(This first appeared in the Wyoming Livestock Roundup in May, 2017)
Last January this paper printed an article that described predictions of warmer temperatures, more winter and spring precipitation, and large increases in evaporation, leading to drier soils. This should have thoroughly frightened readers who depend on sustainable soil productivity. Drier soils hold less soil organic matter (SOM), and soils with less SOM hold less water, defining a drying feedback cycle. The article summarized Wyoming-specific parts of the Third National Climate Assessment released by the National Oceanic and Atmospheric Administration (NOAA). While more precipitation should support more plant growth, increased frequency and intensity of wetting-drying cycles accelerates decomposition and loss of SOM. As quoted from NOAA in the January 14 article, “Even if precipitation amounts increase in the future, rises in temperature will increase evaporation rates, resulting in an increased rate of loss of soil moisture during dry spells.”
The Assessment analyzes past climatic trends and variability, and predicts future Wyoming climate. Models that predict future conditions use known relationships among the composition of the atmosphere, air temperature, and weather. Weather is the wildcard, because warmer air holds more moisture, which translates to more energy, and therefore increases the unpredictability of place-by-place future conditions.
The increased frequency and intensity of wetting-drying cycles sets the stage for loss of SOM. The amount of SOM is generally equivalent to soil health and productivity, since organic matter supports both soil water and nutrient supplying potential.
Soils that are at or near their potential SOM content can withstand drought, heavy rain, heavy grazing, and even climatic fluctuations and still provide forage, crops, water storage, habitat, and many other functions. Loss of SOM reduces this resilience, and with the predicted conditions, it will become more and more difficult to restore resilient soils. If you own some soil that you suspect may contain less SOM than it can hold, or is producing below what you think it should, now is the time to start the slow process of building soil health by adopting management practices that conserve SOM.
Understanding and managing for soil health
The amount of SOM a soil can hold is a reflection of interactions among the climate, landscape, and soil properties like texture, rock content, and alkalinity or acidity. Those factors control the type of plant community present and its productivity, which contributes different forms and amounts of organic materials that become SOM. These factors create complex and variable patterns, such that any particular piece of ground has optimal and sub-optimal management approaches for maintaining soil functions.
There are many approaches for assessing soil health. Some directly quantify the total and most rapidly management-affected components of SOM. Others evaluate properties affected by SOM, such as density, aggregation, water-holding capacity, nutrient content and supplying potential, pH, salinity, and others.
Given underlying variability, there are three basic principles that conserve or build SOM levels in either rangeland or cropland management:
Minimize soil disturbance. Disturbance from tillage or excessive hoof action destroys soil structure, exposes SOM to decomposition, and creates conditions for wind and water erosion. Frequent disturbance causes soil microbial communities to be dominated by opportunistic bacteria that thrive on the readily available nutrients from a deteriorating soil system. Undisturbed systems are often dominated by soil fungi that help to decompose woody materials, cycle nutrients, and form symbiotic relationships that increase plant access to moisture and nutrients. In croplands, minimizing disturbance means transitioning to minimum- or no-till systems, and especially eliminating operations that invert the soil, such as moldboard plowing. In rangelands it means avoiding over grazing, using care in wet conditions, shifting sacrifice areas around, and controlling heavy trailing.
Maintain soil cover. This means minimizing the amount of bare soil. Bare soil increases erosion, while plant residues on the surface decompose in a controlled manner to contribute to SOM. In croplands, it includes maintaining crop residues on the surface and minimizing the amount of time the soil is bare by planting cover crops. Incorporating residues with tillage accelerates decomposition such that much less is converted to SOM. In rangelands, controlling grazing to minimize bare soil, even during winter grazing of senescent plants, helps to build SOM. Some plant communities are patchy and naturally have a lot of bare ground, but they can still be managed to maximize cover and to minimize disturbance of residues and biological soil crusts.
Promote plant diversity. A diversity of different types of vegetation contributes different types of residues, some that decompose rapidly to provide nutrients, others that decompose very slowly, contributing to stable humus SOM, and the whole spectrum in between. In croplands, this means rotating crops, intercropping, and/or planting cover crops. In rangelands, it means maintaining or restoring diverse plant communities, with many species and life forms, including bunch grasses, rhizomatous grasses, forbs, shrubs, biological soil crusts, and trees in some places.
Clearly implementing these principles in croplands requires much more active management and can bring more rapid results compared with rangelands. But actively controlling grazing by fencing or herding to apply specific livestock impacts to specific sites can change conditions rapidly, especially on moist, productive sites with relatively high SOM and productivity.
Climate change predictions suggest that future conditions will increasingly favor loss of SOM that supports healthy, productive range and croplands. Therefore now is the time to build resilient soils by implementing the tried and true soil building concepts of minimum disturbance, soil cover, and plant diversity.