ST. PAUL, Minn. – Targeting optimum fertilizer rates is always important, and it becomes even more important as margins get tighter. Recently, the University of Minnesota reevaluated their nitrogen recommendations to better serve growers. Now, as they have collected more data, they have revisited the recommendations for potassium.
“We have been working on a number of studies since 2011 to evaluate our current [potassium] guidelines,” said Dan Kaiser, associate professor with the University of Minnesota Extension, during a recent phone interview. “The main one has been looking at soil test methods we are using right now, whether or not they are accurately predicting where potassium is needed.”
The recommendation has always been, if there is greater than 160 part per million of potassium on the soil test, that field has adequate potassium. Adding more potassium to those fields was not expected to increase yield.
Surprisingly, the data coming from both Minnesota and Iowa suggests this is not the case.
“Soils in the south-central part of the state and these poorly drained, higher clay soils, we are encountering some issues,” said Kaiser. “There are instances where we are likely overestimating potassium availability in some of these fields.”
Across the state, the university has several test fields and studies going on. They are evaluating the correlation between soil test results and yield response. They are also looking into building potassium levels in different fields.
Over the past year, Kaiser has been digging into all this data, compiling it and reviewing it. Then applying it to the university’s recommendations.
One area of focus was the high clay soils and issues that were seen in those fields.
“That necessitated for us to raise what we had defined as the critical level,” he said. “So now, instead of 160 parts per million on some of these soils, we would say, poorly drained, higher clay soils, south central, south western Minnesota, use around 200 parts per million.”
The main reason for the increase is the uncertainty around the soil test itself.
When a lab processes a soil test, the soil is dried. There is evidence to suggest that the potassium levels are impacted by the drying process in poorly drained, high clay soils, where the potassium soil test can be high, but deficiencies may still occur.
“We have been looking at determining potassium levels of a field using moist soil samples, where the soils samples are not dried ahead of being analyzed,” he said. “It does help a little to kind of clean up the variations across locations, in particular, it tends to better estimate potassium availability in many soils and tends to lead to a more accurate estimation in high clay soils.”
The trouble is that not many labs can do a moist soil test. Also, the test is more labor intensive which will increase the cost of analysis.
“We are looking at whether or not there are some alternative options to it at this given point in time,” he said.
The other change to come from the research impacts soils on the opposite end of the spectrum, the sandier and typically irrigated soils.
“We are seeing less of a need for potassium in some of our very sandy soils – the soils that are 90 percent sand or more,” said Kaiser. “The critical level is likely lower.”
Kaiser has yet to work this information into the university’s recommendations. There are still a few questions that need to be answered before that change is made.
These soils already tend to be very low in potassium, so there is concern that reducing fertilizer applications could affect yield, and more data is needed across a variety of sandy soils.
Kaiser does point out that in these sandy soils, farmers should not worry too much about building potassium levels.
“Potassium can leach. The cation exchange capacity of very sandy soils is low which impacts the amount of potassium which can be held in the soil,” he said. “So, it doesn't really pay to put a high rate down because it will likely leach deeper into the soil profile.”
Growers can find university recommendations for potassium and other nutrients online at extension.umn.edu/nutrient-management.