A Day in the Life of a Soil Sample: Day 2

In a previous newsletter, we began our 3-part series in A Day in the Life of a Soil Sample: Day 1.

Day 2 for a soil sample begins very early in the morning.  The grinding teams start by retrieving the bench sheets that were produced on Day 1 and the sample storage cups and trays corresponding to the lab numbers assigned to the day’s samples.  Before each sample is ground, the sample ID from the original sample bag is double checked assuring that they are in the same order in which they were logged into the sample database.  At this point, should a discrepancy be identified between the sample order and the bench sheets, the grinding team will note any corrections needed and report necessary changes to the office staff.

Now, soil grinding actually begins.  The entire sample is dumped into a flail-type hammer mill which pulverizes the soil.  The sample is held in the grinder for a minimum of three seconds to ensure that it is thoroughly homogenized.  The sample is then released from the grinder through a sieve which removes any particles greater than 2 mm.  The portion of the sample passing through the sieve is poured into a sample cup identified with its corresponding lab number.  The sample is now ready for the next step in the process.

Dried and ground soil samples now move to “scooping”.  A routine soil analysis requires three subsamples of each sample, one for measuring pH, one for organic matter, and one for nutrient extraction.  These subsamples are scooped by hand. Each technician that scoops soil is carefully trained and routinely audited to ensure consistent scooping procedure.  The volume of soil collected with each scoop is specific to the analysis being performed.

The first scoop is used for organic matter analysis.  A single scoop is transferred to a ceramic crucible.  The crucibles are arranged on specially designed trays that are first loaded into a holding oven to dry off any ambient moisture that may have condensed in the soil as it cooled down from the drying/grinding process.  From the holding oven, each crucible is weighed.  The trays are then transferred to ovens that heat the samples to the point where the organic matter is oxidized and released, and held at that temperature for a predefined time period to ensure that the organic materials are thoroughly evolved.  The trays are then removed from the furnace, allowed to cool, and each crucible is reweighed.  The percent change in weight for each sample is automatically calculated in the database and reported as the percent organic matter.  This process is called Loss on Ignition.

The second scoop is used for nutrient extraction.  This scoop is perhaps the most critical, and only our most experienced technicians are trusted with this task.  The scoop is placed into a cup where it is combined with a carefully measured volume of extracting solution (typically Mehlich-3).  The cup containing the soil - solution mixture is then agitated for a predetermined time period to ensure thorough mixing and to allow adequate time for the extracting solution to work, and the slurry is then filtered through a laboratory grade filter paper to remove the soil.  The resulting solution is then transferred to a test tube and analyzed by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) to determine the levels of nutrients present in the solution and, by extension, within the soil.

The third scoop is used to determine the pH and buffer pH of the soil.  This scoop of soil is placed into a disposable cup, and an automatic dispenser adds deionized water into each cup.  The soil-water mixtures then sits for a set period of time before being stirred and measured by a pH electrode.  The pH is determined on 10 samples at a time.  Any samples that require a buffer pH analysis are identified and the Sikora buffer solution is added to the sample and the BpH is measured after a set period of time.  The pH and BpH data is automatically transferred to the database.

Day 2 comes to a close with all of the raw data being generated and sent to the database where it will be ready for quality control review, agronomist review, and reporting on Day 3.

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