What does a soil test measure?

By Gary Sliworsky
OMAFRA Rep.

Not all of the nutrients in soil are in a form available to a crop. The total amount is very large compared to the amount actually available, and in the end it does not relate to uptake, so there is no value in measuring it. Available nutrient amounts are estimated by using extractants.
When a soil sample goes to a laboratory, subsamples get mixed with chemical extractants that withdraw a portion of the total nutrient in order to approximate what is available to plants for uptake. Extensive research testing various chemical extractants has been done to determine those that most closely mimic plant uptake in our Ontario climate and soils. The OMAFRA accredited soil test extractants are:
·phosphorus (P) - sodium bicarbonate
·potassium (K) - ammonium acetate.
It is challenging for a lab test to determine the exact amount of P and K available over a growing season, because it is impossible to predict growing conditions. For example, P and K move very little in soil, so better root growth can enable plants to access more nutrients.
After soil is shaken with the extractant, the amount drawn out of the soil and into the liquid extract is determined using an analyzer. The extract analysis gives concentration in the liquid, which gets converted to weight of nutrient per weight of soil: milligrams nutrient per kilogram soil = parts per million (ppm). This is the soil test value you see on your report.
Where Do Fertilizer Rate Recommendation Come From?
Fertilizer rate recommendations in the Agronomy Guide for Field Crops (OMAFRA Publication 811) are from measured crop responses (yield) to applied nutrients (application rates) in Ontario fields with varying soil test values determined by the accredited extractant. These recommendations can only be used if the OMAF/MRA accredited test is used to extract the soil. If a different extractant is used (e.g. Bray), the nutrient concentration is different, and the OMAF/MRA tables do not apply.
OMAF/MRA fertilization recommendations are derived by using the “sufficiency approach”. Each crop has a specific sufficiency index and a recom­mended application rate for each nutrient. The goal of the sufficiency approach is to apply enough fertilizer to maximize profitability in that given year of application, while minimizing nutrient applications and fertilizer costs at the same time. Of the management strategies, the sufficiency concept will, in general apply the least amount of total nutrient. Many remember how the sufficiency concept works by saying that it “fertilizes the crop.”

Dates to Remember
Jun 13 – RR Regional Abattoir Inc. Annual General Meeting, 7:30 pm at the Abattoir, 26 Byng Street in Emo.