Assessing Rangeland Soil Health to Inform Livestock Management
For row-crop farmers, translating soil test results to management decisions is straightforward – subtract the soil test nutrient status from an estimate of crop needs and apply the difference as fertilizer. For ranchers, it is more like detective work – trying to uncover reasons that rangelands may not be functioning to their potential.
Ranchers could approach soil testing with this question – is degraded soil constraining range productivity, biodiversity or soil water properties in ways that I can address with the management tools at hand?
Productivity, biodiversity and water properties, including infiltration, holding capacity and resistance to erosion, are functions typically assessed to define rangeland soil health. Each soil type has its own potential for performing these functions. Assessing whether a soil is degraded or is functioning at or near its potential requires several steps.
Identify sampling zones
Identifying sampling zones based on soil type and management history is the first step. Soil type varies with topography and underlying geology.
Soil survey maps can help define zones, but might be too large scale. The drawing to the left shows how a landscape might be split into zones that each have different potential for performing functions and different levels of vulnerability to erosion, compaction and degradation.
Identify reference areas
Reference areas perform soil functions at or near the potential for each zone. They provide benchmarks for assessing management effects. Soil survey information and ecological site descriptions can also provide this type of information.
Ideal reference areas are long-term grazing exclosures but can also be areas that receive less pressure than the sampling zone as a whole.
Many soil properties can be assessed directly, without sending samples and your credit card number to a lab.
Fast, simple techniques include soil resistance to penetration with an old hunting knife, since penetrometers are expensive and range soils are usually too hard; soil texture, structure and moisture; surface horizon color and thickness; amount of bare soil; signs of rill and sheet erosion; and visible salt accumulation.
Somewhat more involved techniques include ponded infiltration rate, which correlates well with penetration resistance; soil bulk density, which requires precisely weighing a known volume; soil solution pH and salinity; aggregate stability; calcium carbonate content; plant-available nitrogen and phosphorus content; and others.
These tests, what they mean and where to order supplies are described on my Wyoming Soil Management website, soilmanagement.wordpress.com.
Collect composite samples
Sending samples to a lab can provide an excellent baseline for starting to understand soil health and can help to calibrate your field observations. Some procedures can ensure lab data truly represents soil health.
When sampling, collect at least three composite samples from each zone and associated reference site. The best way is to follow a zig-zag path, placing at least 20 samples into a bucket. In a patchy plant community, collect samples from each patch in proportion to its part of the zone. Thoroughly mix and fill at least one quart zip-lock bag.
The best is to sample the surface horizon, which is most impacted by management. This could vary from two or three inches on slopes or hilltops to a foot or more in swales. Note the average depth for each composite sample. Sampling at constant depth may include subsoil in some samples and not others, giving an inaccurate picture of soil properties.
The best time to sample is mid-summer when everything is dry, as it makes proper handling of samples easier. Disturbance from sampling stimulates decomposition and changes dynamic soil properties, especially with moisture. Variable moisture among samples can skew the results.
Air dry the samples immediately after collecting them. Set them out on a bench or shop floor with the bags rolled open if they’re nearly dry to begin with. If they’re moist, pour them out on a paper plate. They should be dry in 24 to 48 hours. Avoid letting the bags sit out in the sun or in a vehicle. They get very warm and microbes become very active.
Choosing a lab
The standard soil fertility analysis offered by many labs would provide good information. Make sure it includes soil organic matter. The Colorado State University soil testing lab offers a farm and ranch package for $15 or a routine general fertility package for $35.
Other properties that indicate soil function include total nitrogen and phosphorus content, cation exchange capacity, texture and water holding capacity. Skip or ignore the fertilizer recommendations and just compare each sample to the matched reference sample.
The plant-available nutrients provided by the standard test have a very different meaning for rangelands than for croplands. For crops, they mean you can buy less fertilizer. In healthy, undisturbed soils with perennial plant communities, most plant-available nutrients come from decomposing organic material and are taken up by plants and microbes as fast as they’re released.
So, a healthy soil may have high total nitrogen content, for example, but little plant-available nitrogen. Higher contents of plant-available nutrients indicate disturbance where release rates are exceeding uptake rates. This can be caused by physical disturbance that accelerates decomposition or by disruption of plant/microbe populations that slows uptake. Plant-available nutrients, especially nitrogen, are vulnerable to loss by several pathways, and they indicate a degrading soil system.
A subset of the field observations could be done every year and the lab work repeated every five years or so – long enough for management changes to have an effect.
The results of field observations and lab tests might point to needs for deferring grazing to allow plant establishment on vulnerable areas, fencing or herding to change traffic paths or reduce pressure on areas affected by compaction or low organic matter or even active repair of gullies to reduce sediment deposition on low landscape positions.
In future articles, I’ll describe more specifics about translating soil test data to management actions.