With results and recommendations in hand before the ground freezes, growers can use the winter months to plan their fertilizer programs for next year, to order fertilizer and to take advantage of reduced pricing opportunities that may occur.
For results as close as possible to the situation next spring, the ideal time to take fall samples is when soil temperatures drop below 10°C and as close to freeze-up as possible. Microbial processes in the soil slow down as temperatures cool, so mineralization should be minimal below that temperature threshold.
Snow in October is not usually a major problem for fall soil tests. Temperatures often improve before winter sets in for good, so sampling opportunities usually present themselves.
The key with soil tests is to be consistent. If growers usually test in the fall, continue testing in the fall, and sample in similar locations in the field. With GPS-marked coordinates, growers can sample the same general location each time, but it probably shouldn’t always be the same square foot of ground. Otherwise taking a bunch of cores each time will start to degrade that small area so it no longer represents the field.
Part of being consistent is to stick with the same lab. Labs often have their own methods that may produce slightly different results from the same samples, so switching labs may make it difficult to identify long term trends in the data. A list of labs.
Trends that show nutrient reserves going up or down over time are a valuable part of soil testing. Use trend lines, and not just the lab’s recommendations, to adjust fertilizer rates as needed.