July 12, 2017. Published in the Wyoming Livestock Roundup
The answer is YES, but vegetation composition and production are sensitive indicators of soil problems in rangelands. Therefore many issues revealed by soil analyses, such as erosion, compaction, and loss of organic material, may already be obvious to managers who monitor vegetation. Soil sampling and analysis are labor intensive and expensive; ranchers need to know whether the investment will pay off. So the question is: WHICH soil indicators can either reveal production-limiting issues that are not obvious, or predict production problems and guide pre-emptive management decisions. In this column I’ll discuss how to get a start on understanding the interplay among natural constraints and management-induced changes for setting attainable goals and devising effective strategies toward improving soil health and productivity.
We often separate soil properties into those that are inherent and therefore, limit the productivity of a site, or dynamic and therefore, are affected by management, to sort out which soil properties might provide useful information. There is not a sharp boundary between these two concepts, however, which makes this complicated and keeps generations of soil scientists scratching their heads.
Inherent properties are the outcome of complex interactions among five soil forming factors: climate, parent material, topography, organisms, and time. Climate sets the speed of weathering, erosion, leaching, biological activity, and other processes that determine the depth, distribution, and quality of the soil; parent material sets the ease of weathering and the physical and chemical composition of the soil; topography affects the microclimate and erosion or deposition rates; organisms drive decomposition and organic matter accumulation; and time affects how long the other factors interact to develop soil. Time is a variable concept in soil science; soil in a warm moist environment can be deeper and appear older than one forming over the same period in a cool dry environment.
Dynamic properties change, or can be changed, in time scales relevant to management, or what humans think of as long-term. The amount and composition of soil organic matter (SOM), including soil organisms, are important dynamic properties that react to management. SOM controls soil structure (or aggregation) and porosity, which create water infiltration and plant-available water holding capacity, along with facilitating root penetration and movement of soil air. Soils that lose SOM lose structure and become compacted, even if they’re not physically compressed by wheel or livestock traffic. SOM forms as plant materials decompose and mix with surface soils, but can also be redeposited from hillslopes, forming deep, rich, and productive soils in swales.
Understanding how soils are distributed across the landscape, and how that is reflected in the vegetation, forms a basis for recognizing soils that are not functioning to their potential. For example, flat-topped hills are often the oldest spots on a landscape, with the oldest and most highly developed soils, though not necessarily the deepest or most productive ones. Hillslope soils can be well developed but often have surface horizons that are naturally on the move, slowly transporting materials down slope with rainfall events. Hillslopes can support productive vegetation but might be vulnerable, where bare soil and trailing, for example, can accelerate natural erosion processes and degrade soils. Footslopes and swale bottoms are depositional zones where water, SOM, and sediments naturally accumulate to create the most productive landscape positons, but often the youngest soils on the landscape. Accelerated erosion and runoff from hillslopes can accelerate deposition on swale bottoms. Sediments from accelerated erosion can be saline with low SOM and no soil structure, which limits water infiltration, further increasing runoff from the landscape. Ultimately, continuous gullies can form, transmitting water directly from uplands out of the landscape, truncating natural erosion and deposition processes that store water and support productivity. Livestock trails and roads can also divert runoff, cutting off important soil forming processes.
Some simple field soil tests can help understand landscape-soil-vegetation links, recognize where the soil may not be functioning to its potential, and identify places where lab tests might be useful. Links to Web sites that describe these tests can be found on my Wyoming Soil Management site (https://soilmanagement.wordpress.com/).
- Soil texture by feel: important inherent property that controls water infiltration and holding capacity. This simple test using a mud ball to estimate relative proportions of sand-, silt-, and clay-sized particles in the soil takes a little practice but is very informative. Soil texture is an important indicator of site potential. Sandy soils absorb water rapidly but do not hold it for plant uptake, while clayey soils can seal up and cause runoff. A more or less even mix of sand, silt, and clay provides the ideal combination of water infiltration and holding, but this is a property that management must adjust to rather than change.
- Resistance to penetration: indicates soil density and porosity, and correlates to infiltration, root penetration, and SOM content. You can purchase a penetrometer with a gauge on it (which requires moist soil), or you can use an old hunting knife. This is affected by moisture, so all the spots tested should be the same level of moisture; either dry or after a rain. Simply push the knife into the soil and note how much force it requires. Start in a spot that seems like it should be functioning at or near potential. Note differences between soils under shrub canopies, under grasses, and without vegetation. It might take a while, but if you do this enough you start to see patterns emerge, along with anomalies that might correspond to less-that-optimal productivity.
- Soil moisture by feel: indicates relative soil water holding capacity if observed while the soil is still moist after a soaking rain. This is another mud ball test that can show interesting patterns of water storage on the landscape.
- Simple chemical tests:
- Soil pH and salinity are important properties that correspond with nutrient and water availability. Higher than optimal numbers can indicate erosion or sediment deposition from changes to surface hydrology that may be reversible. Small hand-held meters can be purchased for around $100.
- Soil calcium carbonate content causes the soil to become very hard when dry. It’s common to almost all rangeland soils, but high contents can indicate loss of surface soils or SOM. A weak acid solution is used to cause the soil to effervesce, or fizz, with more fizz indicating higher calcium carbonate content.
- Nitrate and phosphate content: important plant nutrients that can be estimated with water quality test strips or inexpensive garden kits from the hardware store. These forms of nitrogen and phosphorus are the end result of organic matter decomposition and are usually taken up by plants and micro-organisms as quickly as they are released. Therefore we expect very low concentrations in healthy rangelands, but the tests can indicate patterns of soil fertility. Higher concentrations can indicate disturbance, where release rates exceed uptake rates. Excess available nutrients can lead to weed invasion and loss of soil fertility.
With practice, making these observations part of range vegetation monitoring can increase an overall awareness of the productivity relative to it’s potential. Collecting some samples for lab analyses of SOM, texture, and other parameters can for a good baseline and help calibrate your field observations. Select sampling areas by landscape position and management level.
While these observations may not lead directly to management decisions, they will support improved understanding of vegetation and productivity patterns on the range. In future columns I’ll provide more details on soil sample schemes and how lab results can inform management.
More information, along with this and past columns, are posted at https://soilmanagement.wordpress.com/.