by KEITH REID

The lab cannot do anything about how the sample is collected in the field, but it can make sure the part of the sample it selects for analysis is as homogeneous as possible. For regular analysis, the samples are air-dried overnight, or longer if the sample is extremely wet. The dried samples are then pulverized and screened to take out any stones or large pieces of crop residue. Nitrate samples are the exception; they are analyzed while still moist, since the drying process can change the nitrate content of the soil.

At this stage, samples are also assigned a lab number or bar code, so they can be tracked through the analytical process and the correct results sent back to the person who submitted the sample. This number is also recorded on the sample submission form.

Every soil sample that goes through a lab will be tested for soil pH. A portion of the sample is scooped into a small paper cup, and then just enough water is added to make a saturated paste. This is mixed with a glass rod and then left for several minutes to equilibrate. The pH is then measured by immersing a pH electrode into the slurry and reading the pH directly off the display on the meter attached to the electrode.

If the pH of the sample is below 6, then the buffer pH is also determined. This is done by adding a buffer solution to the sample in the cup, stirring it and letting it equilibrate for half an hour, and then measuring how much the pH of the buffer solution dropped. This is then used to determine how much ag lime needs to be added to the soil to neutralize the acidity.

Available nutrients from the soil are measured by using an extracting solution to remove the ions in the soil solution, along with a portion of the nutrients that are attached to the soil particles. This extract is then analyzed for the concentration of nutrient that has been extracted.

It is a common misconception that the nutrients extracted in a soil test represent an actual physical quantity of the nutrient that a crop can pull out of the soil. In fact, no chemist has ever been able to imitate exactly the various ways that plant roots interact with the soil, particularly over the wide range of soil types that exist.

Extractants are chosen based on how strong the statistical correlation is between the amount of nutrient they extract and the amount taken up by plants growing in pots containing various soils in greenhouse trials. Extractants can also be compared on how well they predict differences in response to added fertilizer at different soil test levels.

"Phosphorus available to plants in Ontario soils is extracted using a sodium bicarbonate solution, sometimes referred to as the Olsen extractant. Some other jurisdictions use extractants (such as the Bray or Mehlich) that are acidic rather than alkaline, but these are unreliable in the alkaline soils that make up much of Ontario's cropland.

The sodium bicarbonate extractant works well across a wide range of soil pH. The extraction is accomplished by mixing a scoop of soil with the extracting solution in a flask, then shaking and filtering the solution. The filtered solution is then combined with a series of chemicals that develop a blue colour which varies in intensity with the concentration of phosphate. In the lab, this process is done in an auto-analyzer that combines and mixes the chemicals in the correct proportions. The colour intensity is then measured by the absorbance of light at a specific wavelength.

The amount of plant-available potassium and magnesium is measured in much the same way, except that the extracting solution is made with ammonium acetate. The ammonium ions push the other cations off the exchange sites in the soil into the extracting solution, where they can be measured using an atomic absorption spectrophotometer. This instrument measures the absorption of specific wavelengths of light for each element. The same process can be used to measure calcium content, but the samples normally need to be diluted to bring the concentration down to a measurable level.

Organic matter is not part of a normal soil test because it is not used in making fertilizer recommendations, and because it does not change quickly in the soil. The most common methods of determining organic matter are to burn off the organic matter from a sample and measure the difference in weight or to dissolve and oxidize the organic materials in a strong acid. These methods will give slightly different results in most soils.

Micronutrients are often extracted from soils using chelating agents like Diethylenetriaminepentaacetic acid (DTPA) or Ethylenedinitrilotetraacetic acid (EDTA). The biggest challenge with micronutrients is that the quantities are so small and the lowest amount that can be measured is larger than the critical value for crop production. Zinc and manganese are the two micronutrients that can be measured most reliably in Ontario soils.BF