Brent Swart scouts a soybean field at his farm near Spe

Brent Swart, an ISA director from Spencer, scouts a soybean field at his farm. (Joseph L. Murphy/Iowa Soybean Association)

Dry weather may skew soil sample results, experts say

October 22, 2020 | Bethany Baratta

Soil samples collected following the dry conditions this summer—and prior to significant rainfall—may result in lower than expected soil test results for phosphorus (P), potassium (K), and pH.

Those soil sample results, therefore, should be interpreted with caution, according to Antonio Mallarino and John Sawyer, agronomy professors at Iowa State University. Mallarino and Sawyer published a guide to help farmers and agronomists think about how best to interpret those results, especially in areas where drought was apparent.

The guide is available here.


Less nutrient recycling

Normal rainfall leaches plant nutrients into the soil after plants mature (from standing vegetative plant parts) and from crop residue after harvest. This year, Mallarino says the persistent dry weather since late August in some areas may greatly reduce mainly the amount of plant potassium recycled to the soil.  Therefore, unless there is rain before soil sampling (a couple of inches or so)  soil test results will likely show lower amounts of potassium due to less recycling to the soil and less replenishment of the easily exchangeable soil potassium from other soil pools, according to the agronomists.

ISA District 5 Director Tom Vincent conversed with his agronomist a few weeks ago about this very topic. Vincent farms near Perry, which was in the heart of the drought this year.

“We caught a rain before we headed to the field for harvesting,” Vincent said. “Our agronomist thought the rain we got was probably enough for us to pulls samples to get an accurate picture of potassium levels.”

As Mallarino and Sawyer allude to in their publication, drought conditions also result in lower soil pH than with normal rainfall.

An agronomist holds a soil sample in the field.Soil samples provide a great snapshot of the growing conditions in fields. (Joseph L. Murphy/Iowa Soybean Association)


Don’t skip testing

However, says ISA Agronomist Drew Clemmensen, this is usually a minor discrepancy and is not a valid enough reason to skip soil testing altogether.

In addition, Clemmensen suggests farmers think carefully about their post-harvest tillage practices.

“In an attempt to conserve as much moisture as possible, farmers should carefully weigh their tillage options this fall,” he said.

Vincent said he’s made the decision to reduce tillage this fall.

“We’re going to leave the ripper in the shed this year and try to do strip tilling as much as we can,” Vincent said.

Drew Clemenson, an agronomist with the Iowa Soybean Association, collects soil samples a conducts trials to determine the best agronomic practices for farmers facing drought conditions. (Joseph L. Murphy/Iowa Soybean Association)

Staying on schedule

ISA District 1 Director Brent Swart, a farmer and Pioneer agronomist near Spencer, recommends that farmers stay on their soil sampling schedule as a basis for comparison. Soil sampling in the spring versus the fall can further skew soil sampling comparisons due to precipitation.

Farmers should continue to work with their agronomists to determine how to account for a soil test result which may be different than results in previous years, Swart said.

“In the end, with the technology available to you, soil samples shouldn’t be the only variable that you’re using to base your input decisions,” Swart says.

He says yield maps and nutrient removal rate guides also help farmers decide where they need to add fertilizer. But there’s also a farm budget to help guide input decisions, as well as the art of using past experiences and yield information to decide what’s best for the farm.