Soil Test Tweaks

Fall is a good time to pull soil samples. Beyond getting the sample analyzed by a reputable laboratory, growers also need to consider what kind of test is being run -- particularly when it comes to potassium.

Traditionally, soil laboratories dry soil samples, grind them, scoop out a volumetric sample (instead of weighing) and use an extracting method to analyze for nutrients. That has been the tried-and-true method on which soil calibrations and fertilizer recommendations have been based for years.

However, as we learned last year, a soil analysis during a drought period will underestimate the amount of potassium available. The process of heating soil to dry it also affects the potassium level. A new, more accurate testing option for potassium is now available. Called a wet test, the process can be done without drying the soil.

"Potassium is unique in that it ties tightly in dry soils, so Iowa State University pioneered a slurry method used in the 1970s," said Nick Koshnick with Solum, a laboratory in Ames, Iowa, that began offering a slurry-based wet test last year.

Iowa State has also developed a moist testing method that is quicker and doesn't require the preparation of a slurry. However, that test is slower and too tedious for a commercial lab -- although the slurry and moist approaches achieve comparable results.

Antonio Mallarino, soil scientist at Iowa State University, believes the wet soil test is more accurate and he has spent the past decade validating that theory and developing a calibration for that data. "The moist soil test for potassium better predicts a fertilizer response in the field. Both tests (dry and wet) tell us potassium levels, but the moist is better at predicting a crop response," Mallarino told DTN.

Mallarino pointed out that industry still likes the traditional dry test and hasn't broadly adapted the wet test as an option. "Drying soil is commonly done by labs because it used to be a more practical sample handling procedure, and it standardizes soil moisture across all conditions. A new process is not an easy step when you are analyzing thousands of samples a year and still need to be competitive," he added.

Howard Brown, Growmark manager of agronomy services, agreed the wet test provides a more accurate potassium number. "But it is still a soil test and not yet calibrated beyond Iowa so more works needs to be done," he said.

Randy Brown, an agronomist with Winfield Solutions, would like to see that happen. "I think that not using the right procedure to measure potassium accurately is a big issue across the Corn Belt," said Brown. "Soil tests are an index of nutrient level and it is related to a probability of fertilizer response. We need the most accurate tests available."

The wet test is gaining some traction after being reintroduced to the industry last year with the opening of Solum. To conduct the slurry wet test, a weighed sample is combined with water and the same traditional extracts used in routine soil tests.

Koshnick explained that soil scientists have recognized the key is not drying the soil to gain more accurate results. "Results from the dry test are more variable and uncertain and potassium responses are more unpredictable. Natural potassium levels have an effect as well," he added. "When levels are high, the difference between a wet and a dry test is minimal. When native potassium levels are low, the difference between a wet and a dry test are noticeable."

Any time a new soil test is developed, it has to be calibrated. Mallarino explained to DTN that calibration establishes the relationship between a given soil test value measured by the laboratory and the probability of a yield response from adding the nutrient as fertilizer. "When soil tests are in the low or very low range, it tells you how much nutrients to apply to get maximum yield," he said.

Mallarino has been using the wet test to write a calibration for Iowa soils and expects that information to be released yet this fall. "With the wet test, we are developing a tighter calibration curve that better predicts a yield response, especially in the very low, low or optimal (medium) categories. This better predicts an actual yield response," he said.

Farmers need to consider a few things when choosing to run the wet test. "The amount of extracted potassium is lower for the moist test than for the dry test at values usually optimum for crops or lower," Mallarino notes. "But the difference narrows as (soil test) levels increase." He added that labs will still use a dry test to analyze for other important nutrients such as phosphorus, calcium, magnesium and micronutrients.

The west test for potassium may be considered an inconvenience for many of today's soil laboratories. It's an extra procedure and it will also likely add to the cost of analysis. Still, with potash at $600 to $800 per ton, getting the most accurate measurement makes a lot of sense.

(PS/AG/CZ)

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