Advances in soil-testing research has led Iowa State University Extension agronomists to revise recommendations for phosphorus, potassium and lime.

"Field research is conducted continuously to assure that nutrient management suggestions are up to date," says Antonio Mallarino, an ISU Extension agronomist and professor of agronomy. "This research has indicated some recommendations should not be changed, but other recommendations needed significant change to optimize nutrient management to improve the profitability and sustainability of crop production."

NEW SOIL FERTILITY GUIDELINES: As postharvest soil testing and fall fertilization season begins, farmers and fertilizer dealers need to be aware that Iowa State University recently announced changes to its potassium, phosphorus and lime recommendations.

He and John Sawyer, also an agronomy professor and ISU Extension agronomist, have updated the extension publication "A General Guide for Crop Nutrient and Limestone Recommendations in Iowa" (PM 1688). It's available online for free at the Extension Online Store. A discussion of the changes may be viewed online.

What did they change? Mallarino says the most significant changes are:

* Include interpretations for the new moist- and slurry-based test for potassium (K)

* Changes to soil-test interpretations categories for K using dried soil samples

* Adjustments to both crop nutrient concentrations and default crop yields needed to estimate nutrient removal for maintaining soil-test levels in the optimum category

* Discontinued using the P and K subsoil categories for interpretations

He says recent research showed the moist- and slurry-test for K is more reliable at assessing K fertilization needs of crops than the commonly used test based on dried soil samples, even with the improved interpretations for the dry test.

What stayed the same? The ISU agronomists maintained many of the current recommendations farmers use to determine soil fertility, including:

* The general concept of phosphorus (P) and K recommendations are for long-term profitability and reduced risk of yield loss, by emphasizing crop response-based applications for the very low and low soil test classes, and removal-based maintenance based on estimated crop removal with harvest for the optimum soil test class

* Interpretation categories for current tests, such as Bray-P1, the colorimetric version of the Mehlich-3 test, and the ICP (inductively-coupled plasma)

* Amounts of P and K recommended for grain production in the very low and low soil test interpretation categories~~~PAGE_BREAK_HERE~~~

* The soil pH considered sufficient for crops

* Interpretations for micronutrients, which currently include only recommendations for zinc (Zn) in corn or sorghum. Ongoing research studying several micronutrients for corn and soybean has not been completed.

ISU releases new potassium recommendations for field-moist soil measurements

While it has been known for decades that drying can change the chemistry of soil, altering the amount of extractable potassium (K), it's just been in the last two years that Solum entered the market with a commercially viable process for measuring soil in its field moist state.

The new potassium recommendations document from ISU notes, "Research has indicated that the moist K test is more reliable than the test based on dried samples and is a better predictor of crop K fertilization need."

The new test categories are as follows:

 Speaking at a mid-year conference recently, ISU Extension soil scientist Antonio Mallarino, reminded the audience that soil test interpretation classes should be understood in terms of probability of response. "The amounts of K extracted can differ greatly between moist and dried samples, and the differences change greatly across soil series, the soil-test K levels, and soil conditions related to drainage and moisture content cycles." While other tests are still available for potassium, he noted that they encourage the use of the moist test for K because it has more predictive value and a more consistent meaning across different soils and conditions.

Solum Inc. opened a moist soil measurement facility in Ames, Iowa in 2012 to offer this test and other advanced measurements. In collaboration with Winfield Solutions, Mosaic, and other industry partners, Solum has conducted trials in more than 15 states and four countries. For more information about field moist or other soil measurement needs, contact 1-855-GO-Solum (855-467-6586) today.  

About Solum: Solum is making the business of agriculture simpler and more productive. The firm develops software and measurement technology to help growers and their crop advisers manage farming more efficiently and achieve higher crop yields. Solum is headquartered in San Francisco and has a soil measurement lab in Ames, Iowa. For more information, visit the Solum website.