December 19, 2014

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Soil Analysis and Methods

See also:

Analyses

We perform the following analyses (follow the links for a brief description of the methodology for that procedure)*

*Please note that Test Codes S03 through S33 are done on a weighed soil basis, while the tests done under Test Code S40 are done using a soil volume measurement (scoop) rather than a measured weight of soil. The tests under Test Code S40 are the same as the Soil Testing Laboratory’s Routine tests. At this time we cannot offer organic determinations (i.e. pesticide analyses). 

Methodology

Sample drying and grinding (Ref. No. 1)

Moist samples are dried under forced air at room temperature, followed by crushing and sieving through a 2.0 mm stainless steel sieve.  Samples for ammonium determinations are stored frozen or refrigerated and are not dried before the analysis.

Nitrate and Nitrite (Extraction: Ref. No. 2 & 3, Analysis: Ref. No. 4 & 5)

Nitrate and nitrite are extracted by shaking 2 g of air dried soil in 30 mL 0.01 M CaSO4for 15 minutes followed by filtration. The nitrate in the filtrate is measured on a Lachat Quikchem 8500 Flow Injection Analyzer. In this method, the nitrate is reduced to nitrite in a copperized cadmium column. The nitrite then reacts with sulfanilamide under acidic conditions to form a diazo compound. This in turn couples with N-1-Napthylethylenediamine dihydrochloride to form a reddish purple azo dye, which is measured colorimetrically at 520 nm. Nitrite is determined by the same process but without using the copperized cadmium column.

Ammonium (Extraction: Ref. No. 6, Analysis: Ref. No. 7)

Ammonium is extracted by shaking 2 g of moist soil with 30 mL of 2 M KCl for 30 min. The extract is analyzed on a Lachat QuikChem 8500 Flow Injectin Analyzer. In this method, the ammonium reacts with salicylate in the presence of  nitroprusside (catalyst) to form an emerald green complex, which is measured colorimetrically at 660 nm. [Nitrate and nitrite can also be determined from the 2 M KCl extract.]

NOTE: Extraction of moist and frozen samples by manual crushing and sieving is done to prevents changes in levels of ammonium and sometimes nitrate that occur upon drying. The higher the clay content, the more difficult it is to crush and sieve wet, frozen samples. It is recommended that such samples be broken up and mixed as much as possible before freezing. Moisture content also needs to be determined on these samples in order to express the results on a dry weight basis.

Total Nitrogen – Dumas Method (Ref. No. 62 and Ref. No. 63)

This technique uses a LECO FP-528 Nitrogen Analyzer to determine total N in soil materials. A 250-300 mg sample is weighed into a capsule and dropped into an 850o C furnace purged with O2 gas. The combustion products of CO2, H2O and NOx are filtered, cooled by a thermoelectric cooler to condense most of the water, and collected in a large ballast. A 3 cc aliquot of the ballast combustion products is integrated into a He carrier stream and passed through: 1.) a hot copper column where the O2 is removed and the NOx gasses are converted to N2 and 2.) a reagent tube which scrubs the CO2 and remaining H2O from the stream. The N2content is then measured by a thermal conductivity cell against a He background and the result displayed as weight percentage of nitrogen.

Organic Matter (Ref. No. 9)

A 5 gram NCR-13 volumetric scoop of soil is placed into a tared, Kimax beaker, and is dried for 2 h at 105 degrees Celsius and weighed. The sample is then ashed for 2 h at 360 degrees Celsius and reweighed. The resulting loss of weight, as a percentage of the dry soil, is the estimate of organic matter content.

Organic Carbon (Ref. No. 10)

a: Noncalcareous samples: Determined by dry combustion at 900°F and subsequent measurement of CO2evolution by using a Elementar, Inc. VarioMAX C/N Analyzer.

b: Calcareous samples: Determination of Total Carbon by method above and then determination of Organic Carbon by analysis of a subsample which was fumigated with hydrochloric acid for 16 hours in order to remove carbonates.

pH (Ref. No. 11)

Soil pH is determined on a 1:1 (V/V) soil/water mixture composed of a 10 gram NCR-13 volumetric soil scoop and 10 mL double-deionized water. Samples are stirred both before and after a 15 minute equilibration period. pH is measured on a Mettler Toledo Seven-Multi pH meter with an InLab Routine Pro combination electrode, calibrated to pH buffers 4, 7, and 10.

Carbonate, Total (Ref. No. 12)

Carbonates are determined by difference, with an untreated sample analyzed for Total C (see Organic Carbon), followed by Total C determination on a sub-sample which has been fumigated with concentrated hydrochloric acid to remove carbonates. 

Chloride (Extraction: Ref. No. 13, Analysis: Ref. No. 14)

Chloride is extracted by shaking 10 g of air dry soil and 25 mg of charcoal in 30 mL of 0.01 M CaSO4 for 30 minutes. Samples are subsequently centrifuged or filtered and analyzed on a Lachat QuikChem 8500 Flow Injection Analyzer by the mercury (II) thiocyanate method. In this method, chloride sequesters mercury from mercuric thiocyanate thereby liberating thiocyanate ion. The thiocyanate ion forms reddish yellow colored ferric thiocyanate in the presence of excess ferric nitrate, which is measured colorimetrically at 480 nm.

Saturated Extract, Electrical Conductance (Ref. No. 15 & 16)

A saturated soil paste is made by combining deionized water with approximately a 50cc volume of soil. After intermittent stirring and equilibration for 2 hours, the free water is extracted by filtration under a vacuum. Electrical conductance of the filtrate is determined using the Accumet Model 30 conductivity meter, and is expressed as millimhos/cm.

Exchangeable Sodium Percentage, ESP, and/or Sodium Adsorption Ratio, SAR (Ref. No. 17)

Exchangeable sodium percentage (ESP) is used in the classification of saline and alkali soils or to determine if a sodium hazard exists. This method requires the determination of CEC and percent exchangeable sodium (ESP). If soluble sodium is present, the exchangeable value is corrected by analysis of Na in a saturated extract.1 SAR, or sodium adsorption ratio is used to estimate or predict ESP by analyzing for Na, Ca, and Mg in a water saturated soil extract.2SAR is commonly determined on irrigation water to classify the sodium hazard.

Boron, Hot Water Extractable (Extraction: Ref. No. 19 & 20, Analysis: Ref No. 21)

Boron is extracted by boiling under reflux 10 g of air dry soil with 20mL of 0.01M CaCl2 for 5 min. in a fiber digestion apparatus. After slight cooling, the sample is filtered and the filtrate is acidified and analyzed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES)

Sulfur, Total (Ref. No. 10)

A 0.150-0.300 g sample of air dried soil is mixed with Com-Cat Accelerator (LECO trade name for tungsten oxide compound) and combusted at 1350°C with measurement of evolved sulfur dioxide by infrared absorption on a LECO S144-DR Sulfur Analyzer.

Sulfate, Extractable (Ref. No. 23)

Sulfate is extracted by shaking 10 g of air dried soil in 25 mL of 0.008 M Ca(H2PO4)2.H2O for 30 minutes. After adding 0.10 g of activated carbon and shaking for an additional 3 minutes, the mixture is filtered. (Double the solution to soil ratio for peats and mucks.) The filtrate is analyzed on a Lachat QuikChem 8500 Flow Injection Analyzer BaCl is included to cause extracted sulfate to produce a BaSO4 precipitate, which is measured turbidimetrically at 420 nm.

Phosphorus, Bray-1 Extractable, for non-calcareous soils (Ref. No. 24)

Phosphorus is extracted by shaking 1 g of air dried soil in 10 mL of 0.025 M HCl and 0.03 M NH4F for 5 minutes. Phosphorus is determined on the filtrate by the molybdate-blue method using ascorbic acid as a reductant. Color development is measured at 880 nm on a Brinkmann PC 900 probe colorimeter.

Phosphorus, Olsen Bicarbonate Extractable, for calcareous soils (Ref. No. 24)

Phosphorus is extracted by shaking 1 g of air dried soil in 20 ml of 0.5 M NaHCO3, pH 8.5, for 30 minutes. Phosphorus is determined on the filtrate by the molybdate-blue method using ascorbic acid as a reductant. Color development is measured at 900 nm on a Brinkmann PC 900D probe colorimeter.

Total Phosphorus (Ref. No. 25)

Total P Microwave Procedure: digest 0.5 g of air dried soil with 10 mL of HNO3 in a 50 mL quartz vessel using microwave digestion for 6.5 minutes at 175oC. Determination of P, K, Na, Ca, Mg, Al, B, Fe, Mn, Cu, Zn, Cd, Ca, Ni, Pb, Co, Mo, Si, S, As, Ti, Be, Sr, Rb, Li, V, and Ba by ICP-AES.

Cation Exchange Capacity, CEC, by Direct Method (Extraction: Ref. No. 26, Analysis: Ref. No. 27)

CEC is determined when 3 g of air dried soil (1 g peat) is leached with 60 mL 1 M NH4OAc, pH 7, to saturate exchange sites with ammonium ions. Excess free ammonium ions are rinsed from the soil with isopropyl alcohol. The remaining ammonium ions held on cation exchange sites are replaced by leaching the soil with successive aliquots of a solution of 10% KCl acidified to 0.005 N HCl. Ammonium is determined on the KCl leachate by colorimetry on a Lachat QuikChem 8500 Flow Injection Analyzer using the salicylate/nitroprusside method.

Cation Exchange Capacity, CEC, by Summation Method (Extraction: Ref. No. 28, Analysis: Ref. No. 21)

CEC is determined by the summation of exchangeable Ca, Mg, Na, K, and H+. Base cations are extracted by leaching 3 g air dried soil (1 g for peat) with successive aliquots of 1 M NH4OAc, pH 7, to total 60 mL. The concentrations of the base cations in the leachate are determined by ICP-AES. The CEC is calculated from the sum of the base cations and exchangeable H+(see below). Includes calculation for percent base saturation.

Exchangeable Ca, K, Mg, Na (and Mn) by Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES) (Extraction: Ref. No. 30, Analysis: Ref. No. 21)

Base cations are extracted by shaking 3 g of air dried soil in 30mL of 1 M NH4OAc (pH 7) for 30 min. Extracts are centrifuged, and the supernatant is decanted and analyzed by ICP-AES.

Available Potassium, by Atomic Absorption Spectrometer (Extraction (Ref. No. 65)

Available Potassium is extracted by shaking 1 gram of air dried soil in 10mL of 1 M NH4OAc for 5 min. The sample is then filtered, and the filtrate is analyzed by atomic emission in a Perkin Elmer Analyst 100 spectrometer.

Extractable Metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn), DTPA (Extraction: Ref. No. 31, Analysis: Ref. No. 21)

These metals are extracted by shaking 10 g of air dried soil for 2 hours in 20 mL of 0.005MDTPA (diethylenetriaminepentaacetic acid). The filtrate is analyzed for Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn by ICP-AES. A soil to solution ratio of 1:5 or 1:10 is used for peats and mucks.

Extractable Metals, 1 M HNO3 (Extraction Ref. No. 32, Analysis: Ref. No.21)

Metals are extracted by shaking 3 g of air dried soil in 30 mL of 1 N HNO3for 1 hour. After centrifugation, the supernatant analyzed for Al, Cd, Cr , Cu, Fe, Mn, Ni, Pb, and Zn by ICP-AES.

Exchangeable Aluminum (Extraction Ref. No. 33, Analysis: Ref. No. 21  )

A 3 g sample of air dried soil is shaken with 30 mL of 1 MKCl for 30 min. and then centrifuged. The supernatant is analyzed for aluminum by ICP-AES.

Mehlich III Extractable Nutrients (Extraction Ref. No. 34, Analysis: Ref. No. 21)

A 3 g sample of air dried soil is shaken with 30 mL of Mehlich III extracting solution [0.2 N CH3COOH, 0.25 N NH4NO3, 0.015 N NH4F, 0.013 N HNO3, and 0.001 NEDTA] for 5 min. and then centrifuged. The supernatant is analyzed for Ca, Cu, K, Mg, Mn, P, and Zn by ICP-AES.

Total Elemental, HNO3/HClO4 (Digestion: Ref. No. 35, Analysis: Ref. No. 21)

A 0.5-1.0 g sample of air dried soil is digested with 3 mL conc. HNO3 in Folin-Wu tubes in an electrically heated block for 1 h at 145o C. Then 4 mL of HClO4 is added and heated to 240oC for an additional hour. Determination of Al, As, B, Ba, Be, Ca, Cd, Co, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Rb, S, Si, Sr, Ti, V and Zn on the supernatant by ICP-AES.

“Total” Elemental, microwave (Digestion: Ref. No. 36, Analysis: Ref. No. 21)

USEPA 3051 Microwave Procedure: digest 0.5 g of air dried soil with 10 mL of HNO3in a 50 mL quartz vessel using microwave digestion for 4 minutes at 175oC. Determination of Al, B, Ca, Cd, Cr, Cu, Fe, K, Mn, Mg, Na, Ni, P, Pb, Zn by ICP-AES.

Mercury, Total (Digestion: Ref. No. 36, Analysis: Ref. No. 37)

Digestion of 0.5 g air dried sample with 10 mL HNO3 acid in a microwave digestion vessel for 10 minutes. The sample is further digested for 2 h with 0.25 M H2SO4, 5% potassium permanganate & 5% potassium persulfate in a hot water bath at 95oC. After reduction with stannous chloride, the Hg is quantified by the cold vapor technique using atomic absorption spectrophotometry on a mercuryMonitor Elemental Mercury Detector.

Textural Analysis, Hydrometer (Ref. No. 64)

This method determines the approximate proportion of sand (2000-50um), silt (50-2.0um) and clay(<2.0um) particles in a soil.  Forty grams of air dried (crushed to <2mm) soil is shaken for 16 hr with 100 ml of 5% sodium hexametaphosphate.  The suspension is quantitatively transferred to a sedimentation cylinder and brought to a total volume of 1L with deionized water.  After a 2 hr temperature equilibration, the suspension is stirred vigorously for one minute to re-suspend the particles.  An ASTM No. 152H hydrometer is carefully placed in the suspension and used to take two readings, one at 40 sec. and another at 6-8 hours (exact time depends on the temperature of the suspension).   The percentage of sand, silt and clay in the soil is calculated from the resulting hydrometer readings.

Soil Fertility Tests [Scoop Volume] (Ref. Source E)

These tests are the same as those used in the University of Minnesota Soil Testing Laboratory and follow the methods in the bulletin “Recommended Chemical Soil Test Procedures for the North Central Region”. Summary method descriptions are available at the University of Minnesota Soil Testing Laboratory website.