Over the past week I have been asked a few times about what the correct rate of potassium fertilizer would be to apply this fall to prepare for next year’s crop. There is no doubt that the exceptional weather for fall field work and impressive crops across the province have led growers to wonder about this very same question. My first response is always to ask for a soil test report. If a report is not available or not current, you need to have one completed as soon as possible.

I have observed some concerning and surprising numbers from a few of the reports I have seen. If your potassium (K) soil test levels are declining or have reached 100 PPM or less, you will see an economic response to applying potassium fertilizer. If you are at these levels or lower, it’s also very likely you are experiencing problems this fall related to standability, disappointing yields and poor grain quality. Even if your hybrids performed well, not making a plan to correct this problem will undoubtedly lead to crop losses in the future if test levels are limiting.

Potassium is known as the quality nutrient and is critical in nearly all areas of crop development; it’s needed to sustain plant growth, reproduction and overall health. Potassium is directly influential on grain quality, stalk strength, and yield. Plants deficient in potassium are also less resistant to environmental stresses.

Potassium removal from grain corn is generally around 0.28 – 0.30lbs/bu. Likewise, a crop of soybeans can remove up to 1.3lbs/bu. That means that on a 200 bushel corn crop, your grain is removing approximately 60lbs of potassium, and a 60 bu/ac soybean crop can remove as much as 78 lbs/ac of potassium. With average yields this year trending higher, you can expect that you will need to apply additional potassium to cover your removal. If your soil potassium (K) test levels are falling below 100 PPM, you will need to adjust your crop plan and increase your application rate to rebuild your soil reserves. Where soil tests indicate that large amounts of potassium are required, the major portion may be broadcast and incorporated in the fall or spring, however, in the case of lower soil test levels, you would also likely benefit from having some in a band close to the developing seedling within maximum safe rates.

The bottom line is you need to know your current soil test levels. You need to include your crop removal. Plan your fertility program as needed, and plan to build your potassium levels if they have fallen to a point where the nutrient is limiting.

Dave Emery, CCA-ON
Maizex Seeds District Sales Manager, Southwestern Ontario
Twitter: @emeryda