Brenda Tubaña, Barbara Campos, Daniel Forestieri and Jose Mite
Intense crop production has created concerns about soil health and environmental sustainability. The use of cover crops during fallow periods when main crops aren’t planted offers protection for the soil surface from the impact of rainfall, preventing destruction of soil structure. This in turn reduces water runoff and erosion. In addition, the decomposition of cover-crop biomass releases nutrients back to the soil, diversifies soil microbial community and, in the long term, builds up soil organic matter. These all collectively improve soil health and minimize nutrient loss to the environment, along with better control of weeds and diseases and increased availability of water in the soil.
The positive effects of cover crops depend on how much biomass they produce. Cool-season cover crops are planted in the fall and grown until early spring. Low moisture in the fall and freezing temperatures in winter are among the conditions that can limit growth of cover crops and reduce biomass yields. The right planting period gives cover crops a head start in stand establishment and growth, even in years with adverse weather conditions. This is a crucial component of a successful cover cropping system.
A corn-soybean rotation field study established at the LSU AgCenter Central Research Station in Baton Rouge in 2017 has demonstrated the difference that planting date can have on cover-crop biomass accumulation. In this study, different species of cover crops were planted after harvesting the main crop each year. The first planting was in mid-September followed at 30-day intervals by a second planting in October and a third in November. Cover crops were mixes of tillage radish, hairy vetch and crimson clover. Cover-crop biomass and soil samples were collected in February, and evidence of transport of soil nutrients and plant growth was found.
Over the three-year study period (2017-2019), cover crops planted in September produced an average of 2,628 pounds of dry biomass per acre, which is 54% higher than the dry biomass produced by cover crops planted in October (Photo 1). Cover crops planted in November essentially did not produce any higher dry biomass than the native weeds on the nonplanted plots. These observations indicate that cover crops planted in September had more time to grow and accumulate biomass compared to those planted in October and November. With a well-established root system early on, more nutrients were then recovered and stored in cover-crop biomass. Thus, the amount of major plant-essential nutrients recovered in the biomass of cover crops planted in September was higher than the amounts measured in cover crops from other treatments, including the nonplanted plots. These nutrients, which include nitrogen, phosphorus and potassium among others, are released back to the soil upon decomposition of cover-crop biomass. Annually, as much as 40 pounds nitrogen, 14 pounds phosphorus and 84 pounds potassium per acre can be potentially added to the soil from biomass of cover crops planted in September. A considerable addition of these nutrients was also recorded from the biomass produced by cover crops planted in October.
With more accumulated biomass, it is expected that more nutrients will return to the soil. However, there are two important things to remember when accounting for nutrient turnover from cover-crop biomass. First, the decomposition is generally a slow process and highly driven by temperature, moisture and microbial activity. More often, the release of nutrients from biomass is not completed by the time the main crop needs these nutrients. In this study, partially decomposed biomass of cover crops was commonly observed on the soil surface while the main crop growing season was already in progress (Photo 2). The other point is that cover crops are plants that also require essential nutrients for development and growth. Thus, cover crops can potentially compete with the main crop for nutrients, especially in years where plant growth is accelerated in the fall because of ideal temperature and adequate moisture. This is also true in years with a mild winter when plant growth can be extended.
In this study, the presence of cover crops made a positive return in nutrients compared with not having cover crops at all. The cover-crop biomass released as much as 90 pounds of nitrogen to the soil by the time corn was in active vegetative growth. With 120 pounds of soil nitrogen from applied fertilizer, the total available nitrogen for the remainder of the corn growing season in 2020 was 210 pounds of nitrogen per acre. This was significantly higher than the no-cover-crop treatment.
Cover cropping is a best management practice effective only when properly implemented. The outcomes from this study suggest that the planting period within which cover crop growth is optimized is between September and October. Optimal biomass production is crucial to maximize the soil health, yield and environmental benefits from cover cropping.
Brenda Tubaña holds the Jack E. and Henrietta Jones Professorship in the School of Plant, Environmental and Soil Sciences. Barbara Campos is a visiting scholar from Federal University of Uberlandia, Brazil. Daniel Forestieri and Jose Mite are graduate students in the school.
(This article appears in the fall 2020 issue of Louisiana Agriculture.)
Photo 1. An aerial side view of the cover crops taken during biomass sampling in the early spring of 2020. Cover crops were planted in November, September and October 2019. The nonplanted control plot is at the top of the photo. Photo by Daniel Forestieri
Photo 2. Partially decomposed roots of tillage radish cover crop on the soil surface a few weeks after soybean germination. Photo by Brenda Tubaña