The USDA-NRCS (United States Department of Agriculture-Natural Resource Conservation Service) defines soil as (i) the unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants, (ii) the unconsolidated mineral or organic matter on the surface of the Earth that has been subjected to and shows effects of genetic and environmental factors of climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time [1,2]. As an evolving natural resource, soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics. The American Society of Agronomy recently provided an updated soil definition: “The layer(s) of generally loose mineral and/or organic material that are affected by physical, chemical, and/or biological processes at or near the planetary surface and usually hold liquids, gases, and biota and support plants” [3]. The newer definition includes the liquid and gaseous phases rather than just the solid phases.

Currently, soil scientists, agronomists, horticulturalists, animal scientists, and our colleagues in the biological sciences are re-imagining soil as a natural resource which is the biological and physical underpinning of terrestrial ecosystems [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24]. The inherent biology of soil is immense and complex and is critically important to supporting ecosystem stability. Yet, our ability to evaluate soil at the pedon level requires a database that indicates whether the pedon is operating at a level compatible with the soil’s ecosystem service provision potential.

Soil taxonomy and soil mapping are critical conventions to organize our soil knowledge; however, soil mapping and soil taxonomy evolved through mutual interaction. Field evaluation of the soil profile necessitated the introduction of diagnostic soil horizons; that is, soil horizons that have specified and observable field characteristics. These recognizable field features include texture, structure, boundaries, soil color, and redoximorphic features, clay films, organic matter accumulation. These diagnostic horizons and their associated field features have been employed to infer to influence water movement, nutrient provision, carbon and nitrogen cycling, plant anchorage, and an array of other ecosystem services. Thus, soil interpretations became predicated on the soil profile description; however, soil profile descriptions were never intended to be indicative for interpreting the quantitative assessment of ecosystem services.

Since the 1930s and continuing to the present, conservation tillage was advanced to improve water relations in semiarid regions and to reduce soil injury because of soil erosion. Soil scientists soon became aware that conservation tillage tended to increase soil organic carbon levels towards levels prior to cultivation [7,10,15]. Other beneficial soil properties were also supported, such as greater water infiltration. With the advent of synthetic ammoniated nitrogen fertilizers, increases in near-surface soil acidity were observed. Thus, land management became an important concept regarding soil productivity. With the recent rediscovery and then advancement of cover crops, producers are witnessing increases in soil carbon, improved soil structure, positive changes in water relations, and other beneficial properties. Slowly, a shift in our understanding that soil productivity, as influenced by land management, should not be considered uniquely as a function of soil classification, but rather each individual soil requires a baseline where land management-based soil alterations may be either augmented or corrected. Given the array of different land management practices, it is prudent to consider the appropriate collection of baseline data to connect to soil health objectives and specific land management practices.

This manuscript, with an emphasis on beef cattle grazing in the central USA, attempts to survey the literature to: (i) identify important ecosystem services provided by grazing lands, (ii) develop a listing of soil indicators that may be selected to credential soil quality, and (iii) develop guidelines that align soil indicators and changes in grazing management to support the restoration of ecosystem services.

The literature provides numerous definitions of soil quality. Karlen et al. [17] defined soil quality as “the fitness of a specific soil function within its capacity and within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation”. Within their definition is the acknowledgment that there exist innate differences between soils and that soil quality must be based on the individual uniqueness of the existing soil resource. As an example, soil drainage class likely varies among soils in a toposequence (catena), thus the drainage class is a soil property based on landscape position and is not highly influenced by land management. Drainage class may set limits for individual soils in a toposequence, with soils having a poorly drained classification likely having a greater soil organic matter content than adjacent soils having a well-drained classification. However, within each soil, the soil organic matter content may vary within limits because of tillage or manure amendments. Thus, a baseline for assessing soil organic matter changes for the poorly drained soil should differ from that of the well-drained soil.

The USDA-NRCS [3] defined soil quality as “the ability of soil to perform certain functions, such as (1) effectively cycling nutrients, minimizing leaching and runoff, which makes them available to plants, (2) maximizing water-holding capacity and minimizing runoff and erosion, (3) adsorbing and filtering excess nutrients, sediments, and pollutants (4) providing a healthy root environment, (5) providing a stable foundation for structures”. Items 1 through 5 may be defined as soil functions, or as some researcher’s term “ecosystem services”. More recently, Doran and Parkin [21] defined soil quality as: “the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal health”. Thus, the soil quality concept provides a compromise optimization involving ecosystem services and production agriculture.

Soil functions or ecosystem services may be listed, such as (i) moderating and influencing the hydrologic cycle, (ii) anchorage and physical support of plants, (iii) retention and provision of plant nutrients, (iv) detoxification of wastes, decomposition of particulate matter into humus and microbial degradation of agrichemicals, (v) renewal of soil fertility, and (vi) regulation of nutrient/element cycles [8,12,25,26,27]. Kruse [25] reviewed various soil function definitions and their listing of ecosystem services, noting that some authors included biodiversity, social-economic factors, and societal long-term benefits. Thus, the credentialling of soil functions remains an active area of discussion [17,26,27,28].

The global emphasis on sustainable land use and governmental attention towards maintaining soil functions is increasing, frequently co-involved with global issues, such as climate change, regional water shortages, large-scale land degradation attributed to erosion, deforestation, desertification, and heavy metal impact. However, a more immediate and pragmatic need is to develop and authenticate protocols to evaluate the various land management influences on ecosystem services [29,30,31]. Researchers have defined two kinds of soil properties: (i) inherent or use-invariant properties and (ii) dynamic or use-dependent properties [25]. Inherent properties include texture, soil depth, clay mineralogy, cation exchange capacity, drainage class, and thermal regime, which are soil properties related to the combined effects of the five soil-forming factors (parent material, climate, organisms, relief or topography, and time). Dynamic soil properties are soil properties that alter within a reasonable time frame because of land management and these properties include soil organic matter content, bulk density, soil structure, infiltration rate, water holding capacity, nutrient holding capacity, and pH [4,5,6,8,12,26]. Consider soil organic matter content, which is readily altered because of land management attributed to the crops grown and tillage employed, but constrained by inherent factors (texture, climate).

Soil quality parameters, also termed indicators, form a composite set of measurable attributes indicating the intensity of soil function activity [25]. Typically, any changes in a soil quality parameter, as monitored by a specific sampling protocol, may warrant appropriate land management alterations. Three main categories of soil indicators permeate the literature: (i) chemical, (ii) physical, and (iii) biological [21,25]. Chemical indicators provide information on soil solution species and their concentrations, the types and quantity of exchange or adsorption sites, the nutritional status for maintaining plant and animal communities, the presence and activity of soil contaminants, and other soil chemical attributes. Frequently used indicators include: (i) pH, (ii) Eh (oxidation-reduction status), (iii) EC (electrical conductivity), (iv) soil nitrate concentration, and (v) exchangeable or total acidity. Physical indicators provide information primarily on the soil’s hydrologic characteristics. Physical indicators include: (i) aggregate stability, (ii) available water capacity, (iii) bulk density, (iv) infiltration, (v) soil structure classification, (vi) the macropore-micropore distribution, and (vii) enzyme activity. Biological indicators provide information on the soil’s biotic activity. Biological indicators include: (i) soil organic matter content, (ii) active carbon, (iii) respiration, (iv) microbial biomass, and (v) mineralizable nitrogen [21,25]. Dick [32] noted that plant roots secrete extracellular enzymes, thus the rhizosphere is enhanced with phosphatases, nucleases, invertases, urease, catalases, arylsulfatases, and proteases. If investigators are interested in monitoring nutrient cycles, then key enzymes are important indicators: (i) the carbon cycle (amylase, cellulase, lipase, glucosidases, and invertase), (ii) the nitrogen cycle (proteases, amidases, urease, deaminases), (iii) the phosphorus cycle (phosphatases), and (iv) the sulfur cycle (arylsulfatase). Dick [32] also reviewed literature that demonstrated that manure amendments supported increased enzyme activity, whereas nitrogen synthetic fertilizers did not appreciably increase enzyme activity.

The minimum data set is the identification of a series of relevant indicators that permit the monitoring of important soil processes (ecosystem services) because of specific land practices. The relevant indicators require scoring protocols so that diverse indicators may be compared [29,31]. Suppose that pH and earthworm activities are selected indicators, then an earthworm population-scale indicative of low, low-moderate, moderate, high-moderate, and high populations may be created as an indexing scale. Similarly, pH may be indexed as maximum, minimum, and optimum pH levels for various soil organisms: bacteria (maximum is pH 5, minimum is pH 9, optimum is pH 7), actinomycetes (6.5, 9.5, 8), fungi (2, 7, 5), blue-green bacteria (6, 9, greater than 7), and protozoa (5, 8, greater than 7) [33]. Thus, informational comparisons are possible.

Indices of soil quality (ISQ) are rubrics where the various indicators are collectively evaluated to provide a quantitative assessment of where soil monitoring indicates that soil quality is static, improving, or degrading [4,25,26,29]. Thus, an ISQ may be established as: