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Posted 28 May 2015. PMN Crop News.

Invest in a Soil Test, Advises Agronomist

Six things to consider when managing soil fertility

Source: United Soybean Board Press Release.

Chesterfield, Missouri (May 4, 2015)--Soil tests can prove to be useful in many different ways, from predicting lime needs to measuring the impact of fertility programs.


According to a recent webcast from Dave Mengel, Ph.D., professor of agronomy at Kansas State University, the unfortunate fact is, “While there are several benefits to soil testing, less than half of U.S. cropland is sampled regularly, resulting in mismanaged soil fertility.”

Get the most out of your soil by considering these six sampling points:

1. Location, location, location.

Mengel says, “Soil sampling should try to capture all known differences in nutrient levels or productivity.” Make up a single sample of your field by collecting various cores (cylindrical samples of soil) from various points in a field, avoiding unusual areas. And while there is an infinite amount of information that can come from sampling, Mengel cautions that farmers should be realistic. “Consider the potential gain in yield or reduced fertilizer cost versus the cost of sampling and testing. Only invest in what will truly pay off,” he advises.

2. Dig deeper.

Immobile nutrients, like potassium and phosphorus, accumulate in the top few inches of soil. If your sample barely scrapes the surface — about two inches deep — your fertility results may be deceivingly high. This leads many to underestimate their fertilizer needs. Mengel recommends pulling samples from deep in the ground — six to eight inches — to get accurate levels.

3. Strength in numbers.

“The nutrient content of soil can differ across very short distances. It’s not unusual to pick up soil variations that date back as far as World War Two,” says Mengel. Accommodate for outliers due to erosion, landscape position and previous manure and fertilizer occurrences by collecting up to 20 cores for a single sample.

4. Timing is everything.

While Mengel recommends taking samples in the springtime, when soil conditions are good, he stresses that consistency is key. “You can test for nutrients like potassium, phosphorus and zinc at any time, but always test at the same time,” he says. And the best way to get a return on your soil-testing investment is to start well before planting begins. This will give you time to consider the results and adjust fertility plans.

5. Handle with care.

“It’s not entirely uncommon for farmers to send me soil samples packaged in old cake-mix boxes,” Mengel says. Recycling boxes left over from last week’s bake sale may seem environmentally friendly, but remnants of enriched flour can affect phosphorus readings. Avoid contamination from dirty or galvanized buckets, and never oven-dry soil samples. Mengel recommends using durable, plastic lined bags available at many soil-testing labs, or sealable sandwich bags.

6. The best test.

According to Mengel, a good soil test needs to be six things: simple, precise, reproducible, inexpensive, a predictor of nutrient needs and a good representative of the relationship between soil test level and yield. “Meet all these criteria, and be consistent,” he says. Sample the same areas regularly and use the same lab to develop a good soil test history.

Overall, Mengel emphasizes that your investment in soil tests needs to make economic sense. Using a systematic-grid sampling system will give you a lot of data, but it comes at a substantial cost, and provides no benefit if you are not going to adjust your fertilizer application accordingly. And you can certainly test for nickel, an essential nutrient, but there’s never been a significant response to adding it in the field. “Focus your attention on valuable nutrients that will give the greatest return to fertilization.”

The soy checkoff sponsors the Focus on Soybean webcasts through a partnership with the Plant Management Network. U.S. soybean farmers can watch executive-summary versions of these presentations for free at any time.