March 29, 2019
You didn’t soil sample last fall because it turned wet. Will you get the same results if you sample this spring? This is just one question Jason Ackerson gets.
The Purdue University Extension soil specialist interacts with farmers to tell them about updated soil sampling guidelines. He believes the publication AY-368 is the first major update on soil sampling issued by Purdue since Dave Mengel was the soil fertility specialist in the early 1990s.
Here are four questions Ackerson answered recently:
If soils are very wet or very dry, will it affect soil test results? Yes, it’s possible results could be affected. For example, if it’s a wet season, potassium may be affected because there may be less mineralization. However, I would still stay on schedule and pull samples. If potassium levels are a bit short compared to the last time you tested, realize that it could be due to a wet season and not necessarily a true drop in potassium levels.
We recommend being consistent with the time of year you sample. If you normally sample in the fall and sample in the spring instead, results may vary just because of when you sampled.
Which lab should you send samples to for testing? Purdue doesn’t make recommendations on which lab to choose. Work with a lab which is certified by the North American Proficiency Testing Program. Their website lists all accredited labs. Currently, most major labs within Indiana meet certification requirements.
We advise sticking with the same lab each time you test. There are small tweaks and differences in how certain labs carry out specific procedures, even though they still meet national standards. To ensure consistency and not introduce another variable, I would stay with the same lab.
Should we mark points with GPS where we pull cores and return to those same spots the next time we sample? Recording sampling points with GPS is a good idea. You know exactly where you pulled samples. If you’re using the grid sampling method, say on 2.5-acre grids, consider returning to the same spots to pull samples each time. Since grids are arbitrary, unless you adjust them for soil types, you may have more than one soil type in a single grid. Returning to the same points each time ensures you’re sampling the same soil type.
If you’re sampling by management zones set up using yield monitor results and other factors, it shouldn’t matter as much if you don’t change management zones.
In one of your examples, you show sampling on half-acre grids. That doesn’t seem practical since it would be hard for custom-applicating equipment to adjust that rapidly in the real world. Is it practical? No, you’re exactly right. We would never recommend sampling on half-acre grids and then trying to set up a prescription map to apply fertilizer based on half-acre grids. My colleague, Bob Nielsen, had graduate students sample on half-acre grids in a field at the Davis-Purdue farm near Farmland [Ind.] to compare with results from sampling the same field on 2.5-acre grids. It made the point that sampling on 2.5-acre grids doesn’t pick up every change in soil type. Those two maps looked quite different.
However, we weren’t implying that you ought to sample on half-acre grids, even if you had the manpower. Right now, it just isn’t practical.
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