Brenda Tubana, Fultz, Lisa
Brenda Tubaña, Daniel Forestieri, Lisa M. Fultz and Jose Mite
Soil nutrient enrichment is among the known benefits of using cover crops as a management tool. Winter cover crops are grown in fall through winter. During this period, cover crops absorb nutrients from organic matter decomposition or from underutilized fertilizer applied to the previous main crop, which otherwise would have been left in the soil and subjected to loss through surface runoff, erosion and leaching. When the cover crop is terminated early spring, the decaying biomass releases a full range of nutrients in plant-usable forms. In addition, the cascading positive effects of cover crops on the soil microbial community and water-holding capacity can also increase available nutrients for plants.
Nitrogen is the most sought-after nutrient for credit when practicing cover cropping; however, other plant-essential nutrients are released back to the soil when cover-crop biomass decomposes. Depending on the mixes of cover crop species, significant nutrient contribution other than nitrogen is also expected from the biomass. There are differences in rooting depth, yield potential and carbon-to-nitrogen ratio of biomass among cover crop species. Thus, what, how much and when these nutrients in biomass will become available to the main crop are among the questions regarding nutrient contributions of cover crops.
Several studies were initiated in 2017 on producer fields in central and northeast Louisiana and at LSU AgCenter research stations to provide a general understanding on soil nutrient enrichment from cover crops. Mixes of cover crops species consisting of legumes (such as clovers), brassicas (such as forage radish and rapeseed) and grasses were established in the fall at each location. Prior to termination in early spring of the following year, cover crop biomass and soil samples were collected. These samples were analyzed for nutrient composition and concentration.
A summary of the full range of nutrients recovered per year showed that cover crop mixes consisting of hairy vetch, crimson clover and tillage radish recovered substantially higher amount of nutrients than the native weeds from the no-cover-crop plots (Figure 1). Not only nitrogen but also potassium, calcium, magnesium, phosphorus and sulfur were stored in the biomass — about triple the amount stored in biomass of native weeds. In fact, the average potassium stored in cover-crop biomass per acre was 72 pounds, which exceeded nitrogen at 51 pounds. Trace amounts of micronutrients, including iron, zinc, copper, boron, manganese and molybdenum, were also measured in cover-crop and native-weed biomass. Except for iron, all these micronutrients recovered in biomass were higher in plots with cover crops.
The amount of stored nutrients in cover-crop biomass does not proportionately raise nutrient levels in the soil. Many factors affect the rate by which nutrients are released from plant materials, including temperature, microbial activity and soil pH. As a result, the amount and time of release of nutrients vary among soil types, farming practices and weather conditions. The outcomes from these studies generally indicate a boost in soil nutrient content within the first year of cover cropping.
The analysis of soil nutrient content indicated that soil phosphorus and sulfur were at sufficient levels for corn and soybeans with or without cover crops. This was also the case for other plant-essential nutrients. The trend of soil phosphorus and sulfur within the 37-month period, which is equivalent to three crop seasons, demonstrated that the benefit of cover crops on soil nutrient enrichment was apparent (Figure 2). Both soil phosphorus and sulfur levels were raised by 19% to 20% at nine months after planting the first cover crop mixes. This was also approximately three months after the termination of the first cover crops. Although the levels appeared declining with time, the plots with cover crops maintained higher soil phosphorus and sulfur than the plots without cover crops.
The levels of phosphorus, potassium and sulfur in the soil were mostly improved within a two-year period because of cover crops at the three producer sites in central and northeast Louisiana. The most notable was the 56% increase in soil potassium 19 months after the first cover crop planting in 2017. The highest improvements were 15.7% for soil phosphorus and 27.3% for soil sulfur. Soil phosphorus and sulfur levels were lower in fields with cover crops than in fields without cover crops after 16 months and eight months, respectively, of first cover crops. This was likely because of a delayed release of stored phosphorus and sulfur in cover-crop biomass. Unlike potassium, which remains in its available form in plant cells, phosphorus and sulfur are incorporated in organic compounds such as amino acids, proteins and nucleic acid. This means that organically bound phosphorus and sulfur are released first through decomposition before they can be converted into plant-usable forms. Thus, if weather conditions slow down decomposition, the release of phosphorus and sulfur is also delayed.
The outcomes from these studies show cover crop contributions to soil nutrient enrichment could be evident within the first year of practice. The levels of nutrient contribution will vary, depending on the choice of cover crops species, single or in mixes, that may be further compounded by soil types, weather conditions and farming systems. As apparent as it may sound, nutrient contribution from cover crops can also build up with time. With long-term practice, cover crops increase soil organic matter, enhance microbial activity and stabilize soil structure, which collectively improve nutrient cycling and prevent nutrient losses from the soil.
Brenda Tubaña holds the Jack E. and Henrietta Jones Professorship in the School of Plant, Environmental, and Soil Sciences. Lisa M. Fultz is an associate professor in the school, and Daniel Forestieri and Jose Mite are graduate students.
(This article appears in the fall 2020 issue of Louisiana Agriculture.)
Biomass clippings are taken from 1-square-meter sampling areas along with soil samples prior to cover crop termination in early spring. Photo by Daniel Forestieri
Figure 1. Average amount of nutrients recovered in pounds per acre per year by native weeds (no cover crops) and cover crops mixes containing hairy vetch, crimson clover and tillage radish. Note: Boron, manganese and molybdenum were also recovered but at lesser amounts than zinc. Photos by Brenda Tubaña
Figure 2. Trend of soil phosphorus and sulfur in pounds per acre with no cover crop and with cover crops plots within a three-year period. Soil phosphorus and sulfur values are higher in plots with cover crops than with no cover crops. Yellow arrows indicated the months when the main crops were actively growing.