512 Lesson 10 – tour of ISU Soil and Plant Analysis Laboratory


Soil testing is a valuable diagnostic tool
for growers. Soil testing laboratories, such as at Iowa State University, often evaluate
soils for their available nutrient status, organic matter content, pH, and lime requirement. The first step in the process is to ensure that a proper soil sample is obtained. Most
soil testing laboratories have printed materials on how to obtain a good, representative soil sample. After the sample has been received, the laboratory will follow these steps in handling the sample: Receiving and recording the sample, Drying, Grinding, Extraction, Measurement, Interpretation of the results. When handling thousands of samples, keeping careful records is imperative. Each sample received by the Iowa Soil and Plant Analysis Laboratory is assigned a unique number that follows that sample throughout each operation. Soils collected in the field vary in moisture
content. In the past, analysis was performed on dry soils for major soil nutrient determinations. However, recent research has indicated that the soil test potassium values from a moist
sample analysis correlates better with yield response for corn and soybean. Iowa State
offers interpretation of potassium for both dried and moist samples. Most analyses, such as pH, lime requirement, organic matter, and micronutrients, require the sample to be dry. Each sample is maintained in its own bag with its unique laboratory number. The drying usually takes about 16-20 hours at 40 degrees C. Once the samples are dry, they are ground
to pass a 2-mm sieve. Take this sample here and make sure all of it goes into the grinder. For some tests, finer grinding of samples is necessary. Laboratories use 1 to 10 grams of soil for nutrient determinations, so it is important that the soil be uniformly
mixed and that the small subsample is representative of the large sample. For determination of potassium using moist sample analysis, the samples are homogenized and sieved to pass a one quarter inch sieve. The moisture content of the sample is then determined by drying the samples at 105 degrees C in
a drying oven, and weighing is based on the dry-weight equivalent. After the sample is dried and ground, it is
ready for extraction of the nutrients. The first step is to obtain a specific quantity
of soil. Most laboratories do this by volume as shown here. Different size scoops are used
for different analyses. The scoop is placed into the dry, ground soil, the scoop is tapped
three times to settle the soil, and then the top of the scoop is leveled. The choice of extractant to be used is based
on the test being conducted. A soil testing laboratory does not measure the total of plant
nutrients, but rather estimates the plant available nutrients. Thus, a chemical, or
mixture of several chemicals, is used to approximate what is available to plants throughout the
growing season. Based on different soil properties, crops, and weather, it is not surprising to
learn that different labs use different chemicals. Also, different laboratories have different
procedures to determine soil organic matter, different soil-to-water ratios in determining
soil pH, and different means of determining lime requirements. Based on soil properties
and others tests to be run, labs may use more than one extractant. For example, Iowa State has
three different phosphorus tests that it uses. The test methods used at Iowa
Soil and Plant Analysis Laboratory have been field calibrated with crops grown in Iowa
soils, under Iowa climatic conditions. Iowa State lab procedure includes a control
sample, with known values of the components being analyzed, in every tray of 10 unknown samples. Here, the known control sample is being added to the extraction container. It
will follow all the steps of the unknowns. If the soil test values of the control sample
are not within quality control limits, the unknown samples will be re-analyzed. The common extractants or methods used at Iowa State for soil fertility analyses are shown here: The test for pH uses a one to one soil-water ratio; The test for Lime requirement uses the Sikora method; The test for Organic matter content uses Dry combustion; The test for Phosphorus may use the Mehlich-3, the Bray-1, or the Olsen method; The test for Potassium may use Ammonium acetate or Mehlich-3; The test for Zinc uses Diamine-triethylene-penta-acetic acid (commonly known as DTPA) The laboratory can determine many other elements, including nitrate-nitrogen, using the potassium chloride extraction method for the late spring nitrate test, and end-of-season stalk nitrate test. Sample extractants need an opportunity to
react with the soil. Here, samples are being placed on a shaker for a specific time period;
and here, the samples are being filtered. After filtering, the extracted nutrients can be measured.
Even though different equipment may be used in different soil testing laboratories
to measure available nutrients, it is the extracting solution that normally determines
nutrient availability and not the equipment used to make the measurements. Soil testing laboratories use expensive equipment
for analyzing soils. Some of the instruments used at Iowa State and their operating principles
will be discussed. After mixing and equilibrating the soil with
water, a pH electrode is placed into the soil suspension. The electrode is adjusted to read
pH 4 and 7 with standard buffered solutions. The potential of the H ion activity (known
as pH) is measured across a glass membrane and translated to an electrical potential,
indicating the pH of the solution. For lime requirement tests, a buffered solution
is added to the soil-water mixture and equilibrated after pH determination. The buffered solution
removes the potential acidic ions from the cation exchange sites into the solution, where
the pH electrode measures their activity. The decrease in the buffer pH relates to quantity of lime
required to raise the soil pH to 6.0, 6.5 or 6.9. A colorimetric technique is often used for
phosphorus determination. The extracted sample solution is mixed with a coloring reagent
(acid ammonium molybdate) and a reducing compound (ascorbic acid). The coloring reagent and
the reducing compound combine with phosphorous in the sample solution to form phospho-ammonium
molybdate complex that is blue in color. The intensity of the blue color in the complex is proportional to the concentration of phosphorous in the solution. Inorganic nitrate and ammonium nitrogen in solution is
also determined colorimetrically. Iowa State uses a Lachat automated flow injection
analyzer, the instrument shown here, for this determination. A peristaltic pump is used
to aspirate sample and various reagents into the manifold and mix them before passing the
colored product into a flow cell for measurement. Again, the intensity of the color developed
is measured as absorbance and is linearly proportional to the concentrations of nitrate
and ammonium nitrogen in the standards. An atomic absorption spectrometer is often
used to determine potassium, and can be used for other cations as well. In this procedure,
a solution containing potassium is passed into a flame. The potassium atoms in the sample
solution are raised to a higher energy state in the flame. As they cool, they fall back
to their unexcited, or ground energy state, and emit a specific wavelength of light, that
is, 766.49 nm, which is then measured. The intensity of the light is directly proportional to the
concentration of potassium in the sample. The most expensive analytical instrument in
the laboratory is the inductively coupled plasma – atomic emission
spectrophotometer, or ICP-AES unit. The operation is complex, but basically involves ionizing
the sample ions by extreme heat through electromagnetic induction in an inert gas. The principle of
this process is that each element emits energy at specific wavelengths peculiar to its atomic character. The intensity of the energy emitted at the chosen wavelength is proportional to
the amount (that is, the concentration) of that element in the sample being analyzed. Thus, by
determining which wavelengths are emitted by a sample, and by determining the intensities of those wavelengths, the elements can be qualitatively and quantitatively determined
from the given sample relative to a calibration standard. The ICP instrument is quite sensitive
to element concentrations and can measure both metals and non-metals simultaneously.
It is often the unit of choice when multiple analyses are wanted from the same sample. All Iowa State interpretative data can be found in PM-1688 (many other states have similar publications). The values obtained in the laboratory for plant-available nutrients should be thought of as indices, which mean little
without proper interpretation. Interpretation is specific to crops, soils, and local weather conditions
of a region. Iowa State researchers conduct many field trials in different soils
of the state to determine soil test response categories, which indicate the probability
of obtaining a fertilizer response. From the field calibration data, fertilizer recommendations
are made. Table 3 in the PM-1688 publication indicates
the response categories of phosphorus and potassium for Iowa corn production. For example, note
that a Mehlich-3 test value of 12 would be classified as “low” and the recommendation is
to apply 75 lbs per acre of phosphate. The bottom part of the table gives the recommendations
for potassium. An agronomist with experience in working with a specific grower in specific
soils under specific weather and management conditions, and considering the current costs of
fertilizer and value of crop, may wish to adjust this basic recommendation. Soil testing is a valuable tool for the grower
to maximize profit per acre while minimizing negative environmental impacts. It involves
the taking of a good soil sample from the field, carefully analyzing the soil in a reputable
laboratory, collecting localized data that relates the laboratory index to field responses,
and finally making an informed fertilizer and lime recommendation for the crop to be
grown. The ISU Soil and Plant Analysis Laboratory
is located in the Department of Agronomy at Iowa State University in Ames, Iowa.

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