Soil results could signal new approach

Frozen ground in December marked the end of fall-2024 soil sampling in the Midwest. But soil-analysis results are continuing the story that the year told – including noticeably increased phosphorus, potassium and organic matter across enough samples to make wondering minds explore.

“Overall soil-test phosphorus and potassium are higher (in 2024) than in 2023 and much higher than 2020 – the (previous) sampling event for most of this year’s fields,” said Dustin Sawyer, lab director for Rock River Laboratory. “Our suspicion is sample depth, as the 2024 fall soil was dry and hard, making it difficult to collect a full soil core.”

Even if a soil probe goes the full 8 inches for a complete core, in such dry and hard soil the bottom inch can fall out when retrieving the sample, he said. Or the soil becomes compacted at the bottom of the hole, rather than captured in the probe.

The analysis of shallow samples yields increased phosphorus, potassium and organic-matter results because they accumulate in the upper regions of the soil profile. Lower pH is also indicative of shallow samples because of surface acidification – which explains the 0.1 unit lower pH seen in soils analyzed in 2024 by Rock River Laboratory, as compared to 2020. That sort of soil-nutrient stratification occurs because fertilizers and other soil amendments are applied to the surface and need to work downward into the profile.

asonal dryness and drought are different, and play differently on the environment.

“Seasonal dryness is much different than drought and each impacts the soil differently,” Sawyer said. “A prolonged drought acts on the soil chemistry and ties up potassium on the exchange sites while increasing salt content.”

The late-season dryness of 2024 was a double dose of different challenges. It was difficult to pull a full core, and the lack of precipitation meant nutrients didn’t move down into the soil profile.

For those operations that are trying to overcome such challenges, now is a good time to review soil-analysis management – and any necessary changes to the plan. Soil testing is not intended to track nutrients through time, but rather to predict the likelihood of crop response to added fertilizer in the coming crop year.

“A soil test is a snapshot of the soil nutrients, and as such, weather and soil biological activity have a significant impact on the results,” he said. “Unexpected analysis results can happen, and if that occurs while a farm is using the minimum recommended testing interval of once every four years, there is no time to resample in different conditions.”