Eric Rund got oddball soil-test results a few years ago from a strip-till cornfield. Some of the nutrient levels were higher than the Pesotum, IL, farmer expected.

Now near the end of a four-year soil test variability study with University of IllinoisExtension Soil ScientistFabian Fernandez, Rund hopes they’re getting closer to answers from Rund's farm and others on how best to manage soil-test variability on band-applied fields.

"I was getting unexpected phosphorus (P) and potassium (K) soil-test levels which I attributed to inadvertently pulling samples from a previous year's strip," says Rund, who raises food-grade corn and seed soybeans. He’s been strip-tilling corn for 15 years; the last six applying a full corn maintenance level of P and K in the fall, the spring or both.

RTK technology places the fertilizer precisely, “but we may need to use a different method when soil testing where fertilizer has been banded," he says. "The question is where to place the fertilizer. Is it best going back into or very near the old strip to build up a hot spot, or should subsequent bands be moved a certain distance from the old row?"

Rund’s soil tests have been drawn every 7.5 in. across four-row sections at three different depths to gauge nutrient levels. The goal is to see what nutrients exist where in the field, and where plant root masses lie in relation to the nutrients.

Banded fertilizer can be as effective as broadcast applications in helping maximize P and K use by corn plants, according to Fernandez’s research.

Subsurface banding can reduce nutrients in the soil surface where they’re most susceptible to runoff losses. But to determine ideal application rates, soil-test variability must first be minimized, Fernandez says.

Soil tests are still the most important guide to determining the fertility status of a field as long as sampling minimizes errors in the test, Fernandez says.

No universally accepted method for collecting soil samples exists for banded fields, he says. Fernandez recommends using a tube to sample, although a spade or soil auger will work as long as the sample width is constant for the entire sample depth. In fields where exact band locations are known, he suggests staying a few inches away from the band or taking a large number of samples off the band for each sample taken in the band. If the location of the band is unknown, he advises increasing the number of cores making a sample to 20 or 30.  Vary sampling positions, so the band does not bias test results.

"When collecting soil samples, take fewer samples with more cores rather than more samples with fewer cores," Fernandezsays. “If you do take one core in the band, take five or six cores outside the band to make a sample for analysis.Soil-test P and K levels tend to increase in the band and decrease everywhere else in the field. If you always sample in the band, you may not apply enough fertilizer. Sampling between rows would probably mean test levels lower than actual fertility and over-fertilization."

George Rehm, University of Minnesota nutrient management specialist (retired), agrees. "Studies over the years show that in ridge-till and strip-till, soil testing is not easy or straightforward," he says.

If you know where the row is,Rehm says collect cores from 6-8 in. on either side of the row and 6-8 in. deep.

Minnesota field research results indicate the sampling procedure provides the best prediction of required P and K rates when they’re applied in a band. RTK provides better results and allows samples in the same location every year, Rehm says. Rows can be placed so the band is not disturbed.

"K is more variable and harder to accurately measure than P," says Fernandez. "Potassium is in the crop and in the residue after harvest but quickly leaches out into the soil with rain. Fall is the most practical time for sampling and applying P and K because it’s not as busy as the planting season and soil conditions are more favorable. The K soil test tends to be cyclic, typically with low test levels in late summer and early fall and high test levels in late fall to early spring (see chart). P and pH levels are generally not seasonally affected in most soils."

Fernandez encourages farmers to always sample the same time of year and under similar moisture conditions to get more consistent results.In addition, soil-test results should be compared to previous results to be most useful. To minimize variability, use the same test lab every time. "Understanding how various factors can influence the outcome of test results is critical so you do not become overly concerned when the final report is not what you expect,” Fernandez says. “You need to look at a test value; not as an absolute number but rather a number in the center of a range of values.”

Banded Application Tip

Eric Rund, Pesotum, IL, farmer, considers the RTK units on both his tractor and implements essential for a successful strip-till system.

"I can be more successful staying on the strip where fertilizer has been applied using RTK. I’m aware of one farmer that tested turning the RTK off and on alternately on rows across an 80-acre field. He was sometimes 6-8 in. off the strip where he turned off the RTK, and saw an average 5-bu./acre lower yield on those strips. Clearly guidance pays for itself in these situations,” he says.