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).
Table 1.2 Tier 2 Soil Health Indicators and Methods to be Assessed
| Indicator | Method | Reference |
|---|---|---|
| Sodium Adsorption Ratio (SAR) | Saturated paste extract followed by atomic absorption or inductively coupled plasma spectroscopy | Miller et al., 2013 |
| Soil Stability Index | Combination of wet and dry sieving at multiple sieve sizes | Franzluebbers et al., 2000 |
| Active Carbon | Permanganate oxidizable carbon (POXC). Digestion followed by colorimetric measurement | Weil et al., 2003 |
| Soil Protein Index | Autoclaved citrate extractable |
Schindelbeck
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