Potassium vs. Phosphorus: What Your Crops Need Most — and When
Farmers know that nutrients are important to crop growth. However, in this era of tight operating budgets, some may feel forced to choose between them. According to Carrie Laboski, Soil Fertility and Nutrient Management Specialist at the University of Wisconsin, potassium could be more yield limiting than phosphorus in corn and soybean production than what was previously understood.
Meaning that for some soils, potassium could reap more economical returns than expected.
Laboski launched a study in 2011 to update potassium and phosphorus rates for newer corn hybrids and soybean varieties. After analyzing her initial results, her research actually led her to broader insights: she was able to uncover more about the importance of potassium in corn and soybean production.
For her research, she used fields at the University of Wisconsin Agricultural Research Station in Arlington that were testing very low for potassium and phosphorus. Laboski then applied different rates of potassium and phosphorus. For potassium, she used rates of 0, 40, 80, 120 and 160 pounds per acre. At each potassium rate, she applied 0, 30, 60 and 90 pounds per acre of phosphorus.
Results Support Potassium’s Role in Increased Yields
The study was conducted from 2011 to 2016 and the results indicated substantial yield increases of applying potassium every year for corn. Even in the drought year of 2012, Laboski experienced a yield increase of 80 bushels per acre for corn. In other years, the yield increase averaged 200 bushels per acre compared to areas where no potassium fertilizer was applied.
Also, soybean yields increased on average 25 bushels per acre compared to those where no potassium fertilizer was applied. This does not include the drought year of 2012, which had a seven bushel per acre yield increase.
Potassium in Relation to Other Nutrients
Corn yielded 10 to 50 bushels per acre when no potassium was applied on very low potassium testing soils. On similar soils, without applying nitrogen but with adequate potassium and phosphorus levels, she says corn yields would likely be 100 to 150 bushels per acre.
During the research, Laboski found a yield response to applying phosphorus in soybeans when soil test potassium levels were over 75 ppm in 2016 only. Soybean plot photos show that plots with no added potassium and higher levels of phosphorus did not close the row and yielded smaller soybeans at harvest. Soybean plots with no phosphorus and higher levels of potassium closed the row and had higher yields.
There are a couple of possible explanations for the lack of a response to phosphorus in soybeans. One is that potassium soil test levels might need to be higher to see a yield response. Moreover, this research also suggests there may be an interaction between potassium and phosphorus impacting yields. For example, in 2014 the corn plots showed a yield response from applying phosphorus, but only in plots that had 80 pounds per acre of potassium applied per year and which had soil test potassium levels greater than 65 ppm. From this situation, Laboski concludes that for corn, potassium might be more yield limiting than phosphorus. In addition, she thinks potassium levels need to be adequate before a phosphorus response might be found.
Future Research on Potassium’s Role in High Yields
Laboski’s results show that potassium may be more limiting for corn than soybeans. A phosphorus response in soybean was observed in 2016 in plots where soil test potassium was at least 75 ppm. However, in the previous growing season there was no yield response to phosphorus even when soil test potassium was greater than 75 ppm. Thus, yield responses for phosphorus might only happen after potassium levels are eliminated as a limiting factor and appear to be dependent upon growing season weather.
Laboski is continuing her studies at the University of Wisconsin to gain more information about phosphorus and potassium yield responses.
“This research shows how important potassium is for corn and soybean production,” she explains. “Sometimes potassium does not get the attention it should, but this research shows it is key to increasing yields.”
For farmers with limited budgets, she says this means they might be better off if they prioritize their investment in potassium. To ensure adequate applications are being made, researchers across the Midwest are continuing to update the potassium recommendations. When determining application rates, farmers should use soil tests to determine their soil fertility levels and tools like the eKonomics ROI calculator 2.0 to help them estimate their nutrient needs for their corn and soybean crop.
