Producers continue to look for ways to improve soil health using regenerative agricultural practices. When one looks at the five foundational soil health principles (i.e., soil armor, minimizing soil disturbance, plant diversity, continual live plant/root, livestock integration), the one management change that considers all these factors is conversion of conventionally managed farmland to perennial pasture. The soil is armored with aboveground standing biomass and litter plus it has live roots 365 days per year. There is minimal soil disturbance once plants are established because tillage is eliminated. Plant diversity can be easily addressed with a forage mixture; a diverse mixture will benefit grazing livestock by providing a higher-quality diet.
With the above in mind, plus the need to feed 150 head of cows during the growing season, Colorado State University converted a 200-acre pivot from conventionally tilled cropland to perennial pasture in the fall of 2016. Each quarter of the pivot was planted to a different mixture ranging from a three-way grass mix to a 10-way mix consisting of six grasses and four legumes. This was done to account for the plant diversity part of the equation. Grazing commenced midsummer in 2017 using the management-intensive grazing (MiG) approach promoted by Jim Gerrish. In late spring of 2017, prior to commencement of grazing, soil samples were taken from across the pivot and analyzed for various soil health characteristics. Soil samples were also obtained late spring of 2018 following the initial grazing season. Those results were reported in the February 2020 edition of Progressive Forage. In the present article, we will compare initial findings from 2017 to those observed in 2021, after four years of grazing (4.5 years following establishment).
2021 results
To quantify changes in soil health, we measured the following 10 soil variables: bulk density, water-stable aggregates, microbial biomass carbon, beta-glucosidase enzyme activity (relative measure of microbial activity), potentially mineralizable nitrogen, soil organic carbon, pH, electrical conductivity, and phosphorus and potassium concentrations. Each of these variables can be lumped into general categories of either physical, biological, chemical or nutrient to give an overall picture of what is occurring in those different areas of interest as we consider how changes in management affect soil health. Changes happen most rapidly in the top 6 inches of soil, and thus we sampled the 0-to-2-inch and 2-to-6-inch depths.
Physical changes
Bulk density and water-stable aggregates are the two variables that we looked at to quantify physical soil health changes. As most people are aware, tillage can be extremely detrimental to soil aggregates. Continual soil disturbance breaks up aggregates, which are important for providing aeration and channels for water to infiltrate. Without good aggregation, the soil becomes hard and compact and productivity declines. Positive changes have been measured in water-stable aggregates, especially in the top 2 inches with a 48% increase compared to levels in 2017.
The physical factor of most concern under grazing is bulk density, which is essentially a measure of soil compaction. When grazing large animals like cattle that often weigh 1,300 pounds or more, it is not difficult to imagine how they can compact soils. This impact is of most concern when grazing on heavier-textured soils under wet conditions. Going from a tilled situation at the time of seeding, in which the soils were essentially fluffed, to grazing with 150 cows concentrated in small paddocks led to an initial increase in bulk density of 32% in the top 2 inches. This was concerning as the bulk density was approaching 1.7 grams per cubic centimeter after one season of grazing; this bulk density can impede root growth and water infiltration and lead to forage productivity declines. Data from 2021 showed that bulk density has stabilized at about 1.4 grams per cubic centimeter, due in part to the increase in aggregate stability mentioned above.
Biological changes
Soil biological health was quantified by measuring beta-glucosidase enzyme activity (BG), microbial biomass carbon (MBC), potentially mineralizable nitrogen (PMN) and soil organic carbon content (SOC). Levels of all variables increased in the upper 2 inches of soil in the first year, especially BG, MBC and PMN . Soil organic carbon is slower to respond but has increased in both soil layers by about 70% after almost five years. A functioning microbial community with increased levels of nitrogen and carbon are all positive indicators of a healthy soil ecosystem.
Chemical changes
The pH of the soil was around 8.0 at the start of the transition and would not be expected to change much in the short term due to the high buffering capacity of these soils. However, electrical conductivity (EC), which is a measure of the salt content, did show a significant decline over time. This is most likely due to sprinkler irrigation driving salts lower into the profile where they are not a detriment to plant growth.
Nutrient changes
Phosphorus and potassium are two essential nutrients that tend to increase under grazing due to dung and urine deposition, especially under the high stocking densities associated with MiG. Potassium has consistently increased over time, most likely due to deposition in the urine. Phosphorus (P) declined initially following fertilization of P at time of seeding but has since increased in response to concentrated dung deposition. Potassium is generally high in our western soils, but the increase in phosphorus can decrease the need for addition of expensive P fertilizers.
In summary, converting conventionally tilled cropland to perennial pasture that is well managed using a system such as MiG can result in positive and relatively quick changes in soil health.
