yields declining?
Do crops perform less well than those on neighboring farms with similar soils?
Do crops quickly show signs of stress or stunted growth during wet or dry periods?Figure 2.2 Generalized approaches to field evaluations of soil health. USDA‐NRCS photo credits: Field observations – Roberto Luciano; Visual soil evaluations – Roberto Luciano; Test kits – Susan Samson‐Liebig; Sensor‐based measurements – Keith Anderson.
Are there symptoms of nutrient deficiencies?
Are there increased problems with diseases or weeds?
Does the soil appear compacted?
Does it take more power to run field equipment through the soil?
Does the soil crust over easily?
Are there signs of runoff and erosion?
Are there changes in soil color?
On‐the‐ground observations of crop and soil conditions can be supplemented with aerial imagery to help identify potential production and environmental issues (Schepers et al., 2004). Doing so can efficiently guide follow‐up evaluations and potential management interventions in affected areas if image locations are georeferenced.
Descriptive soil health field assessments can be translated into semi‐quantitative formats using soil health scorecards. Developed in the early 1990s (Harris and Bezdicek, 1994), scorecards use stakeholder knowledge and field evaluations to identify relevant soil health indicators and assign an associated ranking as being healthy, impaired, or unhealthy (Romig et al., 1996). Scorecards rely on the evaluator’s senses (e.g., sight, feel, smell) and manager input to discern the quality or character of an attribute, making assessments unique to the individual.
Soil health scorecards and other semi‐quantitative evaluation approaches have been developed for many states in the USA following a collaborative process including farmers, conservationists, and scientists to identify relevant indicators and ranking criteria (USDA‐NRCS, 1999). Property‐specific pocket charts have also been developed based on associations between descriptive assessments and laboratory measurements (e.g, soil color categories with ranges of organic matter content) (Alexander, 1971). However, broad application of pocket charts for soil quality assessment has been limited because of the complexity and variability among soils. In the case of soil color, associations with soil organic matter content vary strongly with texture, soil depth, and land use (Wills et al., 2007), thereby constraining chart use to a limited geographical domain.
Visual Soil Evaluations
Visual soil evaluations are important components of soil health assessment. If done thoroughly and in a quantitative manner, visual soil evaluations can be integrated with broader land evaluation frameworks (Mueller et al., 2012; Pellant et al., 2020). As there are multiple soil evaluation methods currently in use (Ball and Munkholm, 2015), evaluators need to mindful of their intended application and respective strengths and weaknesses. The Visual Evaluation of Soil Structure (VESS) (Ball et al., 2017) and rangeland health assessment protocols (Pellant et al., 2020) represent approaches for cropland and rangeland, respectively.
The VESS was designed for arable production systems (Ball et al., 2017) based on the Peerlkamp Spade Test (Peerlkamp, 1959). It classifies top‐ and sub‐soil attributes into five scoring categories based on the size, shape, and visible porosity of soil aggregates, rooting characteristics, and presence/absence of macropores. VESS requires only a shovel and scoring guidelines (Scotland’s Rural College, 2019), so evaluators can quickly assess soil structure and assign a score between 1 (best) and 5 (worst). The tactile “hands‐on” nature of VESS enables it to work best when the soil is in a friable condition (neither too wet nor too dry). A limitation of VESS is that it does not work well in very sandy soils because of low structural cohesion (Franco et al., 2019).
The VESS has been used globally across a broad range of soil types and production systems to discern management impacts on soil health (Munkholm and Holden, 2015). The scores have also been correlated to various soil physical properties, soil‐atmosphere gas fluxes, and crop yield (Ball et al., 2017), thus confirming its value for soil health assessment.
Rangeland health assessments using three interrelated ecosystem attributes of soil/site stability, hydrological function, and biotic integrity (Pellant et al., 2020) have been shown to help characterize the status and trends of critical soil functions and effectively guide changes in management (Brown and Herrick, 2016). Using a combination of qualitative and quantitative indicators, Pellant et al. (2020) developed criteria for monitoring rangeland health using 17 indicators that include soil‐related evaluations of bare ground, gullies, resistance to erosion/degradation, and compaction. Rangeland health assessments using these criteria have been adopted by public agencies and private landowners throughout the world and have served to improve the systematic understanding of soil quality in rangeland ecosystems (Brown and Herrick, 2016). Effective use of rangeland health assessments, however, requires the evaluator to recognize and correctly identify site characteristics including landscape and temporal variability since evaluations are made relative to an ecological site or its equivalent.
Soil Health Test Kits
Soil test kits can provide land managers with expedient information on the status of soil physical, chemical, and biological properties. While many test kits focus on measurements of soil solution chemistry for horticultural applications, some test kits include a broad suite of measurements with linkages to soil functions relevant to maintaining productivity, regulating/partitioning water flow, and storing/cycling nutrients. The Soil Quality Test Kit (SQTK), developed by Dr. John Doran (USDA‐ARS, retired), includes equipment and supplies for the measurement of select soil properties recognized as components of a minimum data set for monitoring soil health (NRCS, 2001). In the intervening 25 years since its development, the SQTK has been used by NRCS resource soil scientists throughout the USA as a screening tool to guide more in‐depth soil health assessments. The SQTK also prompted the development of additional field‐based assessments for use in rangeland (e.g., soil slake test; Seybold and Herrick, 2001). While test kits provide a means for receiving near immediate feedback about a soil’s status, some tests can be time consuming. Moreover, measurements of soil solution chemistry by test kits require calibration against known standards to ensure accurate results.
Sensor‐based Measurements
Field‐scale soil property mapping, generally used to improve nutrient use efficiencies, can also document the trajectory of some soil health indicators (Mulla and Schepers, 1997; Smith et al., 1993). However, conventional soil sampling and laboratory analyses can be expensive, time consuming, and thus limit its value for making timely adjustments to management. In response, several novel sensor‐based technologies have been developed (Vol. 1, Chapter 8), thus increasing the likelihood of making real‐time soil health assessments.
Electromagnetic, optical, mechanical, electrochemical, airflow, and acoustic sensors for automated field measurements have been adapted to quantify changes in soil physical and chemical properties across agricultural landscapes (Adamchuk et al., 2004). Most sensors are property‐specific, with electromagnetic, mechanical, electrochemical, and airflow types associated with measurements of electrical conductivity, soil resistance, nutrients or pH, and air permeability, respectively. Optical sensors are useful for predicting both chemical and physical properties (Thomasson et al., 2001). Sensor‐based measurements can also be used to quantify
