Laboratory Methods

Methods and Method References – 9/2012

Utilization of soil fertility and other recommendations/use assessments from the Texas A&M AgriLife Extension Service requires strict compliance with the methods listed below. Alteration of sample preparation or method could result in skewed analytical results. The methods listed below are the identical research methods used in the development of the current recommendations and sample assessments.Soil Methods
All soil results expressed on a dry basis.

Preparation of soil samples

All soil samples should be removed from their initial shipping containers, placed in aluminum or other non-porous, non-corrodible shallow containers and oven dried at 65C (plus or minus 2C) in a forced air oven for 16 hours or until dry. Following oven drying, samples are pulverized using a open mesh bottom hammer style soil pulverizer (as a reference only, common manufacturers of these systems include Agvise, Dynacrusher and Humboldt). All soil exiting the pulverizer is screened to remove all particles greater than 2mm. It is vital that the pulverization step is not overly aggressive and break down small rocks or individual soil separates. The use of disk mills, ring and puck mills, mortar and pestles, cone crushers or other fixed opening mills are not appropriate for use when Texas A&M AgriLife Extension Service soil fertility recommendations are utilized.

Mehlich III (Phosphorus and multi-nutrient extractant)

Phosphorus, K, Ca, Mg, Na , S and B (Boron added to test in 2025 onward) are extracted using the Mehlich III extractant and are determined by ICP. The extractant is a dilute acid-fluoride-EDTA solution of pH 2.5 that consists of 0.2 N CH3-COOH-0.25 N NH4NO3-0.015 N NH4F-0.013 N HNO3-0.001 M EDTA. The method estimates plant available pools of the elements listed above and is currently the only method recognized by Texas AgriLife Extension Service. Reported on a dry soil basis only.

References Mehlich, A. 1978. New extractant for soil test evaluation of phosphorus, potassium, magnesium, calcium, sodium, manganese, and zinc. Commun. Soil Sci. Plant Anal. 9(6):477-492.

Mehlich, A. 1984. Mehlich-3 soil test extractant: a modification of Mehlich-2 extractant. Commun. Soil Sci. Plant Anal. 15(12):1409-1416.

Soil pH (referred to as soil water pH)

Soil pH is determined in a 1:2 soil:water extract of the soil using deionized water. Samples are stirred and allowed to equilibrate for a minimum of 30 minutes after adding the water. The actual determination is made using a hydrogen selective electrode. Reported on a dry soil basis only.

Reference

Schofield, R.K. and A.W. Taylor. 1955. The measurement of soil pH. Soil Sci. Soc. Am. Proc. 19:164-167.

Electrical Conductivity (measurement of soluble salts)

Soil electrical conductivity is determined in a 1:2 soil:water extract of the soil using deionized water. Samples are stirred and allowed to equilibrate for a minimum of 30 minutes after adding the water. The actual determination is made using a conductivity probe and reported in umho/cm. Reported on a dry soil basis only.

Reference

Rhoades, J.D. 1982. Soluble salts. p. 167-178. In: A.L. Page, et al. (ed.). Methods of Soil Analysis: Part 2. Agronomy Monogr. 9. 2nd ed. ASA and SSSA, Madison, WI.

NO3-N

Nitrate-nitrogen (NO3-N) is extracted from soils using a 1 N KCl solution. Nitrate is determined by reduction of nitrate (NO3-N) to nitrite (NO2-N) using a cadmium column followed by spectrophotometric measurement. Reported on a dry soil basis only.

Soil nitrate-N is determined using a 10:1 ratio of 1 or 2 M KCl to <2 mm pulverized soil (see soil prepartion section). A minimum of 2 grams of soil is either volumetrically or gravimetrically placed in an extraction cup or flask and orbitally (1 inch orbital throw) agitated at 200 opm for 5 minutes prior to filtration through a Whatman #2 equivalent filter. Analysis this then performed via Cd reduction.

Reference

Keeney, D.R. and D.W. Nelson. 1982. Nitrogen – inorganic forms. p. 643-687. In: A.L. Page, et al. (ed.). Methods of Soil Analysis: Part 2. Agronomy Monogr. 9. 2nd ed. ASA and SSSA, Madison, WI.

Kachurina, O.M., H. Zhang, W.R. Raun, and E.G. Krenzer. 2000. Simultaneous determination of soil aluminum, ammonium- and nitrate-nitrogen using 1 M potassium chloride extraction. Commun. Soil Sci. and Pl. Anal. 31: 893-903.

Micronutrients (DTPA extractable)

The micronutrients (Cu, Fe, Mn and Zn) are extracted using a 0.005 M DTPA, 0.01 M CaCl2 and 0.10 M triethanolamine solution. The analytes are determined by ICP. This method is the only method recognized by Texas A&M AgriLife Extension Service. for the determination of plant available soil Cu, Fe, Mn, and Zn. Reported on a dry soil basis only.

A minimum of 20 grams of soil is either volumetrically or gravimetrically placed in an extraction cup or flask with 40 mls of DTPA extractant and orbitally (1 inch orbital throw) agitated at 180 opm for 120 minutes prior to filtration through a Whatman #2 equivalent filter.

Reference

Lindsay, W.L. and W.A. Norvell. 1978. Development of a DTPA soil test for zinc, iron, manganese, and copper. Soil Sci. Soc. Amer. J. 42:421-428.

Boron (for samples received in 2024 and back, retired starting Jan. 2025) Boron is extracted from soils using a hot-water extraction. The B is determined by ICP. Reported on a dry soil basis only.

A thirty gram sample of pulverized soil is placed in a heavy duty sealable bag along with 60 mls of DI. water. The bag is then place, along with up to 5 additional bags, in a microwavable straight sided container containing 1 liter of preheated hot water (90-99C). Microwave soil slurries to insure that the slurries boil for at least 5 minutes. Remove from microwave and filter through a Whatman #2 equivalent filter prior to ICP analyses.

Reference C. A. de Abreu, M. F. de Abreu, B. van Raij, O. C. Bataglia and J. C. de Andrade, 1994. Extraction of boron from soil by microwave heating for ICP-AES determination. comm. Soil Sci and Plant Anal. 25: 3321-3333. Bingham, F.T. 1982. Boron. p. 431-446. In: A.L. Page, et al. (ed.). Methods of Soil Analysis: Part 2. Agronomy Monogr. 9. 2nd ed. ASA and SSSA, Madison, WI.

Detailed Salinity The detailed salinity analysis is performed by making a saturated paste of the soil sample with deionized water. The pH and conductivity are measured directly in the paste using a hydrogen ion selective electrode and conductivity probe. The water is extracted using a vacuum extractor. The levels of Ca, K, Mg and Na in the water are determined by ICP.

Reference Rhoades, J.D. and M. Clark. 1978. Sampling procedures and chemical methods in use at the U.S. Salinity Laboratory for characterizing Salt-affected soils and waters. U.S. Salinity Laboratory USDA, Riverside CA. p. 11-12. Reported on a dry soil basis only.

NH4-N Ammonium-nitrogen (NH4-N) is extracted from soils using a 1N KCl solution. Ammonium-N is determined spectrophotometrically. Reported on a dry soil basis only.

References

Dorich, R.A. and D.W. Nelson. 1983. Direct colorimetric measurement of ammonium in potassium chloride extracts of soil. Soil. Sci. Soc. Am. J. 47:833-836.

Markus, D.K., J.P. McKinnon, and A.F. Buccafuri. 1985. Automated analysis of nitrite, nitrate, and ammonium nitrogen in soils. Soil. Sci. Soc. Am. J. 49:1208-1215.

Orthophosphate analysis

Orthophosphate (ortho-P) is determined colorimetrically using the ascorbic acid/molybdate reagent referenced below. Ortho-P can be determined in various aqueous extracts such as Mehlich III or 0.01 M CaCl2 or directly in the case of water or runoff samples. Reported on a dry soil basis only.

Reference Murphy, J. and J.P. Riley. 1962. A modified single solution method for determination of phosphates in natural water. Anal. Chim. Acta 27:31-36.

Water Soluble Phosphorus

Water soluble phosphorus is extracted using a 0.01 M CaCl2 solution. The P can then be determined colorimetrically or by ICP. Reported on a dry soil basis only.

Reference Sel-Davis, M.L. P.A. Moore and B.C. Joern. 2000. Determination of water- and/or dilute salt-extractable phosphorus. Southern Cooperative Series Bulletin No. 396.

Organic Carbon/Inorganic Carbon/Total Nitrogen

Organic carbon, inorganic carbon and total nitrogen are determined by a combustion procedure. All soil samples analyzed for total organic carbon, total inorganic carbon and total nitrogen are ground to pass an 80 mesh screen. Reported on a dry soil basis only.

References

Soil, Water and Forage Testing Laboratory research. Organic carbon is determined directly using a total carbon analyzer by reducing the primary sample ignition furnace to 650C.

McGeehan, S.L. and D.V. Naylor. 1988. Automated instrumental analysis of carbon and nitrogen in plant and soil samples. Commun. Soil Sci. Plant Anal. 19:493

Schulte, E.E. and B.G. Hopkins. 1996. Estimation of soil organic matter by weight Loss-On-Ignition. p. 21-32. In: Soil Organic matter: Analysis and Interpretation. (ed.) F.R. Magdoff, M.A. Tabatabai and E.A. Hanlon, Jr. Special publication No. 46. Soil Sci. Soc. Amer. Madison, WI.

Storer, D.A. 1984. A simple high volume ashing procedure for determining soil organic matter. Commun. Soil Sci. Plant Anal. 15:759-772.

Texture Texture is determined using the hydrometer procedure. Reported on a dry soil basis only.

References

Day, P.R. 1965. Particle fractionation and particle-size analysis. p. 545-567. In: C.A. Black, et al. (ed.). Methods of Soil Analysis: Part 1. Agronomy Monogr. 9. ASA and SSSA, Madison, WI.

Murphy, J. and J.P. Riley. 1962. A modified single solution method for determination of phosphates in natural water. Anal. Chim. Acta 27:31-36.Plant Analysis

Total Nitrogen

Plant nitrogen (or protein) is determined by high temperature combustion process. Reported on a dry plant basis only.

References

Sheldrick, B.H. 1986. Test of the Leco CHN-600 Determinator for soil carbon and nitrogen analysis. Can. J. Soil Sci. 66(3):543-545

Sweeney, Rose A. 1989. Generic combustion method for determination of crude protein in feeds: Collaborative Study. J. Assoc. Off. Anal. Chem. 72: 770-774.

Nelson, D.W. and L.E. Sommers. 1973. Determination of total nitrogen in plant material. Agron. J. 65:109-112.

McGeehan, S.L. and D.V. Naylor. 1988. Automated instrumental analysis of carbon and nitrogen in plant and soil samples. Commun. Soil Sci. Plant Anal. 19:493

NO3-N

Nitrate is extracted from plant samples using a 1 N KCl solution. Nitrate is determined by reduction of nitrate (NO3-N) to nitrite (NO3-N) using a cadmium column followed by spectrophotometric measurement. Reported on a dry plant basis only.

References

Keeney, D.R. and D.W. Nelson. 1982. Nitrogen – inorganic forms. p. 643-687. In: A.L. Page, et al. (ed.). Methods of Soil Analysis: Part 2. Agronomy Monogr. 9. 2nd ed. ASA and SSSA, Madison, WI.

Plant Minerals (Excluding N and Cl)

Plant minerals (B, Ca, Cu, Fe, K, Mg, Mn, Na, P, S, and Zn) are determined by ICP analysis of a nitric acid digest. Reported on a dry plant basis only.

References

Isaac, R.A. and W.C. Johnson. 1975. Collaborative study of wet and dry ashing techniques for the elemental analysis of plant tissue by atomic absorption spectrophotometry. J. Assoc. Off. Anal. Chem. 58: 436-440.

Havlin, J.L. and P.N. Soltanpour. 1989. A nitric acid and plant digest method for use with inductively coupled plasma spectrometry. Commun. Soil Sci. Plant Anal. 14:969-980.Forage Analysis

Acid Detergent Fiber (ADF)-Neutral Detergent Fiber (NDF)

Fiber is determined gravimetrically following a liquid digestion. Reported on a dry plant basis only.

References

Komarek, A.R. 1993. An improved filtering technique for the analysis of neutral detergent and acid detergent fiber utilizing the filter bag technique. Publication #101. Ankom Company®, Fairport, NY 14450.

References

Havlin, J.L. and P.N. Soltanpour. 1989. A nitric acid and plant digest method for use with inductively coupled plasma spectrometry. Commun. Soil Sci. Plant Anal. 14:969-980.

McGeehan, S.L. and D.V. Naylor. 1988. Automated instrumental analysis of carbon and nitrogen in plant and soil samples. Commun. Soil Sci. Plant Anal. 19:493

Nelson, D.W. and L.E. Sommers. 1973. Determination of total nitrogen in plant material. Agron. J. 65:109-112.

Parkinson, J.A. and S.E. Allen. 1975. A wet oxidation procedure suitable for the determination of nitrogen and mineral nutrients in biological material. Commun. Soil Sci. Plant Anal. 6(1): 1-11.

Sheldrick, B.H. 1986. Test of the Leco CHN-600 Determinator for soil carbon and nitrogen analysis. Can. J. Soil Sci. 66(3):543-545

Sweeney, Rose A. 1989. Generic combustion method for determination of crude protein in feeds: Collaborative Study. J. Assoc. Off. Anal. Chem. 72: 770-774.Biosolids

Biosolids include such things as manure, litter, compost and effluent samples. These samples are typically analyzed for total nitrogen, organic carbon and total minerals. For solid samples, total nitrogen and organic carbon are determined by a combustion process. The minerals (B, Ca, Cu, Fe, K, Mg, Mn, Na, P, S, and Zn) are determined by ICP analysis of a nitric acid digest. For liquid samples, the samples are digested using a sulfuric acid digest (modified Kjeldahl). Total N is determined spectrophotometrically, and the minerals are determined by ICP. Manures and high solid content materials are reported on a dry basis together with the moisure content of the as-received sample. Fluid effluents and other biosolids are reported on a wet basis with moisture contents.

References

Havlin, J.L. and P.N. Soltanpour. 1989. A nitric acid and plant digest method for use with inductively coupled plasma spectrometry. Commun. Soil Sci. Plant Anal. 14:969-980.

McGeehan, S.L. and D.V. Naylor. 1988. Automated instrumental analysis of carbon and nitrogen in plant and soil samples. Commun. Soil Sci. Plant Anal. 19:493

Nelson, D.W. and L.E. Sommers. 1973. Determination of total nitrogen in plant material. Agron. J. 65:109-112.

Parkinson, J.A. and S.E. Allen. 1975. A wet oxidation procedure suitable for the determination of nitrogen and mineral nutrients in biological material. Commun. Soil Sci. Plant Anal. 6(1): 1-11.

Sheldrick, B.H. 1986. Test of the Leco CHN-600 Determinator for soil carbon and nitrogen analysis. Can. J. Soil Sci. 66(3):543-545

Sweeney, Rose A. 1989. Generic combustion method for determination of crude protein in feeds: Collaborative Study. J. Assoc. Off. Anal. Chem. 72: 770-774.Water

Water pH is measured directly using a hydrogen ion selective electrode.

Reference

Franson, M.A.H. (ed.). 1989. 4500-H+ pH Value. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C.

Conductivity

Water conductivity is measured directly using a conductivity probe.

Reference

Franson, M.A.H. (ed.). 1989. 2510 CONDUCTIVITY. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C.

NO3-N

Nitrate-N is measured directly by reduction of nitrite (NO2-N) to nitrate using a cadmium column followed by spectrophotometric measurement.

References

Franson, M.A.H. (ed.). 1989. 4500-NO3- NITROGEN (NITRATE). Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C.

Keeney, D.R. and D.W. Nelson. 1982. Nitrogen – inorganic forms. p. 643-687. In: A.L. Page, et al. (ed.). Methods of Soil Analysis: Part 2. Agronomy Monogr. 9. 2nd ed. ASA and SSSA, Madison, WI.

NH4-N

Ammonium-N is measured directly by spectrophotometric measurement.

Reference

Franson, M.A.H. (ed.). 1989. 4500-NH3 H. Automated Phenate Method. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C.

Chloride

Chloride is determined via ion chromotography.

References

Metrohm AG, Herisau, Switzerland.

J.D. Pfaff, C.A. Brockhoff and J.W.0’ DeH, The Determina- tion of Inorganic Anions in Water by Ion Chromatography. Method 300.0, 1991, U.S. Environmental Protection Agency, Environmental Monitoring Systems Lab., Cincinnati, Ohio, USA.

Metals by ICP

ICP metals (Ca, Mg, Na, K, B, S, P, Fe, Zn, Cu, Mn, As, Ba, Ni, Cd, Pb and Cr) are determined directly by ICP analysis.

Reference

Franson, M.A.H. (ed.). 1989. 3120 METALS BY PLASMA EMISSION SPECTROSCOPY. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C.

Carbonates/Bicarbonates/Hardness/Alkalinity

Carbonates/bicarbonates are determined by acid titration using sulfuric acid. Water alkalinity is calculated from the carbonate/bicarbonate concentrations. Hardness is calculated from the Ca and Mg concentrations.

References

Franson, M.A.H. (ed.). 1989. 2320 ALKALINITY. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C.

Franson, M.A.H. (ed.). 1989. 2340 HARDNESS. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C.

Fresenius, W., K.E. Quentin and W. Schneider (eds.) 1988. 3.2.9. Carbonic acid, hydrogen carbonate and carbonate. Water Analysis. Springer-Verlag Berlin Heidelberg.

Fluoride

Fluoride is determined directly using a fluoride ion selective electrode.

Reference

Franson, M.A.H. (ed.). 1989. 4500-F- FLUORIDE. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C.

Total Dissolved Salts and Charge Balance

Total dissolved salts (TDS) are calculated by summing the ppm anions and cations. The charge balance is calculated by dividing the sum of the cations by the sum of the anions.

Reference

Franson, M.A.H. (ed.). 1989. 1030 F. Checking Correctness of Analyses. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C.

Total Solids

Total solids are determined gravimetrically.

References

Franson, M.A.H. (ed.). 1989. 2540 B. Total Solids Dried at 103-105 ºC. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C.

Fresenius, W., K.E. Quentin and W. Schneider (eds.) 1988. 3.2.9. Carbonic acid, hydrogen carbonate and carbonate. Water Analysis. Springer-Verlag Berlin Heidelberg.