Читать книгу Laboratory Methods for Soil Health Analysis, Volume 2 - Группа авторов - Страница 16

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).

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 et al., 2016
B‐Glucosidase	Assay incubation followed by colorimetric measurement	Tabatabai, 1994
N‐acetyl‐B‐D‐glucosaminidase	Assay incubation followed by colorimetric measurement	Deng and Popova, 2011
Phosphomonoesterase	Assay incubation followed by colorimetric measurement	Acosta‐Martínez and Tabatabai, 2011
Arylsulfatase	Assay incubation followed by colorimetric measurement	Klose et al., 2011
Phospholipid Fatty Acid (PLFA)	Bligh‐Dyer extractant, solid phase extraction, transesterification; gas chromatography	Buyer and Sasser, 2012
Genomics	18S, 16S or ITS analysis or a combination of 16S and 18S/ITS and/or shotgun metagenomics	Earth Microbiome Project 500; Marotz et al., 2017 Thompson et al., 2017
Reflectance	Diffuse reflectance spectroscopy	Veum et al., 2015

Several layers of certifications or validations may be in place to ensure data quality for commercial analytical labs. For example, the Agricultural Laboratory Proficiency (ALP) Program provides soil samples to laboratories for analysis and then assesses that laboratory’s results relative to the test soil’s known properties. In turn, the ALP program is accredited by the American National Standards Institute/American Society of Quality Control National Accrediting Board (ANAB) through testing of ISO/IEC 17043, itself an international standard of the International Organization for Standardization and the International Electrotechnical Commission for laboratory proficiency testing that determines the performance of individual laboratories for specific tests or measurements and is used to monitor laboratories’ continuing performance through interlaboratory comparisons. Thus, commercial laboratories conducting analyses for soil health indicators select measurements and indicators that are understood in the research community to relate to processes in soil important to soil health; and methods that are affordable to the farming community, as well as verifiable for accuracy and precision (= reliability to the producer) by independent tests.