Analytical Methods

Soil analysis can provide important information about physical conditions, fertility (nutrient) status, and chemical properties that affect a soil’s quality as well as suitability for growing plants.

Four steps associated with soil testing include: 

  • soil sample collection
  • laboratory analysis
  • interpretation of results
  • potential management recommendations

Soil Sample Collection

 The first step in soil analysis is soil sample collection. It’s important to realize that only a tiny portion of a field is actually analyzed in the laboratory. Thus, collecting a representative soil sample is critical for accurate results. An accurate and reliable analysis of soil samples (physical, chemical, biochemical, microbiological) presupposses a well designed and executed sampling strategy. If sample collection technique is poor, if samples are not representative, or if sample handling is careless, then the reliability of the data will be questionable and any decisions or conclusions based on the data will be suspect.

Soil samples should be immediately air-dried at room temperature for two to three days and should not be heated or dried in an oven. If samples cannot be dried immediately, they can be refrigerated for several days and taken to a laboratory as soon as possible. The primary consideration for timing of soil sample collection is convenience. Collect samples early enough to allow for interpretation and soil management adjustments. Status of some soil nutrients can change quickly, whereas others do not. For example, phosphorus levels in soil are unlikely to change rapidly and frequent testing is unnecessary. Nitrogen levels, on the other hand, change very quickly and only very recent tests will reflect current plant-available levels.

ISO methods that ensure the collection of representative soil samples are:

ISO 10381-1:2002, which sets out the general principles to be applied in the design of sampling programmes for the purpose of characterizing and controlling soil quality and identifying sources and effects of contamination of soil and related material, with emphasis on

procedures required to locate points from which samples may be taken for examination or at which instruments may be installed for in situ measurement including statistical implications, 

  • procedures for determining how much sample to collect and whether to combine samples, 
  • methods of collecting samples, 
  • methods for containing, storing and transporting samples to prevent deterioration - contamination.

ISO 10381-2:2002 gives information on typical equipment that is applicable in particular sampling situations to enable correct sampling procedures to be carried out and representative samples to be collected. Guidance is given on the selection of the equipment and the techniques to use to enable both disturbed and undisturbed samples to be correctly taken at different depths. The guidance provided is intended to assist in the collection of samples for soil quality for agricultural purposes and also provide guidance for the collection of samples for contamination investigations which will require different techniques and skills. It also gives guidance on techniques for taking and storing soil samples so that these can subsequently be examined for the purpose of providing information on soil quality and makes reference to some aspects of the collection of samples of groundwater and soil gas as part of a soil sampling programme.

ISO 10381-2:2002 is not applicable to the sampling of hard strata such as bedrock. Techniques to collect information on soil quality without taking samples, such as geophysical methods, are not covered by this ISO. It also specifically does not cover investigations for geotechnical purposes, though where redevelopment of a site is envisaged the soil quality investigation and the geotechnical investigation may be beneficially combined. ISO 10381-3:2001 gives guidance on safety during sampling. Together with the application of the above Standard Methods, ISO 19258:2005 method regarding the determination of background values is also recommended. The method gives guidance on the principles and main methods for the determination of pedo-geochemical background values and background  values for inorganic and organic substances in soils; on strategies for sampling and data processing and identifies methods for sampling and analysis.

After collection, soil samples are transferred lob for physical, chemical, micorbioloigical and biochemical analysis. Measures should be taken for the conservation of samples qualiy up to their transport to the laboratory

Soil Physical Analysis

Soil physical analysis generally includes the identification of:

Soil Chemical Analysis 

 Measurements, which involve characterization of the soil solution and its constituents and of the composition of the inorganic phases in soils, are broadly termed chemical. A typical chemical analysis involves the identfication o the following parameters :

The above parameters are commonly measured by all soil laboratories and are typical for the identification of soil quality and fertility. However, other parameters are also significant but used mainly for the characterization of a polluted soil system and for other specific purposes. These could be :

The available method for the determination of total elements content is contained in the ISO14869-2:2002 which specifies a method for the dissolution by alkaline fusion of total contents for Na, K, Mg, Ca, Ti, Mn, Fe, Al and Si, however this list is not exhaustive, and other elements are applicable for determination  

Soil Biochemical Analysis

  • Soil dehydrogenase activity is determined using 1 g of soil, and the reduction of piodonitrotetrazolium chloride (INT) to p-iodonitrotetrazolium formazan is measured by a modification of the method reported by Von Mersi and Schinner (1991) and modified by García et al. (1993). Soil DHA is expressed as μg INTF g-1 soil h-1.
  • Soil urease activity is determined by the buffered method of Kandeler and Gerber (1988). In this procedure, 0.5 ml of a solution of 0.48% urea and 4 ml of borate buffer at pH 10 are added to 1 g of soil in hermetically sealed flasks, and then incubated for 2 hours at 37º C. The ammonium content of the centrifuged extracts is determined using a colorimetric method. Controls are prepared without substrate to determine the ammonium produced in the absence of added urea.
  • Alkaline phosphatase and β-glucosidase activities are determined following the methods reported by Tabatabai & Bremner (1969) and Eivazi & Tabatabai (1988), respectively, adding to 0.5 g of soil 2 ml of MUB (modified universal buffer) pH 11 and 0.5 ml of 0.025 M p-nitrophenyl phosphate for APA assay, or 2 ml of MUB pH 6 and 0.5 ml of 0.025M pnitrophenyl β-D-glucopiranoside for β-GA assay. Then the mixtures are incubated at 37º C for 1 hour, after which the enzymatic reactions are stopped by cooling on ice for 15 min. Then, 0.5 ml of 0.5 M CaCl2 and 2 ml of 0.5 M NaOH, for alkaline phosphatase activity assay or 2 ml of 0.1 M Tris(hydroxymethyl)aminomethane-sodium hydroxide (THAM-NaOH) pH 12 for β-glucosidase activity assay, are added. In the control, the respective substrates were added after the addition of CaCl2 and NaOH.

Soil Biological Analysis

  • Microbial biomass C is generally determined by the fumigation-extraction method. Ten grams of sample are fumigated with chloroform and another 10 g are fumigated. Carbon are extracted with 40 ml of 0.5 M K2SO4 solution from fumigated and non-fumigated samples and measured in the centrifuged and filtered extract using a soluble-organic C analyzer (Shimadzu TOC-5050A). Microbial biomass C (Cmic) is calculated by the expression: Cmic=Cextr x 2.66, where Cextr is the difference between the C extracted from fumigated samples and C extracted from non-fumigated samples.
  •  Soil basal respiration is analyzed by placing 50 g of soil moistened at 30-40% of its water-holding capacity (water potential: -0.055 Mpa) in hermetically sealed flasks and incubating for 31 days at 28º C. The CO2 evolved is periodically measured, every day for the first 4 days and then weekly, using an infrared gas analyzer (Toray PG-100, Toray Engineering Co. Ltd., Japan). The data are summed to give a cumulative amount of CO2 evolved after 31 days of incubation, and basal soil respiration is expressed as mg CO2–C kg-1 soil per day.
  •  Adenosine triphosphate (ATP) is extracted from soil using the Webster et al. (1984) procedure and measured as recommended by Ciardi and Nannipieri (1990). Twenty milliliters of a phosphoric acid extractant is added to 1 g of soil, and the closed flask is shaken in a cool bath. Then the mixture is filtered through ash-less cellulose filter and an aliquot is used to measure the ATP content by the luciferin-luciferase assay in a luminometer (Optocomp 1, MGM Instruments, Inc.)

ISO 16072:2002 Soil quality - Laboratory methods for determination of microbial soil respiration

 

Toxicity Tests

  • Phytotoxicity assay by testing seed germination (Barley and rye-grass). Soil and water extracts are prepared according to the standard procedure developed by Zucconi et al. (1981), i.e., 40 ml of water are added to 4 g of samples and stirred mechanically for 60 min, centrifuged and filtered. Ten seeds of barley (Hordeum vulgare) and 15 of rye-grass (Lolium perenne) are placed on a layer of filter paper in 10 cm Petri dishes and two ml of extract are added. Distilled water is used as a control. Seed germination and root length in each plate are measured after 72 h incubation in the darkness at 28 ºC. Five replicates are made for every treatment. The germination index (GI) after exposure to the extracts was calculated as follows. GI (%) = G(%) x L(%) where G(%) and L(%) are, respectively,  the germination and root elongation percentages relative to the control seeds.
  •  Ecotoxicity assayby usingluminiscent bacterium (Photobacterium phosphoreum). The aim of this assay is the determination of the toxicity of samples through the luminescent activity of these bacteria. The decrease of light emission of these bacteria is proportional to the sample toxicity. The sample is extracted with distilled water by mechanical stirring for 24 hours. After centrifugation and filtration of extract, the pH is adjusted between 6 and 8 and salted with NaCl2 until concentration of 2%. Then the reconstituted foto-bacteria are added to the extract and incubated for 15 min at 15ºC. Control samples with 200 ml of foto-bacteria suspension and blank sample with 200 ml of NaCl2are prepared. After incubation the samples are measured in the luminometer.

Literature

  • Amoozegar, A. and Warrick, A.W. Hydraulic conductivity of saturated soils :field methods. In: In Methods of soil Analysis-Part 1: Physical and Mineralogical methods. Klute A. (Ed.). American Society of Agronomy, Inc., Soil Science Society of America, Inc., Madison, Wisconsin, USA, pp. 735-770. 
  • Bingham, F.T. Boron. In: Methods of Soil Analysis-Part 2: Chemical and Microbiological Properties. Page, A.L., Miller, R.H., and Keeney, D.R. (Eds.), American Society of Agronomy, Inc., Soil Science Society of America, Inc., Madison, Wisconsin, USA. pp.443-447.
  • Bruce, R.R., and Luxmoore, R.J. 1990. Water retention: Field methods. In: In Methods of soil Analysis-Part 1: Physical and Mineralogical methods. Klute A. (Ed.). American Society of Agronomy, Inc., Soil Science Society of America, Inc., Madison, Wisconsin, USA, pp. 663-686.
  • Danielson, R.E., and Sutherland, P.L. 1990. Porosity. In : In Methods of soil Analysis-Part 1: Physical and Mineralogical methods. Klute A. (Ed.). American Society of Agronomy, Inc., Soil Science Society of America, Inc., Madison, Wisconsin, USA, pp. 443-461.
  • Ciardi, C., Nannipieri, P., 1990. A comparison of methods for measuring ATP in soil. Soil Biol. Biochem. 22, 725–727.
  • Eivazi F, Tabatabai MA (1988). Glucosidases and galactosidases in soils. Soil Biol. Biochem. 20: 601-606.
  • Garcia C., Hernandez T., Costa C., Ceccanti B. and Ganni A. 1993. Hydrolases in the organic matter fractions of sewage sludge: changes with composting. Biores.Technol 45, 47-52.
  • Green, R.E., Ahuja, L.R., and Chong, S.K. 1990. Hydraulic conductivity, diffusivity, and sorptivity of unsaturated soils:field methods. In : In Methods of soil Analysis-Part 1: Physical and Mineralogical methods. Klute A. (Ed.). American Society of Agronomy, Inc., Soil Science Society of America, Inc., Madison, Wisconsin, USA, pp. 771-798.
  • Kemper, W.D., and Rosenau, R.C. 1990. Aggregate stability and size distribution. In : In Methods of soil Analysis-Part 1: Physical and Mineralogical methods. Klute A. (Ed.). American Society of Agronomy, Inc., Soil Science Society of America, Inc., Madison, Wisconsin, USA, pp. 425-442. 30.
  • Klute, A., and Dirksen, C. 1990. Hydraulic conductivity and diffusivity:Laboratory methods. In: In Methods of soil Analysis-Part 1: Physical and Mineralogical methods. Klute A. (Ed.). American Society of Agronomy, Inc., Soil Science Society of America, Inc., Madison, Wisconsin, USA, pp. 687-734.
  • Page, A.L., Miller, R.H., and Keeney, D.R.,1982. American Society of Agronomy, Inc., Soil Science Society of America, Inc., Madison, Wisconsin, USA.
  • Rhoades, J.D. 1982. Soluble salts. In : Methods of Soil Analysis-Part 2: Chemical and Microbiological Properties. Page, A.L., Miller, R.H., and Keeney, D.R. (Eds.), American Society of Agronomy, Inc., Soil Science Society of America, Inc., Madison, Wisconsin, USA. pp. 167-179.
  • Tabatabai, M.A., Bremner, J.M. 1969. Use of p-nitrophenyl phosphate for assay of soil phosphatase activity. Soil Biol Biochem., 1, 301-307.
  • Taylor, S.A., and Jackson, R.D. 1990, Temperature. In Methods of soil Analysis-Part 1: Physical and Mineralogical methods. Klute A. (Ed.). American Society of Agronomy, Inc., Soil Science Society of America, Inc., Madison, Wisconsin, USA : pp. 927-968
  • Von Mersi W., Schinner, F. 1992. An improved and accurate method for determining dehydrogenase activity of soil with iodonitrotetrazolium chloride. Biol. Fertil. Soils, 11, 216-220
  • Zucconi, F., M. Forte, A. Monac and M. de Beritodi, 1981. Biological evaluation of compost maturity. Biocycle, 22: 27-29.

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