Crop Nutrition Research From the Fields

Why did K levels go down in Fall 2016 when farmers were maintaining or building soil levels?

eKonomics News Team

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You’re not the only one who has reported this phenomenon. I’ve heard this question several times in the last couple of years. While I would struggle to give you a clear reason for your particular scenario, I can share some potential causes of decreased soil test potassium where building has occurred.

Let me first start with soil sampling strategy. I’m not sure what approach is being employed – grid, zone, grid/zone, etc., or the resolution of the sampling – 10 acres, 5 acres, 2.5 acres, etc. One thing I caution farmers and consultants about is the number of samples that are collected to represent the area of interest. Too few samples to constitute a composite (which commonly occurs at more intense sampling approaches) can have large potential errors. Essentially, the sampler is not collecting a large enough sample to get a good approximation of the mean (see our video on spatial variability). So make sure that you collect an adequate number of samples to get a good representation.

Fall moisture patterns can also dramatically affect soil test potassium levels (assuming sampling occurs in the fall). If soil sampling occurs during a dry stretch, soil test potassium can be decreased (I have seen this myself in field research). Soil test potassium level tends to follow soil moisture. This may partly be explained by decreased residue breakdown during dry periods. Corn stover specifically contains a lot of potassium, but if that residue is not broken down due to limited moisture, less potassium is resupplied to the soil system. This can cause a lower-than-expected soil test level.

Another potential issue is soil sampling depth. This is especially important in conservation tillage systems. Nutrients do stratify in conservation tillage approaches causing higher nutrient concentration at the soil surface. Inconsistent depth of sampling can lead to some really confusing results. Make certain that soil samples are collected to the appropriate depth.

Soil potassium levels do change from one sampling season to another, and sometimes we cannot explain the source of the variation. I say this all the time – I am more interested in changes in soil test levels over a few years than over a single year. If you continue to see soil test levels declining (when you should be supplying enough to maintain/build) then something needs to be adjusted.

The final thought would be – are fertilizer applications actually keeping up with nutrient removal? This can be easily tested by collecting random grain samples and determining the nutrient concentration for your crop (there are book numbers out there, but they are averages).