One disadvantageof deep banding phosphorus (P) and potassium (K) is that it makes it harder to accurately measure field fertility. Do you collect soil samples within the band? Outside the band?

“That’s an unanswered question,” says George Rehm, a retired University of Minnesota Extension soil scientist.

Soil test levels of P and K tend to increase in the band, even with maintenance rates, and decrease everywhere else in the field, says Fabian Fernandez, University of Illinois Extension soil scientist. “If you always sample in the band, you may under-apply fertilizer. Sampling between rows would result in lower test levels than the actual fertility of the field and would lead to over-fertilization.”

Lynn Lagerstedt, a crop consultant from Adams, MN, works with several growers who deep band immobile nutrients. As a practical matter, he doesn’t usually see a big difference between cores taken from within the corn row or half way between the rows, indicating “the fertilizer injected under the row has all been used by the plants. That tells me I don’t have to worry about sampling right over the band.”

The best advice currently is to collect soil samples about 6 in. beside the band and 6-8 in. deep, Rehm says. It’s also a good idea to increase the number of cores per sample in banded fields, says Richard Wolkowski, a University of Wisconsin Extension soil scientist. Collect “at least 10 and perhaps 15-20. Mix well in a bucket, since you will end up saving just a portion of what is collected.”

Rehm notes that “It takes a long time for soil test levels to decline, even if you don’t add any fertilizer.” And corn residue is a good source of recycled K, Wolkowski adds.

Beyond that, Rehm says, the purpose of soil sampling is evolving with the adoption of precision-agriculture methods, such as georeferenced soil sampling. “In the past, soil sampling has been used as a predictive tool. With a move to banded application, I see soil sampling used more and more as a monitoring tool.” 

February 2011