Soil stability is a sticky situation

The author is a soil scientist with the USDA Agricultural Research Service in Raleigh, N.C.

In the February 2023 issue of this column, I introduced the forces binding soil into water-stable aggregates. Small particles like clay and silt are active in forming aggregates because they have an abundance of chemically and physically reactive surfaces, whereas larger sand-sized particles are just not as reactive to form strong bonds.

Water-stable soil aggregates are bound together by microbially produced glues, so having an active soil biology community in your pasture will be essential to build these aggregates and keep soil from washing away. Unless they are well-aggregated, small soil particles will be the first to wash away in heavy rainstorms. Water-stable aggregates, on the other hand, form larger clusters that won’t erode as easily. Even if soil does not wash away, having weak aggregates that disintegrate with pounding rain could lead to surface sealing.

Soil stability index is the fraction of aggregates that remain intact even after exposing them to the erosive energy in moving water. Having a high soil stability index is an important factor when evaluating agricultural sustainability because keeping soil on the farm and out of waterways is a desired conservation outcome.

On-farm research

To assess the soil stability index, soil samples from pasture-based livestock farms in Virginia were taken in 2022. Soil from a total of 187 fields on 31 farms was sampled with half of the fields representing grazed pastures. The other half was a combination of woodlands, conventionally tilled, and no-till cropland. The pastures were not all managed the same, so the factors that contributed the most to changes in soil stability index were characterized.

On average, soil stability index was lowest under conventionally tilled cropland (52%) and greatest under grazed pastures (89%). No-till cropland (78%) and woodlands (85%) were intermediate. These results support the value of conservation management to keep soil intact and prevent it from eroding away from the farm. Results are similar to those shared in the February 2023 issue of this column from a study on research stations in North Carolina.

Several management variables influenced soil stability index in pastures in the study. Results summarizing the factors of pasture age, stocking rate, stocking method, and nitrogen fertilizer inputs are shown in Figure 1. The first thing to note is that even at the lowest levels of these factors, soil stability index was typically performing as good or better than in other land uses.

More stability over time

Soil stability index improved with pasture age, likely because of the occurrence of several important changes, like more glues from roots and decomposing soil microorganisms and more drying and rewetting over time. Simply gaining more grass roots to explore the surface soil over time can be important to stabilize soil as well.

Soil stability index was optimized with a stocking rate of approximately one mature cow per acre. Both lower and higher stocking rates reduced the soil stability index. This suggests some livestock traffic and defoliation of the sward will be beneficial, but too little or too much is detrimental.

Stocking method did not statistically influence soil stability index in this study. It may be that too few fields with continuous stocking were available to provide such a robust comparison. Most fields were under some rotational stocking; however, a trending effect was that too much livestock impact at one time could lead to poorer soil stability.

Finally, more nitrogen fertilization led to a decline in soil stability index. This result is likely from greater fertility allowing heavier livestock traffic to consume more of an abundance of forage. Moderate grazing levels in a system with a somewhat slower regrowth period would appear to be a successful alternative. Overall, older pastures with a moderate stocking rate that rely on as much internal nutrient cycling as possible can be managed with a variety of stocking methods to achieve a high soil stability index so that nutrient-rich surface soil stays on the farm and out of waterways.

Well-managed pastures should certainly be considered a good conservation approach, and finding the right mix of grazing practices and pasture management for your farm will be critical. If you’re unsure what is the right mix for you, ask your local county extension agent, your state forage specialist, or a qualified adviser for advice. •