The right time for fall soil testing

September 28, 2016 - Issue 27

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 to be 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 threshold.

A note on timing: Rain on warm soil can cause a short spike in free nitrate 2-3 days after the weather event as soil microbes break down residue. This is temporary, and will subside. If your soil test results come back with N levels that are abnormally high, especially when compared against extraction from grain yields, look at the weather conditions in the few days before the sample was taken, and consider a re-sample if time allows.

The key with soil tests is to be consistent. If you usually test in the fall, continue testing in the fall, and sample in similar locations in the field. (If you have GPS marked coordinates, you can sample the same places each time.) Then when you get the results, concentrate on the trends. Are reserves going up or down over time? Use the trend line to reassess fertilizer rates.

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