Kathleen Bridges, Fultz, Lisa
Kathleen M. Bridges and Lisa M. Fultz
Most grazed pastures in Louisiana usually have a perennial warm-season grass such as bermudagrass or bahiagrass that provides forage for the cattle most of the year. In the cool months, beef cattle producers will often use cool-season annuals, usually a mix of annual ryegrass and clover. In recent years, there has been a growing interest in improving soil health, and one of the methods recommended is increasing vegetative diversity. The objective of this study was to determine how incorporating a mix of nine different cool-season annuals (legumes, brassicas and grasses) in a perennial warm-season pasture would affect soil health with time.
The study was conducted on a commercial pasture where no synthetic fertilizers or lime had been applied for at least eight years. The pasture was divided into 10-acre paddocks. The cattle herd of approximately 175 head was rotated through these paddocks every 12 to 24 hours. This allowed vegetation about 30 days to rest and recover. To evaluate the soil health of these pastures, soil samples were collected in March and October starting in October 2015 and ending in March 2018. Because soil health is comprehensive, the soil samples were analyzed for physical, chemical and biological properties every year of the study.
Organic matter in the top 6 inches of soil averaged 8.5%. It remained unchanged with time but varied across topography (summit, backslope and footslope) (Figure 1). This reinforces the importance of recommended soil sampling procedures, which emphasize taking multiple cores across an area to get a representative sample. Considering that this silt loam soil is generally 2% organic matter, this pasture management has resulted in a high amount of organic matter. Ammonium and nitrate are inorganic forms of nitrogen that are readily plant-available. Nitrate-nitrogen decreased over time while ammonium varied at each sampling (Figure 2). However, total nitrogen remained unchanged over time, indicating that nitrogen was present in the soil as organic forms such as protein. For these organic forms of nitrogen to be converted to plant-available forms, soil microorganisms consume and decompose the organic material in a process called mineralization. This soil microbial process is measured through enzyme activity. The enzyme activity increased over time, indicating that soil microorganisms increased the mineralization process, which provided nutrients like nitrogen for forage uptake (Figure 3). Most of the other measured soil parameters either fluctuated with the season or increased with time, but overall remained steady.
This study not only observed soil health over time but also reinforced the importance of representative soil sampling. In general, the footslopes, or depressions, were areas that had high concentrations of nitrogen, phosphorus and sulfur. This is likely due to runoff of manure or soil from the higher-elevation areas. Backslopes tend to have lower soil nutrient content than summits or footslopes. In general, the steeper the hill, the less nutrient content on the slope. Incorporating perennial plants with deeper root systems on slopes can help to hold the soil and soil nutrients in place.
Pasture management that incorporates annual and perennial forages as well as rotational grazing can result in relatively high organic matter and active soil biology. This can lead to reduced inputs such as synthetic fertilizers and lime. Even though there was considerable variability across the pasture topography and with seasons, soil health of this pasture system was improved with time.
Kathleen M. Bridges is a former graduate student in the School of Plant, Environmental and Soil Sciences and currently a postdoctoral researcher at Ohio State University, and Lisa M. Fultz is an associate professor in the school.
(This article appears in the fall 2020 issue of Louisiana Agriculture.)
Figure 1. To get a representative soil sample, the sampling procedure must include the pasture’s topography, including summit, backslope and footslope.
Figure 2. While ammonium increased over the last two sampling times, nitrate decreased greatly.
Figure 3. Two types of enzymes, NAGase and Glucosidase, increased over time.