By Jeff Bubrink
Ag & Natural Resources Extension Educator, Purdue Extension LaGrange County

LAGRANGE — In the past several weeks, I have had several soil test results shared with me. One trend I have noticed is that most of those test results showed phosphorus (P) and potassium (K) tests that were very high or high. With the profitability of corn and soybeans down significantly in 2024-25, there is a real possibility those fields will not need P or K applied anytime soon, saving the farmer some money.

Jeff Burbrink

So, what exactly is a high soil test for P or K? Bear with me, it’s complicated!

Generally speaking, if the lab that does your soil test uses the Bray P-1 method for phosphorus, any test over 30 parts per million is considered high enough to support the corn and beans without additional fertilizer. Some labs use another method called Mehlich 3P. If your tests are over 40 ppm using the Mehlich test, you have plenty of P in your soil.

Potassium levels are a bit more complicated. Potassium can be a very mobile nutrient if the soils are sandy or lack a lot of organic matter. The nutrient holding capacity is measured be a test called cation exchange capacity. A low CEC (5 or less) means the soil cannot hang onto the extra K, so the amount of K recommended annually is less.

Many labs use a test called the ammonium acetate test to measure K. If your soil has a low CEC of 5 or less, any soil test above 115 ppm for K using the AA test require no additional K fertilizer. The Mehlich 3-K test equivalent for that is 131 PPM.

If the soil’s CEC is greater than 5, an AA test of 150 PPM or more for K suggests you will not see a benefit for additional K. The Mehlich 3K equivalent of the AA test is 171 PPM or more.

The bottom line is that fields testing high or very high for P or K are places you might be able to cut some costs in 2025. And in some fields, the level of nutrients is so high that the savings could be calculated for years, not just next year!

Want more information? Download the TriState Fertilizer Recommendations. You will also see they have fertilizer recommendations for alfalfa and wheat also.