In the Texas High Plains soil health practices are becoming more and more important for farmers to keep their soils productive and fertile for the coming generations. Katie Lewis, assistant professor of soil chemistry and fertility at Texas A&M University, said highly functioning soil supports the biological activity through chemical and physical parameters and farmers need to consider conservation practices because of the sandy soils and erodibility issues in many areas of the High Plains.
“What’s happening when we implement either a crop rotation with a crop with higher biomass production or a cover crop into these systems is we’re increasing the soil organic matter and because we’re adding that residue to the soil, in the long term, we should see improved soil structure,” she said. “This reduces crusting as well as increases filtration and water-holding capacity, evaporation and soil moisture losses because our soil surface is covered more often than not.”
Lewis said the most important aspect farmers in the Texas High Plains should focus on is reducing soil erosion.
“We don’t want to be sharing our soil nutrients with our neighbor 10 miles away, so whether it’s through tillage practices or a cover crop, reducing that erosion will make us better off because the top soil containing the majority of the organic material as well as the nutrients in our soil, will stay in place.”
Lewis said farmers have improved erosion from the 1930’s Dust Bowl time period by as much as 40% with a reduction in tillage and the use of cover crops and crop rotation, however wind erosion is still a concern. In the last five or six years, there has been a strong push toward the use of cover crops because over time they improve soil health. Lewis said mixed species cover crops including: radish, hairy vetch and winter pea have become popular. Wheat and rye have been used routinely across the High Plains of Texas; however, many do not necessarily consider them to be a cover crop. Lewis said the major concern with cover crops is soil moisture use. The biggest benefit she has noticed after 20 years of mixed species cover crop plot research is a reduction in soil pH, as well as greater availability of micro and macronutrients.
“When we look at total nitrogen, we are also seeing an increase with the added organic material,” Lewis added.
When it pertains to tillage and cover crops, Lewis said farmers need to look at the big picture of costs and efficiency.
“A lot of people would say with any sort of no-till practice you are going to get a reduction in input costs, but you also have to consider that your herbicide costs are probably going to be a little greater,” she explained. “You are having to terminate the cover crop, so that’s an additional cost. You do have less passes across the field, but when you look at input costs, specifically with mixed species cover crops, they’re pretty much equal with conventional tillage, and in some cases greater because of the costs of those mixed species, particularly hairy vetch. Also, where you have a cover crop, you will see increased incidents of things like root knot nematode compared to a conventional till system or a crop rotation.”
With cover crops, Lewis said she has started to notice some potential immobilization that is occurring because of the large amount of organic material that has been added to the soil, which stimulates microbial activity and then immobilizes some of the nitrogen and phosphorous that is potentially available. Immobilization is when microorganisms utilize the plant available forms of nitrogen in the soil as ammonium and nitrate to continue breaking down organic material. Throughout the growing season, Lewis said they see greater soil moisture available with the cover crop in place because evaporation losses are decreased, the soil water holding capacity is improved, improving infiltration.
“When we talk about soil organic matter, most times you’ll hear that it provides nutrients or you’ll be able to reduce the amount of nutrients that you’re applying,” she said. “If you have organic material or matter there’s going to be a mineralization that occurs, which is the opposite of immobilization, and releases ammonium and nitrate into the soil.”
Organic nitrogen, if it has been immobilized, is not going to be plant available. The only plant available forms are going to be ammonium and nitrate. It also matters when this nitrogen is available.
“As a rule of thumb, what we use to determine whether or not immobilization or mineralization is going to occur, is dependent on the carbon to nitrogen ratio of the organic material that you’re adding to the soil,” Lewis explained. “Say you have rye residue that has a carbon to nitrogen ratio that’s greater than 30-to-1, most likely the majority of the fertilizer nitrogen, ammonium and nitrate, that you add to the soil is going to be immobilized, so you’re going to see a reduction in plant available forms of nitrogen. However, if you add something like manure to your soil, which has a carbon to nitrogen ratio of less than 20-to-1, then it is going to result in net mineralization or the release in ammonium and nitrate and you’re going to see an increase in the concentrations of those into your soil.”
Where we have immobilization occurring, this is where you could potentially have some nitrogen deficiency symptomology occurring. Lewis said it can take anywhere from eight to 15 weeks for the microbes to reach a point where they are releasing greater amounts of nitrogen than what they’re immobilizing.
“The termination time of cover crops is crucial because you want to ensure that you give that residue a sufficient amount of time to reach the point where the microbes are releasing nitrogen through mineralization rather than taking it up and storing it within their microbial cells, which we refer to as immobilization,” she said.
For farmers trying to determine the best soil health strategies for their operation, Lewis says to consider soil type and how much you are willing to amend your management strategies to fit a specific soil health tactic.
“When it comes to deciding what to do on your farm, my recommendation is if you have a sandy soil with limited irrigation capacity, that would be a case where a rotation would be potentially more profitable than implementing your cover crop, but on a more clay soil with higher irrigation capacity, you could be more successful with a cover crop,” Lewis explained. “However, it’s not to say that it is not feasible to use a cover crop on a sandy soil, it’s just having to alter your management strategies, whether it be irrigation or possibly nutrient management.”
Lacey Newlin can be reached at 620-227-1871 or [email protected].
