offer researchers and others who need scientifically justifiable procedures a good selection for current use, comparison, testing in different locations and agricultural production conditions, and further refinement.
Measurements and methods in Tables 1.1 and 1.2 are the subjects of ongoing research being conducted by the SHI with university, government, and private‐sector partners with funding (2017–2020) from the Foundation for Food and Agriculture Research, General Mills, The Samuel Roberts Noble Foundation, and matching‐fund sources. The indicators under investigation by NRCS are a subset of those being evaluated by SHI, and both organizations coordinated to use the same methods for those specific indicators.
What Do Commercial Analytical Laboratories Need?
The primary interest of researchers usually is a level of accuracy, precision, and explanatory linkage to processes occurring in soil, so that results can be used to explain and predict soil health in a way that leads to new ways of managing the soil resource. In most cases, the limits on accuracy and precision, and the QA/QC procedures to ensure desired data quality and curation, are specified by the individual researcher as needed for the goals of the research and as constrained by the research budget.
Analytical laboratories that measure soil properties for a fee are also concerned with accuracy and precision that reflect the reliability and reputation of their service. Relationships between measurements and soil processes elucidated in research laboratories underlie a service lab’s analytical offerings, but in most cases, such relationships have been worked out by the research community. Although cost is certainly a consideration in a research budget, a service lab must offer analyses in a consistent, cost‐effective, and competitive way to remain in business. Selection of specific methods often relies on recommendations from researchers at universities located within the general region from which a service lab draws customers; such methods are most likely to yield reliable results for the region in which they were developed.
Table 1.1 Tier 1 Soil Health Indicators and Methods to be Assessed.
| Indicator | Method | Reference |
|---|---|---|
| Soil pH | 1:2 soil:water, standard pH electrode system | Thomas, 1996 |
| Soil Electrical Conductivity (EC) | 1:2 soil:water, standard electrical conductivity meter system | Rhoades, 1996 |
| Cation Exchange Capacity (CEC) | Sum of cations: Soil pH ≥ 7.2: use ammonium acetate extractant; Soil pH < 7.2: use Mehlich 3 extractant | Knudsen et al., 1982 Sikora and Moore, 2014 |
| % Base Saturation (BS) | Calculation: For soil pH ≥ 7.2: use ammonium acetate extractant; for soil pH < 7.2: use Mehlich 3 extractant | Knudsen et al., 1982 Sikora and Moore, 2014 |
| Extractable Phosphorus | Soil pH ≥ 7.2: use sodium bicarbonate extractant; Soil pH < 7.2: use Mehlich 3 extractant | Olsen and Sommers, 1982 Sikora and Moore, 2014 |
| Extractable Potassium, Calcium, Magnesium, Sodium | pH ≥ 7.2: use ammonium acetate extractant; Soil pH < 7.2: use Mehlich 3 extractant | Knudsen et al., 1982 Sikora and Moore, 2014 |
| Extractable Iron, Zinc, Manganese, Copper | DTPA extractant derivatives | Lindsay and Norvell, 1978 |
| Total Nitrogen | Dry combustion | Nelson and Sommers, 1996 |
| Soil Organic Carbon (SOC) | Dry combustion; corrected for inorganic C, if present, using pressure calcimeter | Nelson and Sommers, 1996 Sherrod et al., 2002 |
| Soil Texture | Pipette method with a minimum of 3 size classes. Weight/volume measurements | Gee and Bauder, 1986 |
| Aggregate Stability | Wet sieve procedure. Weight measurement Water slaking image recognition | Kemper and Roseneau, 1986 Mikha and Rice, 2004 Fajardo et al., 2016 |
| Available Water Holding Capacity | Ceramic plate method measured at –33 kPa (–10 kPa for sandy soils) and –1500 kPa | Klute, 1986 |
| Bulk Density (BD) | Core method: diameter to be determined, (most likely 2‐inch or 5.08 cm) | Blake and Hartge, 1986 |
| Saturated Hydraulic Conductivity | Two‐ponding head method in field with Saturo | Reynolds and Elrick, 1990 |
| Crop Yield | Obtained from historical and current plot yield data provided by site manager | |
| Short‐Term Carbon Mineralization | 4‐d incubation followed by CO2–C evolution and capture at 50% water‐filled pore space. | Zibilske, 1994 |
| Potentially Mineralizable Nitrogen | Short‐term anaerobic incubation with ammonium and nitrate measured colorimetrically pre‐ and post‐incubation | Bundy and Meisinger, 1994 |
Service labs must maintain consistent quality of data if they are to remain in business. A farmer must have confidence that analyses conducted in different years or on different parts of the farm reflect real properties of the soil, and if changes in a measurement are occurring, that these really do reflect changes in soil on the farm. Service labs may strive to achieve this reliability through associations with organizations that provide independent testing and verification of laboratory results.
One example of laboratory validation is offered through the North American Proficiency Testing (NAPT) Program delivered by the Soil Science Society of America. The NAPT program supports soil, plant, and water testing laboratories by providing interlaboratory sample exchanges and statistical analyses of data. American and Canadian experts from scientific organizations, state (U.S.) and provincial (Canada) departments of agriculture, regional working groups, and public and private analytical labs provide organization and oversight (SSSA, 2020).
