Rangeland soil health indicators: Presentation at the Wyoming Stock Growers Association fall meeting in Casper, November 28, 2017

112817 WSGA Casper Rangeland Soil Health indicators

Click above to see the slides from my presentation at the Wyoming Stock Growers Association fall meeting in Casper, November 28, 2017. This presentation at the Rangeland Soil Health workshop presents a stepwise guide to soil health indicators and focuses on the first level: simple observations of surface conditions, topsoil thickness, and resistance to penetration with a knife.

1.Identify good monitoring and reference locations;

2.Observations of soil depth, structure, surface conditions, water holding capacity that only require a tape, shovel, knife, and notebook;

3.Field measurements of water infiltration, bulk density, aggregate stability, and nutrient content that provide harder numbers, but require more tools and more time, and;

4.Sampling for lab tests when the need is indicated.


Rangeland soil health: how to assess it in ways that inform livestock management

First published in the Wyoming Livestock Roundup

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 the following steps:

Identify sampling zones. Identify sampling zones based on soil type and management history. Soil type varies with topography and underlying geology. Soil survey maps can help define zones, but might be too large scale. The drawing 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.

block diagramFrom Ohio State University Extension Soil Sampling Fact Sheet.

Identify reference areas. Reference areas are performing 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.

Field observations. 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 (penetrometers are expensive and range soils are usually too hard), soil texture, structure, moisture (after a nice rain), 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 (this correlates well with penetration resistance), soil bulk density (requires precisely weighing a known volume), soil solution pH and salinity (as electrical conductivity; requires an electrode device), aggregate stability, calcium carbonate (free lime) 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 Web site (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. How and where to sample: 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. How deep to sample: the best is to sample the surface horizon, which is most impacted by management. This could vary from 2 or 3 inches on slopes or hill tops 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. When to sample: mid-summer when everything is dry 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. Sample handling: 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 set out in the sun or in a vehicle. They get very warm and microbes become very active.

Ohio State Extension Soil Sampling Fact Sheet.

What to have the lab analyze. 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 & Ranch Package ($15.00) or a Routine general fertility package ($35.00)(prices appear on the lab submission forms. 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.

How often to sample. 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.

Data to management: can soil health information inform range management decisions?

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/).

  1. 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.
  2. 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.
  3. 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.
  4. Simple chemical tests:
    1. 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.
    2. 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.
    3. 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/.

Now is the time to fix your soil!

(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.

Integrating livestock into sugarbeet-barley rotations: Barley after barley for soil health and profit

(This first appeared in the Wyoming Livestock Roundup in November, 2015)

Driving around and talking to producers in the Big Horn Basin this fall, I noticed an increasing trend toward replanting barley for fall and winter grazing. Some producers are also lengthening rotations and including two to three years of alfalfa for seed or forage.

Sugarbeet-barley rotations are notoriously destructive to soil health because of intensive tillage, long periods of bare soil, and the every-other-year frequency of heavily consumptive sugarbeets. Several research papers have shown that the less often sugarbeets are grown, the healthier is the soil. One of the latest reports is from our on-farm study in the Powell area, in which both soil health and sugarbeet yield improved in the order sugarbeet-barley, sugarbeet-bean-barley, sugarbeet-barley-alfalfa-alfalfa (Hurisso et al., 2015).

Farmers are realizing that the early harvest of barley creates a window of opportunity. Barley can be replanted right behind the combine to grow a tall, green forage crop before freeze up. Some farmers irrigate and fertilize to maximize forage production. Others include a mix of other cover crop species to benefit both the soil and the livestock.

The system works especially well with sprinkler irrigation because of reduced requirements for tillage, leveling, and bedding, but if those operations can wait until spring, it should benefit furrow systems as well.

We’re catching up with farmers to evaluate the benefits, but barley is known to be a great cover crop. It grows very rapidly, competes voraciously with weeds, is very salt tolerant, and has prolific fine roots that improve soil tilth and organic matter content. Barley can produce more biomass in a shorter time than any other cereal crop. It can form a deep, fine root system that improves soil structure and scavenges valuable nutrients. Grown together with a legume like field pea, barley can stimulate more nitrogen fixation and capture it in easily decomposed biomass. A USDA Sustainable Agriculture Research and Education (SARE) on-line publication provides an excellent description of barley and many other cover crops (http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition).

Several research studies have shown that grazing does not reduce soil health benefits of cover crops. To the contrary, a recent study in the southeastern US found that grazing sped turnover rates of cover crop biomass, increasing biologically active carbon and soil microbial biomass without affecting the amount of potentially available nitrogen (Franzluebbers and Stuedemann, 2015). The benefits were greater under reduced tillage, which is another growing trend under sprinkler irrigated systems in Wyoming. We’re making plans to study interactions among cover crops, tillage, and grazing in this part of the country, but we expect similar beneficial outcomes.

If grazing is not part of the picture, replanting barley could still be worth the effort, especially if other cover crops like nitrogen fixers and deep rooted radishes are included. Planting just-harvested barley seed is a very cheap way to get a cover crop in the ground, and following the combine with the planter maximizes the time for growth before frost. Simply allowing volunteer barley to thrive and provide cover during winter could result in better soil health and productivity. The cover crops capture and cycle residual soil nitrogen and phosphorus from fertilizers to reduce losses from leaching, runoff, or volatilization.

Overall, replanting barley after barley looks to be an excellent way to save soil, cycle nutrients, and integrate livestock grazing into sugarbeet production systems. The benefits might be even greater if legumes and other cover crops are mixed with the barley seed. Expanding rotations to include perennial crops like alfalfa also has clear benefits for soil health and productivity.


Contact the author for copies of literature referred to in this article:

Franzluebbers and Stuedemann, 2015, Journal of Soil and Water Conservation 70(6):365-373.

Hurisso et al., 2015, Soil Science Society of America Journal: doi:10.2136/sssaj2015.02.0073.

Cheatgrass effects on soil health

(This first appeared in the Wyoming Livestock Roundup in July, 2015)

Driving across the green Wyoming rangelands this June, the reddish patches that indicate cheatgrass is having a good year remind me of the summer 14 years ago I spent up to my eyeballs in the weedy annual grass. Cheatgrass grows well in Great Basin of Utah and Nevada, but not that tall; I spent much of that summer standing in five-foot deep holes with my head protruding at ground level in a sea of cheatgrass.

As a researcher at Utah State University, I was collecting data on how conversion from sage-steppe grassland to persistent monocultures of the exotic winter annual grass impacts soil health and organic matter processes. I started by working with long-time Great Basin ecologists like Neil West and Steve Monsen to locate seven long-term infestations with matched areas dominated by native range vegetation.

With help from a crew of muscle-bound Utah college students, I dug holes at each of the 14 locations and carefully described the soil to make sure the paired sites were well matched, and to look for differences attributable to the weed infestation, like porosity and root characteristics. Then I took samples back to the lab where I measured many properties that underlie soil health and productivity, such as density, texture, alkalinity, organic carbon and nitrogen, microbial activity, and easily decomposable soil organic matter.

It turned out that looking at soil under paired cheatgrass and native range in the Great Basin was almost like looking at soil under paired wheat fields and native range in eastern Wyoming. Cheatgrass grows a thick mat of shallow, very fine roots that die each summer, aerating the soil and adding carbon-rich organic material in almost the same manner as tilling to incorporate wheat straw. Soil microbes thrive on the added air and the carbon energy source, and their rate of decomposing organic material multiplies.

The initial effect of increased microbial activity is increased mineralization of soil organic matter, making more nutrients available for the crop (or the weed). But after a few years of enhanced conditions caused by disturbance, microbes start eating themselves out of house and home, depleting soil organic matter that accumulated under millennia of perennial grassland vegetation. Loss of organic matter degrades all the properties that contribute to soil health, including building soil structure, enhancing water infiltration and storage, providing a time-release nutrient supply, and supporting sustained productivity.

In cropland, organic matter loss means that more and more tillage and more and more fertilizer might be necessary to keep growing crops. In cheatgrass-infested rangelands, it means that the older and more monocultural the infestation, the more difficult it might be to restore desirable vegetation. Cheatgrass tends to convert slow, conservative nutrient cycles of native grass and shrublands to flashy, leaky nutrient cycles where mineral nutrients accumulate to create conditions for rapid cheatgrass growth (just add water), or to be lost with erosion, leaching, or volatilization. In native perennial plant communities, a diversity of plants and microbes turn mineral nutrients into more stable organic forms almost all year.

In annual cropping, a great practice is to try to keep plants growing and covering the soil all year, or especially to convert fields to perennial cover, like hay or pasture, for a few years. This minimizes disturbance (aeration) and shifts the system back toward accumulating, rather than losing, soil organic matter.

The extensive nature of cheatgrass infestation, often on hot, south slopes and rough ground, makes restoration difficult. Range managers in the Great Basin have had some success “re-perennializing” cheatgrass stands by seeding introduced cool-season bunchgrasses like pubescent wheatgrass or crested wheatgrass. After the bunchgrasses become established and reduce competition from cheatgrass, managers increase the diversity by weakening the introduced grasses with fire, heavy grazing, herbicides, mechanical treatment, or a combination, and then planting native grasses, forbs, and shrubs.

Rigorous fire prevention is crucial to restoring cheatgrass infestations to diverse, desirable plant communities. Once a seed bank is established, cheatgrass may always be there in the understory, filling the bare spaces and creating a continuous fine fuel load so that when a fire occurs, it can be much larger and more destructive, and the plant community quickly converts back to cheatgrass.

A comprehensive guide to cheatgrass management in the Rocky Mountain region can be found on the University of Wyoming Extension publications Web site (http://www.wyoextension.org/publications). Search on “cheatgrass” or bulletin number B-1246.

For more detailed information on the study discussed here, see the papers posted at:

Cheatgrass Invasion Alters Soil Morphology and Organic Matter Dynamics in Big Sagebrush Steppe Rangelands. Wildland Shrub Conference Proceedings.

Soil morphology and organic matter dynamics under cheatgrass and sagebrush-steppe plant communities. Journal of Arid Environments.

Mediterranean annual grasses in western North America: Kids in a candy store. Plant and Soil.

Occasional tillage: How persistent are soil health benefits of no till?

(This first appeared in the Wyoming Livestock Roundup in June, 2015)

No-till crop production slows decomposition of crop residues, allowing redevelopment of soil aggregates and increasing the nutrient and water supplying potential of the soil. More moisture and nutrients reduce the need for fallow and often allow more diverse crop rotations. Even perennial forage production for hay and pasture benefit from minimizing or eliminating tillage when stands are renewed.

Such benefits make true-believer no-till practitioners steadfastly committed to keeping steel out of the ground. But sometimes there are good reasons for dusting off the tillage equipment. No till can cause low-mobility nutrients like phosphorous to accumulate near soil surface; without careful traffic management it can cause soil compaction; weeds can get away; or high-yield years can create excess crop residue that is difficult to plant into. The question is how much soil health, recovered after years of no till, is lost with occasional tillage?

Even a single tillage operation can increase soil aeration and accelerate loss of carbon and nitrogen, resulting in immediate bursts of carbon dioxide (CO2) and nitrous oxide (N2O), which are very sensitive indicators of soil disturbance and organic matter loss. Carbon dioxide and N2O are greenhouse gases that drive global warming, and agricultural practices that include frequent tillage are important contributors. Soils under no till tend to store more carbon and nitrogen so that losses as CO2 and N2O are reduced.

So, would a single tillage event in a long-term no-till field create huge bursts of CO2 and N2O when stored organic matter is exposed to air? To find out, we conducted an experiment at the University of Wyoming Sustainable Agriculture Research and Extension Center (SAREC) near Lingle.

We collected and compared soil air samples and surface soil samples from winter wheat-fallow systems that have been under no-till and frequent tillage management since UW bought the research station 11 years earlier. Small plots in each of the study fields were established to create three treatments: 1) no-till; 2) one-time till in no-till; and 3) summer tillage (typical) fallows. Air and soil samples were collected before and immediately after a one-time pass with a tandem disk that loosened the dry soil to 4-inch depth in the one-time till in no-till and the frequently tilled plots, and concurrently without tillage in the no-till plots. Carbon dioxide and N2O emissions, as well as concentrations of easily decomposed forms of organic matter were determined 0, 1, 5, 25, and 50 hours after tillage. To collect soil air, we drove a sharpened six-inch long piece of eight-inch diameter PVC pipe about three inches into the soil and left it there throughout the experiment. At each sampling time we placed a PVC cap fitted with a septum onto the base and sealed it with a rubber gasket. We used a syringe to collect soil air samples from the canister three times, at 0, 15, and 30 minutes, placing each sample in to a glass vacuum vial. The samples were analyzed by gas chromatography, and emissions calculated in micrograms per square meter of each gas.

Results indicate that CO2 emissions from one-time tillage in no-till were 30 to 40% lower than from the frequently tilled soils. The one-time tilled soils also had less soluble organic carbon and mineral nitrogen, indicating less disruption of organic matter. Surprisingly, the values from one-time tilled no-till soils were much closer to those from the no-till plots that we didn’t till than to the frequently tilled plots. This suggests that soil organic matter stored in the long-term no-till soil was resistant to the single summer tillage operation. Importantly, tillage did not affect the magnitude of N2O emissions in any of the treatments, suggesting that if performed during dry summer, this operation did not contribute to nitrogen loss as gas.

The results suggest that the 11 years of no till facilitated formation of stable soil aggregates that protected organic matter and were not destroyed by the tillage operation, and also formation of resistant organic compounds, like humus, that are not easily decomposed. Experience shows that repeated tillage would breakdown those materials, compromising the resilience gained during the years of no till. But occasional summer tillage to accomplish particular management objectives may not destroy the benefits of long-term no till, at least on the silt-loam soils at SAREC.

A more in-depth article about this research can be found here:

Greenhouse Gas Fluxes and Soil Carbon and Nitrogen Following Single Summer Tillage Event, International Journal of Plant and Soil Sciences

Don’t Volatilize your bank account: Avoid N fertilizer losses

(This first appeared in the Wyoming Livestock Roundup in April, 2015)

Ah springtime! Birds are singing, days are longer, even in Laramie it’s warming up, and our thoughts turn to…. fertilizer. Well, if it’s not the first thing on your mind, it’s probably close. That’s because if you’re an ag producer you have to lay out a lot of money for plant food. Here’s some food for thought: half or more of the nitrogen you apply can escape from the soil as ammonia gas; a process called volatilization. Nitrogen has many forms during its journey through the nitrogen cycle, including organic forms in plants and microbes, soluble mineral forms (nitrate and ammonium) used by plants, and gases like di-nitrogen (most of our air) and ammonia – many opportunities for loss. Cycling of our other big fertilizer investment – phosphorus – is less complicated. With no gaseous form, it transforms among organic, fixed, and plant-available forms. Phosphorus fixation in calcareous soils is another source of heartburn for growers, but that’s a different story. This article describes factors that cause ammonia volatilization of nitrogen fertilizer, and how to minimize those losses.

Nitrogen in any type of fertilizer can be lost to volatilization, but the problem is most prominent, and most studied, in urea and urea-containing fertilizers (like UAN 28 and 32 solution). Each transformation in the nitrogen cycle is mediated by soil microbes, so conditions that favor microbial activity – the same optimum temperature and moisture as for plant growth – accelerate nitrogen cycling, including ammonia volatilization. When urea hits the soil it must go through a process called “hydrolysis” to become available to plants as ammonium . This process is catalyzed by a natural enzyme from soil microbes called urease. Hydrolysis raises pH in the immediate microsite around the urea, which favors ammonia gas over ammonium. Thus volatilization is a bigger problem for alkaline soils (like yours!) than for acidic soils in higher rainfall regions.

If warm temperatures speed volatilization, then putting down urea on cool days in fall or early spring, or on the snow, should be a safe practice, right? Wrong. For many years that was assumed to be so, but recent research in Montana shows that as much as 44 percent of nitrogen applied as urea between October and April was lost to volatilization, and the average loss across 23 study sites was 16 percent. This is enough to really hurt yields and represents money drifting up off your field and out of site in the Wyoming wind. Even though the cool temperatures slow down volatilization, all the other transformations, including plant and microbial uptake, are slowed down too, so ammonium is left exposed to the volatilization pathway for a long period. Surface crop residues also increase potential ammonia volatilization because they keep the soil surface moist, reduce the amount of urea diffusing into the soil, and have high urease activity.

Minimizing nitrogen volatilization

Choosing a nitrogen source with lower risk of volatilization is one way to minimize losses, but of the two with low risk factors, one (liquid anhydrous ammonia) is not widely used in Wyoming, and the other (ammonium nitrate) has limited availability because of explosive properties. Other alternatives to urea, like 28 or 32 solution, ammonium sulfate, and others each have lower volatilization potential, but in alkaline soils under the wrong environmental conditions the potential is still high. The Montana study showed that urease inhibitors like NBPT, the active ingredient in Agrotain®, slowed volatilization, especially under cold dry conditions. Other enhanced-efficiency urea fertilizers can slow volatilization and can work well for no-till or perennial crops under the right conditions, but proper placement and timing of fertilizer is the best route.

Broadcasting without incorporating to at least two inches should be avoided. The best alternative to incorporating fertilizer is to broadcast it just before a single rain or irrigation event of at least ½ inch; enough to dissolve and carry the fertilizer into the soil. Subsurface banding to at least two inches can be effective, but the slit produced by banding needs to be closed tight to avoid loss of ammonia from the concentrated band of urea. In conservation tillage or no-till systems fertilizers should be knifed into the soil below the residue and the slit closed tightly. That also goes for fertilizing perennial pastures or hayfields. Applying liquid nitrogen with a spoke-wheel injector has been effective for these situations.

Montana State University has produced some excellent guides on managing nitrogen fertilizers to minimize volatilization. You can access them by going to http://www.uwyo.edu/soilfert/ and clicking on Soil Links> Montana State University Soil Fertility Program>Ammonia Volatilization.

Soil quality and the benefits of crop rotation, reduced tillage, and manure application

(This first appeared in the Wyoming Livestock Roundup in February, 2015)

Two underlying strategies toward improving soil quality include conserving soil organic matter by reducing tillage and adding organic material by applying composted manure or other amendments. Crop rotation also builds soil quality by supplying diverse crop residues, especially if perennial nitrogen-fixing legume crops, such as alfalfa, are included. Starting in 2009 we conducted a four-year experiment on irrigated fields at the University of Wyoming Sustainable Agriculture Research and Extension center near Lingle to evaluate how rapidly combining crop rotations, reduced tillage, and manure application would improve soil quality on depleted soils that had been under intensively tilled continuous corn for many years.

We divided the area into 12 one-acre plots and converted four plots to a conventionally managed rotation of dry beans-corn-sugarbeet-corn, four plots to reduced-tillage under the same rotation, and four plots to an organically managed rotation of alfalfa-alfalfa-corn-dry beans. Tillage was similar among the conventional and organic plots, and fertility management was similar among the conventional and reduced-tillage plots. Tillage on the conventional and organic plots included moldboard plow, disk, and harrow for seedbed preparation and cultivation as needed for weed control. The reduced-tillage plots were tilled once with a Landstar machine (Kuhn Krause, Inc., Hutchinson, KS). Conventional and reduced-tillage plots were fertilized with commercial fertilizers based on soil-test-based recommendations, while the organic plots were fertilized with composted and fresh cattle manure applied based on crop needs for soil N (about 5 tons per acre). Yields were comparable among the three systems, and economic analysis indicates that the organic rotation is more profitable than the other two if premiums for organic-certified crops are considered. To assess effects on soil quality, we measured properties that respond rapidly to management changes, including soil microbial populations and functional groups like fungi and bacteria, soluble organic carbon and nitrogen compounds, and easily decomposed organic materials.

Results indicate that converting from monocropped corn to crop rotations had the largest positive impacts of the three strategies, regardless how they were managed, and that reduced tillage, manure, and alfalfa in rotations added to the positive effects. At the end of the four-year study total microbial biomass in the soil had tripled or quadrupled in all three systems, with the largest increase in the organic system. The most striking increases in microbes were among fungal species that break down crop residues, with five- to eight-fold increases in that group. Mycorrhizal fungi also increased in all three systems by factors of two to four. Mycorrhizal fungi form symbiotic relationships with plants and support improved uptake of water and nutrients, especially phosphorus. Both types of fungi form extensive networks of microscopic hyphae that hold soil particles together and improve soil structure, porosity, and water infiltration. Their presence indicates more stable and resilient soils. Overall microbial diversity also increased by 50 to 70% over the four years, indicating establishment of a rich soil biological system with abundant beneficial organisms.

These increases in microbial numbers and diversity correspond to increases in decomposable organic material that serves as substrate – or food – for microbes. Dissolved organic carbon, which is essentially sugar derived from initial decomposition of plant residues, increased six-fold by the end of the fourth year, with larger increases under organic and reduced-tillage than conventional management. Dissolved organic nitrogen, however, did not increase appreciably, meaning that the carbon-to-nitrogen ratio of the most microbially available substrates doubled or tripled in the three systems. This favors beneficial fungi over bacteria, and probably results from diverse types of crop residues. The amount of easily decomposable organic material more than doubled in the organic system but stayed relatively stable in the reduced-tillage and conventional systems, indicating that the combination of manure and alfalfa in rotation began to build soil organic matter more rapidly than either the crop rotations alone or reduced tillage.

Results of this four-year study emphasize the importance of rotating crops for maintaining soil quality and productivity. Even with two years of corn and one of sugarbeet – both highly consumptive crops – in the four-year rotation, soil microbial activity and organic matter components increased significantly compared with conditions after years of continuous corn. Combining rotation with reduced tillage further boosted microbial activity, as did including alfalfa and manure applications in the rotation, but rotation itself had the largest impact on soil quality.

Results of this study were published in the open access scientific journal PLoS ONE and can be found on line by searching on PLoS ONE 9(8): e103901. Parts of this article were published previously in the on-line Western Nutrient Digest Newsletter.