in sample collection across multiple sites, among different sampling crews, and over time (Boone et al., 1999). There is also less concern about biases associated with sampling depth provided the increments meet assessment objectives and accurately account for site characteristics and management attributes. Sampling by uniform depth increment also allows evaluators to know how many samples will be collected at the beginning of a project. Knowing this is beneficial for budgeting and for planning labor requirements (Boone et al., 1999).
One caveat associated with uniform sampling is the potential to miss important profile differences, especially if depth increments are large (e.g., ≥30 cm). Uniform sampling can also be problematic during near‐surface assessments if soil properties are strongly stratified with depth (Bowman and Halvorson, 1998). Failure to adjust sampling depth for near‐surface stratification can result in misleading management recommendations (Reeves and Liebig, 2016). Therefore, if near‐surface stratification is suspected, using small depth increments for the top 30 cm of the soil profile is recommended.
Conversely, a distinct advantage of uniform sampling is the opportunity to quantify soil bulk density. In addition to being a useful measure of soil physical condition, soil bulk density enables conversion of concentration data to a volumetric basis, thereby permitting expression of results on an area basis for a given depth increment (Dick et al., 1996). Soil bulk density values are also essential for calculating nutrient stocks using the “equivalent soil mass” method (Ellert and Bettany, 1995). This method accounts for differences in genetic horizon thickness and/or soil bulk density differences among treatments by calculating a standard soil mass before computing nutrient stocks.
Timing and Frequency of Sampling
Appropriate timing and frequency of sampling for soil health assessments will vary based on evaluator goals, indicators chosen, environmental conditions, management operations, and available resources. Evaluators must gauge tradeoffs associated with quantifying seasonal variability versus conducting an assessment at a single point in time. If done appropriately, the latter sampling option would select a time guided by knowledge of seasonal and/or annual variability and the timing of management operations. As shown by previous soil health evaluations, selecting an optimal sampling time is difficult (Wuest, 2015; Pikul et al., 2006; Mikha et al., 2006; Wienhold et al., 2006). Accordingly, sampling time decisions should be based by assessment objectives, recognizing that both management disturbances and environmental conditions can lead to misleading outcomes if not properly accounted for.
It is also important to recognize that due to temporal variability in every soil health indicator, the benefit to cost ratio will be low if sampling occurs too frequently. Annual sampling may be desirable for questions focusing on biological indicators, but for long‐term soil health changes, sampling every three to five years or at the end of a recurring period inherent to the production system (e.g., following a crop rotation or grazing cycle) will generally be sufficient. It is also important to collect samples at approximately the same time of year or under similar weather patterns to minimize variation in soil water content and temperature (Dick et al., 1996).
Sample Collection, Processing, and Archival
After deciding to implement a soil health assessment, it is important to carefully consider and document every aspect of the experimental design, sampling protocols, and how the samples will be handled. Each set of protocols is inherently project‐specific, underscoring the importance of thorough documentation for future reference. As a guideline for this chapter, protocols adapted from the USDA‐ARS GRACEnet (Greenhouse gas Reduction through Agricultural Carbon Enhancement network) project are listed below (Liebig et al, 2010). Additional guidelines can also be found in Boone et al. (1999) and Dick et al. (1996).
Sampling decisions will vary based on assessment objective(s), geographic location, investigator preferences, and/or agroecosystem attributes. For initial soil health samplings, extra care is warranted since those data will ultimately be referenced as baseline data against which long‐term changes in soil properties are measured. After selecting an appropriate sampling design, the best approach for sample collection in order to meet project goals and soil conditions should be determined and documented in the metadata. For example, mechanical coring devices (handheld or machine‐driven) will often be used because they permit rapid collection of soil samples with a uniform cross‐sectional area. However, for soils with high near‐surface sand content or excessive stones, a compliant cavity method may be a preferable approach (USDA‐NRCS, 2004).
After carefully collecting soil samples, they should be placed in labeled plastic bags, sealed, stored in coolers with ice packs, and transported promptly to a laboratory where they can be held in cold storage (5 °C) until processed. Thick‐gauge polyethylene or double bags may be required to limit moisture loss.
Processing protocols should minimize changes in soil properties. For biological attributes, storage time even at 5 °C should be minimized. If extended storage of biological samples is necessary, freezing at −20 °C is recommended over air‐drying (Sun et al., 2015). For chemical soil health indicators, samples can be air‐dried at 35 °C for 3 to 4 d before sieving to remove rocks, root fragments, and non‐soil material. Some soil physical indicators (e.g. bulk density) should be determined using non‐disturbed samples, while coarse (~8 mm) sieving can be used to prepare samples for aggregate stability analysis (Vol. 2, Chapters 4 and 5).
Though frequently overlooked, archiving of soil samples is critically important, especially for long‐term studies. Archived soil samples provide ‘time capsules’ for assessing temporal changes in soil properties and are particularly valuable as new analytical capabilities are developed (Boone et al., 1999). The amount of soil archived will vary by evaluator goals, available storage space, and projected future needs (e.g. research vs production‐scale monitoring), but in general several hundred grams of air‐dried soil should be archived from ‘time‐zero’ with additional amounts from key subsequent samplings. Archived soil samples should be kept in air‐tight, non‐reactive containers with secure lids and permanent labels. Samples should also be kept in a dry, secure location with moderate temperature conditions and a low probability of water or fire damage.
Field Evaluations
Field evaluations of soil health can often provide timely insights into soil condition (Fig. 2.2). They can affirm the efficacy of current and previous soil and crop management decisions (e.g., tillage intensity, rotations, manure application) and thus help guide management changes to better align with goals of the land manager. Field evaluations can also help discern the value of more intensive, costly follow‐up assessments, and may confirm (or refute) findings from previous laboratory analyses.
Soil‐related information gathered during a field evaluation is strongly influenced by the approach taken, and much like the selection of sampling designs, the evaluator should be aware of tradeoffs associated with the selected approach. Therefore, attributes of field evaluations and their capacity to meet stated objectives should be carefully considered by the land manager prior to initiating assessments.
General Field Observations
Field‐scale soil health assessments should begin with general field observations such as aboveground biomass, plant growth characteristics and soil conditions. Since these observations are generally part of normal field management practices, they are a logical first step during soil health assessments to determine if more detailed, follow‐up evaluations are warranted. Moreover, much of this information may be obtained through conversations with the land manager. Common field observations outlined by Magdoff and Van Es (2009) include:
