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Dips Into Soil Nutrient Bank May Not Show This Year, But ...

Withdrawals will eventually show up.

One strategy that is getting lots of talk this spring is drawing down the bank of soil nutrients built up when fertilizer wasn't as expensive. That means either skipping fertilizer applications for everything but nitrogen for corn altogether, or cutting back significantly this year. A soil test expert has one word of caution: if you do that, just realize that sooner or later it may show up in soil test results.

There are soils where phosphate and potash levels, one or both, are so high that it would take more than one season to begin to draw them down to a level that could affect yields. That's based on information collected by a number of agronomists, and published in tables of recommended fertilizer rates depending upon soil nutrient results from the Tri-State Fertilizer Recommendation guidelines. Those recommendations were developed a few years ago by agronomists from three universities- Purdue, Ohio State and Michigan State.

Randall Warden, Director of client services for A & L Great Lakes Labs in Ft. Wayne, Ind., says that many farmers who rely on nutrients banked in the soil won't notice any changes in trends in nutrient levels this year, assuming that all areas of the field where they don't apply fertilizer are already testing high to very high for the nutrient they choose not to apply. That's because many soil testing programs currently only call for resampling the same field every four years, he notes. So what you do in one season won't necessarily make a difference in the trend.

"If you draw from the bank this year, but then catch up next year, say if fertilizer prices aren't as high, then when you test again you may not notice the difference," he notes. Warden says that what counts in soil testing are the trends over time. That's the best gauge of what's happening with a specific nutrient in a field, or certain area of the field. The shift toward precision farming and precision sampling has also made a difference in uncovering more areas where soil nutrient levels are either higher or lower than what was detected using conventional sampling methods of the past, he notes.

What often happens, however, Warden notes, is that when someone draws from the nutrient bank one year, he may never get around to catching up by applying more in another year. If that's the case, then a routine soil sampling program, even one where soils are only tested once every four years, will eventually detect a trend of decreasing soil test values.

His comments make food for thought as planting season and the last opportunity to apply fertilizer for this year approaches. The secret, of course, is having adequate soil test results to guide you when making these decisions.

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