Our Soil Testing Methods
We use a variety of instruments to analyze soil samples including:
- atomic absorption spectrophotometers;
- Inductively Coupled Plasma Spectrometers (ICPs);
- Alpkem Rapid Flow Analyzers;
- colorimeters; and
- general laboratory equipment.
Brief descriptions of our soil testing methods are listed below.
Routine Soil Tests
Soil Tests Made Upon Request
Florist/Greenhouse Tests
At the laboratory, each sample is assigned a 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 overnight in a cabinet equipped with a heating element
and an exhaust fan to remove moisture-laden air. The temperature in the
cabinet does not exceed 97° F in order to approximate air-drying conditions.
Samples are crushed with a mechanical grinder equipped with a porcelain
mortar and stainless steel auger. They are subsequently passed through
a stainless steel 10-mesh sieve to remove larger clods 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.
Organic matter is determined by ashing a 5 gram scoop of the soil sample
at 360oC for 2 hours in a muffle furnace. The loss by weight
of the sample during this ignition is calculated as the organic matter.
Results are reported as per-cent organic matter by weight in the soil.
The pH is determined using a glass and reference electrode with a pH
meter on a 1:1 suspension (5 gram scoop of soil to 5 milliliters water).
Samples of mineral soils with pH values of less than 6.0 are analyzed
further for the following lime requirement test.
The SMP 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 a pH meter, after the sample
has been stirred intermittently for 15 minutes.
Bray-1 Method
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 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 ammonium molybdate-hydrochloric acid solution
and then aminonaphthol-sulfonic acid solution. The color is measured by
an absorption spectrophotometer at 640 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.
Olsen Method
For highly calcareous soils (pH greater than 7.4), the Olsen sodium bicarbonate
method is used. A 1 gram scoop of 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 successive additions
of an ammonium molybdate-sulfuric acid solution and then an ascorbic acid
solution and measured with a fiberoptic probe colorimeter at 882 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,
neutral, ammonium acetate with a 1 gram scoop of soil and shaken for 5
minutes. The exchangeable potassium is measured by analyzing the filtered
extract on an atomic absorption spectrophotometer 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 of a 1:1 suspension. Three 10 gram scoops of soil and 30
milliliters of deionized water are measured into large test tubes and
shaken for 30 minutes. The electrical conductivity of this slurry is determined
with a dip cell and Solu Bridge, and reported as 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 150 grams 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 a Solu Bridge and reported as millimhos
per centimeter (mmohs/cm).
Readily soluble and adsorbed sulfates are extracted with a monocalcium
phosphate [Ca(H2PO4)2] solution containing
500 part per million of phosphorus. A 10 gram scoop of soil is treated
with 25 milliliters of extracting solution and shaken for 30 minutes.
The sulfate content in the filtrate is determined turbidimetrically by
the addition of BaCl2. The results are reported as parts per
million (ppm) of extractable sulfur (S) in the soil.
Zinc, copper, iron, and manganese (Zn, Cu, Fe, and Mn) is determined
by treating a 10 gram scoop of soil with 20 milliliters of DTPA extracting
solution (0.005 molar DTPA, 0.1 molar TEA, and 0.01 molar CaCl2,
adjusted to pH 7.3). After shaking for two hours, the soil is filtered
and the extract analyzed for metals with 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 0.01 M
CaCl2 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 continuous flow 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 soil and 20 milliliters of 0.1 % CaCl2. H2O
(calcium chloride) 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, neutral, ammonium acetate with a 10 gram scoop of 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 a 3 gram sample 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. A blue
color is developed with successive additions of 1) ammonium molybdate-sulfuric
acid solution and then 2) ascorbic acid solution. The intensity of blue
color, corresponding to the amount of P in the extract, is measured with
a fiberoptic probe colorimeter at 882 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.
Reference
Recommended Chemical Soil Test Procedures for the North Central Region.
1988. North Central Regional Publication No. 221. NDSU Bull. No. 499.
Soil Testing Lab, Rm. 135 Crops Research Bldg.,
1902 Dudley Ave, St Paul, MN 55108-6089
Email: soiltest@umn.edu,
Phone: 612 625-3101, FAX: 612 624-3420
Last updated:
October 22, 2007
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