Soil health involves the fundamentals associated with soil, crops, and overall agricultural management practices that serve to enhance good soil system function. The concept of soil health basically addresses how well a soil is functioning. A simple analogy is with human health and as we see with a healthy person there is good overall system function and there are many interactive components in soil systems, like the human health system.
Soil health has been defined by the U.S. Department of Agriculture Natural Resource Conservation Service as “The continued capacity of a soil to function as a vital living ecosystem that sustains plants, animals, and humans” (USDA-NRCS, 2022).
The principles of soil health include soil quality parameters that serve to maximize biodiversity, soil cover, encouraging strong plant root system development, reduced tillage and practices that minimize soil disturbance, and will develop good soil conditions that are conducive to better infiltration and soil water holding capacity, sequester carbon (C), improve nutrient cycling, enhance ecosystem services, and many other benefits that are associated with healthy soils. Thus, soil health represents an extremely broad set of concepts and functions, and it is a good way to present the holistic aspects of soil systems.
As an agronomist and soil scientist working professionally for over 45 years, I find it interesting that what is now commonly referred to as “soil health” might be a new term but it is not a recent discovery. What is often referred to as soil health has often been described for many years as “soil quality” (NRCS, 2015).
Soil quality is the ability of a soil to perform the basic functions necessary for its intended use. These soil system functions typically include:
● sustaining biological diversity, activity, and productivity
● regulation of water and solute flow
● filtering, buffering, degrading organic and inorganic materials
● storing and cycling nutrients and carbon
● providing physical stability and support
Soils have been recognized as living entities for many decades and we commonly address soil function in terms of the three categories of physical, chemical, and biological properties. However, these categories overlap in function and are not always clearly defined as an independent soil property in a soil system since each can affect multiple soil functions.
Thus, soil health serves to integrate the dynamic functions of a soil system among the physical, chemical, and biological properties associated within a given soil system (Figure 1).
Figure 1. Soil health and the integration of fundamental soil
properties. Source: University of Tennessee Cooperative Extension.
One of the principal aspects of soil health is the emphasis on the relationship between soils and soil systems to human health via the function of soils as a fundamental component of terrestrial ecosystems, Figure 2.
Figure 2. The broad and integrated aspects of soil health and relationship to human health. Source: van Es and Frost, 2016.
In agricultural systems these relationships are commonly recognized but this new emphasis is good in my view because of the capacity to bring the importance of soil systems into a better realm of understanding and appreciation by the public and non-agriculturalists who have never really thought about this before. It is also important for us to recognize that whatever we do to impact a soil system in one aspect, it will have impacts on other aspects as well.
The study of soil fertility in the context of soil-plant relationships has often served to integrate the physical, chemical, and biological properties in relation to plant or crop growth. The concepts of soil health commonly place an emphasis on soil carbon (C) content, particularly in relation to stable organic carbon (SOC). This is related to the common reference to soil organic matter (SOM) content in soil health discussions.
It is important to distinguish the difference between organic materials and organic matter in soils. Organic materials include crop residues and SOM is the stable, residual forms of C compounds left in soils following microbial decomposition. For example, after the harvesting of a crop the crop residues are organic materials and they do not equate to SOM. The SOM is the final result of the crop residue breakdown and it is usually a very small fraction of the total organic material that was originally deposited.
Soil organic C represents the net balance of inputs and outputs of C to the soil over time. Inputs of C into a soil system consist largely of root exudates, residues of leaves, stems, and roots, and it also includes the deposition of materials transported by wind and water. In agricultural systems these inputs can include organic amendments such as manure, compost, biosolids, biochar, etc. to supply nutrients or organic matter.
Inputs of C into the soil system are counterbalanced by the C outputs which are dominated by the mineralization of SOC to carbon dioxide by microbes. In an agricultural soil system, microbial degradation and the transformation of plant inputs creates a complex of microbially derived organic compounds in the soil (Grandy and Neff, 2008). Outputs also include any harvested crops, residue burning and erosion.
Typically, soil health measurements focus on the soil surface properties, usually the upper foot (30 cm) of the soil surface. Thus, the transport of C deeper into the soil profile by water or tillage pedoturbation would result in a decrease in the measured SOC in the surface.
In a desert agricultural setting, it is important to review the basics of C cycling in soil systems as shown in Figure 3.
Figure 3. Soil carbon cycle. Source: Lavallee and Cotrufo, Colorado State University, 2020.
Desert crop production systems can produce large amounts of organic material, plant structures and residues. However, much of the C captured in plant residues and incorporated into the soil is soon lost to the atmosphere due to the high amounts of solar energy inputs, an abundance of soil microbes, with sufficient soil moisture and aeration as a function of good soil drainage, and a good nutrient supply (particularly nitrogen).
These are all functions of healthy soils, which are common in desert agricultural settings. The result is a relatively low level of stable organic matter (SOM), usually less than 2% on a soil mass basis and most often ~ 1% or less in desert agricultural soils. Even if difficult, trying to enhance SOM accumulation in desert soils is a worthy goal.
It is important to understand the basic concepts and complexities associated with soil health and to consider agronomic aspects (soil and crop factors) of soil and crop system management that serve to enhance healthy soils for both short and long-term productivity and sustainability.
References
Lavallee, J. and F. Cotrufo, 2020. Soil carbon is a valuable resource, but all soil carbon is not created equal. The Conversation and Colorado State University, 2020.
https://theconversation.com/soil-carbon-is-a-valuable-resource-but-all-soil-carbon-is-not-created-equal-129175
Grandy, A.S., Neff, J.C., 2008. Molecular C dynamics downstream: the biochemical
decomposition sequence and its impact on soil organic matter structure and
function. The Science of the Total Environment 404, 297–307.
https://doi.org/10.1016/j.scitotenv.2007.11.013.
USDA-Natural Resource Conservation Service (NRCS). 2015. Soil Quality Indicators Physical, Chemical, and Biological Indicators for Soil Quality Assessment and Management.
https://www.nrcs.usda.gov/sites/default/files/2022-10/indicator_sheet_guide_sheet.pdf
USDA-NRCS. 2022. https://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/health/
van Es, H. and P. Frost. 2016. Gaining Ground on Soil Health. Tata Cornell Institute.
https://tci.cornell.edu/?blog=gaining-ground-on-soil-health
This study was conducted at the Yuma Valley Agricultural Center. The soil was a silty clay loam (7-56-37 sand-silt-clay, pH 7.2, O.M. 0.7%). Spinach ‘Meerkat’ was seeded, then sprinkler-irrigated to germinate seed Jan 13, 2025 on beds with 84 in. between bed centers and containing 30 lines of seed per bed. All irrigation water was supplied by sprinkler irrigation. Treatments were replicated four times in a randomized complete block design. Replicate plots consisted of 15 ft lengths of bed separated by 3 ft lengths of nontreated bed. Treatments were applied with a CO2 backpack sprayer that delivered 50 gal/acre at 40 psi to flat-fan nozzles.
Downy mildew (caused by Peronospora farinosa f. sp. spinaciae)was first observed in plots on Mar 5 and final reading was taken on March 6 and March 7, 2025. Spray date for each treatments are listed in excel file with the results.
Disease severity was recorded by determining the percentage of infected leaves present within three 1-ft2areas within each of the four replicate plots per treatment. The number of spinach leaves in a 1-ft2area of bed was approximately 144. The percentage were then changed to 1-10scale, with 1 being 10% infection and 10 being 100% infection.
The data (found in the accompanying Excel file) illustrate the degree of disease reduction obtained by applications of the various tested fungicides. Products that provided most effective control against the disease include Orondis ultra, Zampro, Stargus, Cevya, Eject .Please see table for other treatments with significant disease suppression/control. No phytotoxicity was observed in any of the treatments in this trial.
Last month, we investigated the use of applying steam to the soil to control weeds in baby leaf spinach. In the study, we utilized the prototype steam applicator described in previous UA Veg IPM articles to inject steam into the soil prior to planting. The concept is to heat the soil to levels sufficient to kill soilborne pathogens and weed seeds (140 °F > 20 minutes). The device is principally comprised of a 35 BHP steam generator mounted on an elongated bed shaper (Fig. 1). The apparatus applies steam via shank injection and from cone shaped ports on top of the bed shaper. After cooling (<1 day), the crop is planted into the disinfested soil.
In the trial, the unit was configured so that one of the unit’s narrow-bed bed shapers (42” wide) was outfitted with fourteen steam injection shanks positioned to inject steam in the soil at a depth of about 2” (Fig. 2). Seven of the injectors were in a rank towards the front of the bed shaper with each injector spaced about 3.5” apart. A second rank of 7 injectors, also spaced 3.5” apart, was positioned in-line with the first rank towards the rear of the bed shaper. The steam applicator was underpowered to treat the entire 22” wide bedtop as the machine was designed to treat two, 4” wide bands of soil (8” total). As a consequence, travel speed was slow, 0.15 mph, to ensure target temperatures were met.
Results showed that steam treatment provided outstanding weed control of nearly 100% (Table 1, Fig. 3). The predominant species at the site were nettleleaf goosefoot and common purslane. This is a very impressive result however work rates were low (0.05 ac/hr) and fuel use/cost was high ($1,000/ac).
This was our first trial investigating the use of soil applied steam to control weeds in high density crops and think these operational parameters can be significantly improved. As stated previously, the unit was equipped with a steam generator with insufficient steam generation capacity. Equipping the device with a higher powered steam generator so that two beds can be treated at the same time would double the work rate. Also, steam was injected at a depth of 2” and at a travel speed was such that the amount of steam applied was adequate to control essentially all (100%) of the weeds. It is logical that shallower treatment depths and faster travel speeds could be utilized and still provide adequate control. Finally, it is estimated that travel speed can be at least doubled by operating the device when initial soil temperatures are high (>120 °F) as compared to this study where soil temperatures were relatively low (90 °F) since much less heat energy is needed to raise soil temperatures to target levels. If travel speed were doubled and two beds were treated during a pass, work rate would be improved to 0.21 ac/hr and fuel costs would be about $500/ac. These numbers are much more reasonable and show potential if high levels of weed control can be maintained. Over the next couple of months and throughout the summer, we plan to investigate these and other ways to improve the efficiency of steam application.
Acknowledgements
This work is supported by the Arizona Specialty Crop Block Grant Program and the Crop Production and Pest Management grant no. 2021-70006-35761 /project accession no. 1027435 from USDA-NIFA. We appreciate their support. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
Fig. 1. Band-steam applicator principally comprising a 35 BHP steam generator mounted on a bed-shaper applicator sled.
Fig. 2. Steam applicator sled a) top view and b) bottom view.
|
Table 1. Weed density and control with steam in baby leaf spinach trial. |
||
|
Treatment |
Weed Density |
Weed Control |
|
|
(#/ft2) |
(%) |
|
Steam |
0.05 |
99.4 |
|
Untreated |
8.23 |
--- |
Fig. 3. Weed control with steam in spinach. Steam was applied to the soil at a 2” depth via shank injection prior to planting (a) and untreated control (b).
What is the difference between resistance to herbicide and tolerance? Sometimes we use the terms inconsistently or interchangeably. For example, some manufacturers of transgenic varieties refer to them as herbicide-tolerant entities and other authors refer to them as resistant-varieties.
The WSSA (1998) helped us define this concept:
Tolerance is the “inherent ability of a species to reproduce and survive after herbicide treatment”. This means no selection or genetic manipulation occurred to create it, the plant is “naturally tolerant”. Some weeds are pulled around to some herbicides due to morphological, physiological, and genetic plant characteristics.
Resistance is the “ability of a plant to survive and reproduce following an exposure to an herbicide dose that is normally lethal to the wild type” of that species1. Resistant weeds appear due to genetic selection by herbicide over a period of time. This usually take several lifecycles.
We are evaluating some fields at the Yuma Mesa in Yuma, AZ for possible Pigweed (Amaranthus palmeri) resistance to glyphosate. In one case after the application of glyphosate we can see dead plants as well as perfectly healthy plants next to each other, which indicates the presence of resistant individuals. In other case plants do not show symptoms of herbicide activity except when applying an extremely high rate (8x) of glyphosate.
It is very important to examine the field history. For instance, if the field is coming from citrus and only glyphosate was used frequently it is possible you inherited the problem of weed resistance. Will continue with our evaluations and keep you informed of our findings.
Figure 1. Susceptible and Non-Susceptible Pigweed (Amaranthus palmeri) after
glyphosate application.
Figure 2. Pigweed (Amaranthus palmeri) population not showing symptoms
after glyphosate application.
Reference:
