We use a variety of instruments to analyze soil samples including:
- atomic absorption spectrophotometers;
- Inductively Coupled Plasma Spectrometers (ICPs);
- Alpkem Rapid Flow Analyzers;
- Lachat Flow Injection Analyzer;
- colorimeters; and
- general laboratory equipment.
Brief descriptions of our soil testing methods are listed below.
Routine Soil Tests
- Estimated texture category
- Soil pH (1:1 suspension)
- Lime requirement (Sikora buffer index)
- Extractable phosphorus (Bray-1 extractant, Olsen-sodium bicarbonate extractant)
- Available potassium (ammonium acetate extractant)
- Organic matter (%) (loss on ignition)
Soil Tests Made Upon Request
- Soluble salts (electrical conductance, 1:1 soil suspension, saturation extract)
- Extractable sulfur (calcium phosphate extractant)
- Extractable zinc (DTPA extractant)
- Extractable zinc, copper, iron, and manganese (DTPA extractant)
- Nitrate-nitrogen (0.01 M CaSO4 extractant)
- Exchangeable magnesium and calcium (ammonium acetate extractant)
- Hot water extractable boron (0.1 % CaCl2.H2O)
- Environmental Lead (1 N HNO3 extractant)
- Nutrient Management Phosphorus (Olsen method)
At the laboratory, each sample is assigned an identification number, transferred to a paper bag, and then placed in a metal tray. Every 12th sample is a quality control sample, either a check sample of known chemical properties to ensure accuracy, or a duplicate sample to evaluate laboratory precision.
Samples are dried rapidly under forced air, the temperature not exceeding 95oF.
Dried samples are crushed with a mechanical grinder, and passed through a stainless steel 10-mesh 2.0 mm sieve to remove stones and unwanted debris.
The relative amounts of sand, silt, and clay are estimated by the feel of the soil in a moist condition. The soils are then classified into three categories: C (coarse textures of sand, loamy sand and sandy loam), M (medium textures of loam and silt loam), and F (fine textures of clay loam, silty clay loam, silty clay and clay.
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.
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. Mineral soils with pH values of less than 6.0 are analyzed further for the following lime requirement test.
The Sikora Buffer Index (lime requirement test) is determined by adding 10 milliliters of buffer solution to the above 1:1 sample. The Buffer Index of the suspension is determined with the pH meter, after the sample has been stirred intermittently for 15 minutes.
The soil phosphorus measured is that which is extracted by a solution consisting of 0.025 normal HCl and 0.03 normal NH4F, referred to as Bray-1 extractant. A 1 gram scoop of air-dried soil and 10 milliliters of extractant are shaken for 5 minutes. The amount of phosphorus extracted is determined by measuring the intensity of the blue color developed in the filtrate when treated with molybadate- ascorbic acid reagent. The color is measured by a Brinkman PC 900 probe colorimeter at 880 nm. The result is reported in parts per million (ppm) phosphorus (P) in the soil. The phosphorus measured does not represent all of the phosphorus that may be available for plant growth; e.g., some fraction of the organic phosphorus not measured may become available upon mineralization. The upper reporting limit for this test is 100 ppm. For situations such as nutrient management decisions where a higher value is needed, see the nutrient management test below.
For highly calcareous soils (pH greater than 7.4), the Olsen sodium bicarbonate method is used. A 1 gram scoop of air-dried soil and 20 milliliters of 0.5 molar sodium bicarbonate (NaHCO3) solution are shaken for 30 minutes. Blue color in the filtered extract is developed with molybdate- ascorbic acid reagent and measured with the Brinkman PC 900 probe colorimeter at 880 nm. Results are reported as parts per million (ppm) phosphorus (P) in the soil. As with the Bray P-1 test, potentially available organic P is not measured by the test. The upper reporting limit for this test is 50 ppm. For situations such as nutrient management decisions where a higher value is needed, see the nutrient management test below.
Potassium is extracted from the soil by mixing 10 milliliters of 1 normal ammonium acetate, pH7 with a 1 gram scoop of air-dried soil and shaken for 5 minutes. The available potassium is measured by analyzing the filtered extract on an atomic absorption spectrometer set on emission mode at 776 nm. The results are reported as parts per million (ppm) of potassium (K) in the soil.
Soil samples are evaluated for salinity by first determining the electrical conductivity (E.C.) of a 1:1 suspension. One 10 gram scoop is mixed with 10 mL deionized water to form a slurry, which is stirred intermittently for 30 minutes. The E.C. is determined by the Accumet model 30 conductivity meter, and is reported in units of millimhos per centimeter. (mmohs/cm).
Slightly to strongly saline soils (conductivity more than 0.9 millimhos) are subjected to a more precise test. A saturated soil paste is prepared by slowly adding deionized water to about 50 CC’s of soil until the mixture is a thick paste. After an equilibration time of two hours, the saturation paste is filtered under suction. The electrical conductivity is determined on the filtrate with the Accumet Model 30 conductivity meter, and is reported as millimhos per centimeter (mmohs/cm).
Readily soluble and adsorbed sulfates are extracted with a monocalcium phosphate [Ca(H2PO4)2] solution containing 500 parts per million of phosphorus. A 10 gram scoop of air-dried soil is treated with 25 milliliters of extracting solution and shaken for 30 minutes. The sulfate in the filtrate precipitates with Barium (added in the form of BaCl2) and the SO4 concentration is determined turbidimetrically on the Lachat QuickChem 8500 Flow Injection Analyzer. The results are reported as parts per million (ppm) of extractable Sulfate-S in the soil.
Concentrations of zinc, copper, iron, and manganese (Zn, Cu, Fe, and Mn) are determined by treating a 10 gram scoop of air-dried of soil with 20 milliliters of DTPA (Diethylenetriamine-pentaacetic acid) extracting solution. After shaking for two hours, the sample is filtered and the extract analyzed by an inductively coupled plasma atomic emission spectrophotometer (ICP-AES). The results are reported as parts per million (ppm) for each metal in soil.
Nitrate-nitrogen is determined by adding 60 milliliters of KCl extracting solution to a 2 gram scoop of soil and shaken for 15 minutes. The nitrate level in the filtered extract is measured on a Lachat QuickChem 8500 Flow Injection Analyzer by the cadmium reduction method. The results are reported as pounds per acre (lbs/A) of nitrate-nitrogen (NO3-N) in the top 2 feet of soil or as parts per million (ppm) nitrate-nitrogen (NO3-N) in the soil for all other depths.
A 10 gram scoop of air-dried soil and 20 milliliters of 0.01 M CaCl2. H2O (calcium chloride dihydrate) solution are boiled in a metal container for 5 minutes under reflux using a fiber digestion condenser apparatus. Boron in the filtered extract is determined with an inductively coupled plasma atomic emmission spectrometer (ICP-AES). The results are reported as parts per million (ppm) of boron (B) in the soil.
Calcium and magnesium are extracted from the soil by mixing 10 milliliters of 1 normal, pH7, ammonium acetate with a 10 gram scoop of air-dried soil and shaking for 5 minutes. The filtered extract is analyzed with an inductively coupled plasma atomic emmission spectrometer (ICP-AES) for calcium and magnesium. The results are reported in parts per million (ppm) calcium (Ca) and magnesium (Mg) in the soil.
This measure of lead in the soil is determined from 3 grams of air-dried soil that is shaken in 30 milliliters of 1 molar nitric acid (HNO3) for 1 hour at 180 opm. The mixture is then centrifuged and the supernatant is analyzed for lead with an inductively coupled plasma atomic emmission spectrophotometer (ICP-AES). Results are reported in parts per million (ppm) lead (Pb) in the soil. Although this method does not measure total lead, it measures the fraction that is environmentally available and that which is potentially harmful to human health upon ingestion.
For situations involving soil phosphorus and nutrient management decisions where extractable P is likely to exceed 100 ppm on the Bray test and 50 ppm on the Olsen test. The range for the nutrient management P test is 20-250 ppm. This test is basically the same as the Olsen P test, but with a dilution of the extract before colorimetric analysis. In detail: A 1 gram scoop of soil and 20 milliliters of 0.5 molar sodium bicarbonate (NaHCO3) solution are shaken for 30 minutes. The mixture is filtered and the filtrate is diluted 5 X with sodium bicarbonate. Color is developed with the molybdate-blue/ascorbic acid method. The intensity of blue color, corresponding to the amount of P in the extract, is measured with the Brinkman PC 900 Probe Colorimeter at 880 nm. Results are reported as parts per million (ppm) extractable phosphorus (P) in the soil.
The Spurway extract is a weak acetic acid extraction of the potting media. A representative sample of the media is seived through a 2 mm seive to remove the larger pieces of bark, stones and other relatively inert materials. A 5 cc subsample is placed into a 50 mL Erlenmeyer flask and 25 mL of 1% acetic acid solution is added with a volumetric buret. The mixture is shaken for 1 minute and then filtered. The filtrate is analyzed for important plant nutrients: nitrate and ammonia levels are determined by colorimetry, and phosphorus, potassium, magnesium, sodium, iron, manganese, zinc, copper, molydenum, and boron are analyzed by ICP-AES. Results are reported as ppm (mg/L) in the media extract. A separate 15 g subsample is wetted with 75 mL of deionized water, stirred every 10 minutes for 40 minutes and the solution analyzed for electrical conductivity. The media pH is determined on a separate 5 cc subsample that is wetted with 5 mL of deionized water, allowed to stand for 15 minutes, and the pH determined using a pH meter with reference and glass electrodes.
The Saturated Media Extract is a water extract of the entire media sample. No attempt is made to sieve the sample before analysis. A representative sample of the media is wetted with deionized water until water just barely stands on the surface. The mixture is allowed to stand for 90 minutes and then filtered under suction. The filtrate is analyzed for important plant nutrients: nitrate and ammonia levels are determined by colorimetry, and phosphorus, potassium, magnesium, sodium, iron, manganese, zinc, copper, molydenum, and boron are all analyzed by ICP-AES. Results are reported as ppm (mg/L) in the media extract. The electrical conductivity is also determined on the above filtrate. The media pH is determined on a separte 5 cc subsample that is wetted with 5 mL of deionized water, allowed to stand for 15 minutes, and the pH determined using a pH meter with reference and glass electrodes.
Recommended Chemical Soil Test Procedures for the North Central Region. 1988. North Central Regional Publication No. 221. NDSU Bull. No. 499.