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Moist Soil Sample Provides Precision, Better Accuracy in Estimating PotassiumMoist Soil Sample Provides Precision, Better Accuracy in Estimating Potassium

Test and Sidedress the Same Day | Rapid Field Moist Sample Analysis is Now Available and Paying Dividends

Jim Ruen

March 27, 2013

6 Min Read


Ignored for decades, moist sample analysis saves money and boosts yields. Soil labs and scientists alike abandoned field moist soil sampling decades ago. Common until the 1980s, field moist soil sampling was replaced with a faster and easier "dry and grind" procedure. The Iowa State University (ISU) soils lab discontinued the moist test in 1988, not because dry was better, but only because “drying soil was a more practical procedure and standardized soil moisture across all conditions," says Antonio Mallarino, ISU professor, soil fertility and nutrient management.

However, his 1990s field research showed much variability with potassium (K) soil testing when using the dry and grind procedure. “The dry test was better in some conditions than others; however it was difficult to predict the circumstances when it would be better. Soil conditions, drainage and texture all affected results."

Mallarino has confirmed that field moist test of K is more accurate, after 300 field corn and soybean trials conducted since the early 2000s.

Peter Scharf has found similar results, after testing field moist and dry sampling for nitrate and ammonia. The University of Missouri agronomy professor concludes, "Dry always comes out higher. There is even a difference with oven-dried versus air-dried samples: Oven-dried provides an additional bump."


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But is the "bump" available for crops? Scharf notes that ammonia and K have an identical charge and behave the same way. K attracts water, creating the same size particle as ammonia, and both structures get caught between layers of clay. He theorizes that when the clay particles dry during sampling, they release both K and nitrogen (N), a release that doesn't happen under field conditions.


Rapid field analysis

What’s available to the plant is what counts, agrees ISU’s Mallarino. “It is a miracle that soil tests work at all and that we can correlate results to what a plant needs in the field."

Until recently, both lines of thought were literally academic. There was no point in university researchers making recommendations for a process that wasn't used. That’s now changed with the introduction of rapid field moist sample analysis by Solum, through its soils lab in Ames, Iowa.

ISU is again offering this test, and will publish new interpretations for use with field moist sample analysis.

High-tech analysis equipment like flow injection analyzers, inductively coupled plasma optical emission spectrometers and pH robots are revolutionizing soil testing. Doors to the new labopened on Sept. 17, 2012. Solum ran between 2,000 and 2,500 samples a day in its first week, with results out in three days or fewer of samples’ arrival.

"Using our cloud-based system, customers can log in and actually watch their sample go through the system and see final results as they’re produced," says Leon Zinck, Solum national sales manager. "The industry went to dry samples for high-volume handling and efficiency. Our system provides quick, high throughput capacity with field moist samples."

Solum also introduced a rapid nitrate testing system using optical bin sensor technology that can be field-installed in local ag retail facilities. Growers in south-central Minnesota were among the first to field test both technologies this past year. Central Valley Co-op (CVC) used Solum's labs for moist soil sampling and its field lab for nitrate tests.


Five-minute nitrate field results

"The No-Wait Nitrate field lab gave us an accurate nitrate reading in five minutes," says Irwin Arndt, CVC agronomy sales manager. "We’d tried early season testing for N in the past, but it took too long to get results back. With Solum's system, we take the samples, run them and can apply that day."

Samples were taken according to productivity zones, which were then sidedressed for optimum yields. In some cases no need for sidedressing was indicated. Control strips were sidedressed according to traditional protocols. Sampling was carried out at stages V3 through V5 for application needs and again at V8 for verification.

"Where no need was indicated, yields were unaffected," says Arndt. "We also saw no yield advantage to standard sidedressing. Where a need for additional N was indicated, we saw the best yields ever, with 10-20-bu. increases over no application control strips. Our growers ended up putting on less N than they would have normally, but used it where it was needed. I think in most cases they used around 80% of what they had planned to use."

Arndt says growers were equally pleased with results from field moist sampling for K. Cooperators chosen for the comparison had good fertility programs. However when the field moist samples came back for highly productive areas in fields, K levels were lower than conventional tests indicated.

"We have really been trying to push yields in these fields, but with conventional soil sampling, we weren't getting the yields we wanted," he says. "When we applied potash VRT on 2.5-acre grids according to the moist samples, we were applying more in the areas where it was needed. We saw yields climb dramatically, up to 260-270 bu. in areas where we had never seen those yields before. It is tough to tell, but we expect a 5% yield increase with field-moist sampling over conventional dry."

Field moist sampling isn't a silver bullet, but Mallarino feels it is more precise and more accurate in estimating actually available K. He has seen no difference between wet and dry samples for P, calcium, magnesium, pH, buffer pH or other minerals. As a longtime advocate of moist sampling, he’s pleased to see other states gathering data for K recommendations on their soils, as well. While the original data used for fertilizer recommendations in the 1960s and 1970s used moist samples, fresh data needs to be gathered, he says.

"Soils, varieties and hybrids are different across states,” says Mallarino. "We need to do fresh field calibrations using field trials and gathering yield data."

Mallarino and Arndt agree that farmers maintaining high K levels in their fields may not see a difference in yield response. "If you maintain optimum or high levels, this test may not affect what you are doing. The issue is with a portion of fields or entire fields where levels are low," explains Mallarino. "The moist test is a better predictor. It may not be a problem when growers are making money and applying everything, whether needed or not. When crop prices drop and people need to really fine-tune their inputs, that's when even a minor difference in testing will really help."

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