Soil Testing: Behind the Scenes

Soil Testing: Behind the Scenes

A look into how agricultural laboratories ensure accurate results

Many growers rely on soil tests for nutrient recommendations to apply to the next growing season, but do not know all the hard work that goes on behind the scenes to achieve those accurate results. John Lee is a soil scientist at North Dakota-based AGVISE Laboratories, a leading Midwest agricultural laboratory. Lee emphasizes that an agricultural testing lab is not a place where lab technicians merely swirl around a few test tubes, but rather involves a process that requires many experienced technicians and sophisticated equipment, therefore he took us inside the operation.

Lab technicians dry, grind soils and then use multiple extracting solutions and very hot temperatures to determine soil test levels. All of this is done while maintaining Quality Control protocols. At AGVISE in Northwood, North Dakota, the general process begins with the sample being put into a dryer and heated to no higher than 105° Fahrenheit to minimize any issues with potassium (K) fixation.

"Once the sample is dry, it goes through the grinding process," says Lee. "The sample is put through a hammer mill specifically designed for soil samples. We recognize there is a lot of effort involved in collecting a soil sample, and whoever is submitting the sample feels it is representative. Because of this, we use the entire sample which can be up to 10 pounds from some customers. After the sample has been dried and ground, we then pass it through a 1-millimeter sieve. This produces a very well blended and fine material that is tested in the laboratory.


One of the central tools used in the lab to analyze nutrient levels is the inductively coupled argon plasma instrument (ICP). "It uses a plasma torch to heat up the soil extract which excites the electrons in the solution to a higher energy state," says Lee.

Once the electrons release that energy and come down to their base energy state, they give off a specific wavelength of light for each nutrient. The ICP measures the amount of each wavelength released to determine the amount of each nutrient in that soil sample.

Testing the soil organic matter content is also important because it indicates the ability of the soil to hold water, resist crusting and erosion. The organic matter testing method called loss on ignition (LOI) starts by weighing a small portion of the dried sample and then heating it to 360° Celsius for two hours. During that two hours the easily oxidizable carbon is burned off. A final weight is recorded after this stage and the amount of weight lost is used to calculate the percent of organic matter in that sample.

"During analysis in the laboratory, quality control samples are tested alongside grower samples," says Lee. "For each nutrient analysis, the technicians know what the test value for the quality control soil sample is supposed to be. Every 10th sample tested in the laboratory is a quality control sample to ensure all aspects of the testing are done correctly and then the data is run through a program based on normal soil test ranges for their region. Any samples that are found to be outside of those ranges are brought to the attention of one of our Soil Scientists and the samples are reran."

When all the test data has passed the quality assurance process, it is then posted to the website for customers to access. The customers then utilize this information to help make fertility decisions for the next crop year.

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