This article first appeared in the Wyoming Livestock Roundup in July, 2014
Overgrazing – repeatedly removing most or all of the vegetation – causes loss of roots, which leads to soil compaction, runoff, erosion, and ultimately huge reductions in productivity. But this article isn’t about overgrazing. It’s about how proper management affects soil health and productivity.
Healthy rangeland soils have thick, dark surface horizons that absorb water and supply nutrients to diverse and productive vegetation. Where does that black soil come from? It comes from diverse and productive vegetation! Virtually all the aboveground plant material produced each year and about a quarter of the roots turn over – or decompose – to build soil organic matter and provide nutrients to the next crop. Over thousands of years, this feedback loop created soils resilient to all kinds of disturbance, and the farther east you go in the prairie region, the more resilient they are. But they have their limits. Over the past century, repeated plowing caused loss of around two-thirds of the original organic matter under Great Plains grasslands.
The cycle of growth and decay that sustains productive grazing land hinges on return of adequate amounts of diverse plant residues to the soil. Plant communities with rich mixtures of bunchgrasses, rhizomatous grasses, tap-rooted forbs, nitrogen fixers, and shrubs of many species contribute a wide variety of residues to the soil that decompose at different rates and times. This supports a huge diversity of soil microbes that cycle plant materials over the entire year, providing nutrients to growing plants. Plant residues that decompose rapidly provide nutrients, while those that are more resistant to decomposition tend to become stable soil organic matter – humus – that gives the soil a dark color and sponge-like qualities that enhance moisture holding potential and resilience to disturbance.
The amount of residue left uneaten by livestock to return to the soil is one important part of maintaining soil health. This part of the equation became starkly clear to me in my previous job in the California annual grasslands, where all the plants die each summer and regenerate from seed each winter and spring. Removing too much of the plant material has immediate effects on productivity by causing erosion, poor germination, and shifts toward less desirable species (like ripgut brome!). Livestock producers there are very aware of their RDM – Residual Dry Matter. In Wyoming most of our rangeland plants are perennial so the effect is not as immediate; plants regrow from their roots so germination conditions are less important. But removing too much plant material, even when plants are dormant in the mid-summer or winter, will have the same effects, leaving soil exposed to erosion and depleting soil organic matter.
The number of species and types of plants – the diversity – is another important part of soil health in grazing lands. A great deal of research in many types of plant communities has shown that more species lead to more fertile soil, more productivity, and less weed invasion. Mixtures of plants that grow at different rates, mature at different times, and have different root structures improve animal performance, and also increase soil microbial activity. Pastures that are all one species, either from conversion to introduced grasses like crested wheatgrass or invasion of weeds like cheatgrass, tend to end up with more shallow, less productive soils with limited biological activity.
In a study of native-plant-dominated, crested-wheatgrass-converted, and cheatgrass-invaded sagebrush grassland soils in Utah, I found that soil under the native plants had a wide variety of roots, the most organic material, especially the type that turns over rapidly, and also the most microbial activity. In that soil, lack of disturbance protects easily decomposed material and regulates the availability of nutrients to plants. The soil under cheatgrass had lost organic material because the dense mass of very fine roots dies each year, leaving thousands of pores that aerate the soil and accelerate decomposition of organic materials. This causes pulses of nutrient availability that favor annual weedy vegetation; almost like tillage. The soil under the 50-year old crested wheatgrass stands was intermediate in soil organic matter content and microbial activity, probably because the presence of only one type of plant residue and roots, that all grow and turnover the same way, limits microbial diversity and causes distinct pulses alternating with periods of little microbial activity.
So how do we manage grazing to improve soil health and productivity? Proper management to maintain and enhance plant production is also the best way to maintain and enhance soil health. In native rangelands, proper rest and grazing at different times of the year can maintain or enhance diversity. In improved pastures and management-intensive situations, interseeding, especially with nitrogen-fixing legumes, and then managing to maintain them, can improve both productivity and soil health. Keeping animals off wet soils, and avoiding prolonged concentration (trying not to create sacrifice areas) are other important ways to maintain the soil that sustains productive grazing lands and healthy, growing animals.
Please contact me for more information and reading materials on this subject.