a cotton farmer sees “worms” in his fields, it is usually a bad thing. Boll worms are a major pest which eats the crop. But when we started sampling producer fields for a research project, we found beneficial earthworms living in cotton fields.
Planting winter wheat in row middles has become a standard practice to protect cotton seedling from blowing sand in the spring. We are finding that the wheat residue is an important food source for earthworms and microorganisms. Without something to eat and adequate water, earthworms die or fail to reproduce. With the new interest among farmers in planting cover crop blends and rotating cotton with corn, earthworms will probably have a more balanced diet in the future.
We are studying three methods of judging the soil health in cotton fields- soil respiration, active carbon amount, and phospholipid fatty acids. A healthy soil has microorganisms which digest organic material (food) and respire carbon dioxide gas, just like we do. To measure soil respiration, we drive metal rings in the soil, cap them with a plastic lid for 30 minutes and use a syringe to pull an air sample through a inline CO2 indicator tube. As you might expect, microbial respiration tends to be highest in fields which had winter cover crops. Respiration is highest in May and June when there is new residue material and soil moisture from spring rains. When the soil becomes hot and dry the microbes and earthworms become less active.
The main focus of the research project is studying soil active carbon levels in Missouri cotton soils. It can take many years of reduced tillage and careful residue management to significantly increase the percentage soil organic matter just 1%. But the active carbon fraction of organic matter changes more quickly and has the greatest impact of crop growth. David Dunn, manager of the Delta Center Soil Test lab, and I are working together testing for active carbon. We are using a method developed by the Natural Resource Conservation Service. We are tweaking the procedure to help identify differences. In the procedure, the amount of soil used in the extraction procedure is critical. In the process of doing this research, we learned that the Soil Health Lab in Columbia Missouri has a new procedure called a Phospholipid Fatty Acid (PLFA) test which estimates the amount of each group of microbes in a soil sample. Donna Brandt (573-882-0941) is a research specialist at the lab who tests the samples. Below are some results from our cotton fields in 2013.
We still have much to learn about promoting microorganism populations in the soil. Mycrorrhiza fungi which work symbiotically with plants to help roots take up more nutrients are found more often in perennial crops than cotton. Gram negative bacteria tend to be highest in the soil surface/root zone. They occur in the highest proportion in soils with organic inputs such as manure or cover crops with no-tillage. Gram positive and actinomycetes usually increase proportionally with soil depth and are less responsive to fresh organic inputs.