By: Dustin Sawyer, MS
Frozen ground in December marked the end of Fall 2024 soil sampling in the Midwest US. But soil analysis results are continuing the story that this year told - including noticeably higher phosphorus (P), potassium (K), and organic matter (OM) across enough samples to make wondering minds explore. Overall soil test P and K are higher this year than 2023 and much higher than 2020 - the last sampling event for most of this year’s fields. 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 the soil probe goes the full 8 inches for a complete core, in such dry, hard soil, the bottom inch can fall out when retrieving the sample. Or, the soil gets compacted at the bottom of the hole, rather than captured in the probe. The analysis of shallow samples yields higher P, K, and OM results, as these 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 by Rock River Laboratory in 2024, as compared to 2020. This 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.
Seasonal dryness and drought are different and play differently on the environment. Seasonal dryness is much different than drought and each impacts the soil differently. A prolonged drought acts on the soil chemistry and ties up K on the exchange sites while increasing salt content. Meanwhile, 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, it’s a good time to review soil analysis management - and any necessary changes to the plan. Soil testing is not intended to track nutrients over 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. 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.
If tracking nutrients is an operation’s goal, testing at least every other year, not every four years, will produce a better picture. More frequent soil analysis would help overcome such odd weather challenges, as these data points could be removed without a scramble - especially in the case of permitted operations. An even more intensive and accurate testing protocol is testing every year, which would offer the opportunity to manage land off of the four- or five-year rolling averages for a comprehensive understanding of how nutrients are changing over time. In the case of operations guided by Nutrient Management Plans (NMP), I suggest yearly analysis or every-other-year analysis to secure enough information to avoid environmental challenges.
The environmental ramifications associated with the quality of a soil analysis are considerable, as key decisions hinge on this data. Securing more data can paint a more informed picture to base such decisions - reaping rewards for both growers and their land’s long-term sustainability. Winter is a great time to plan out soil analysis frequency for the coming years and determine what decisions can be accurately made with the data available as the plan is carried out.
