How Much Phosphorus And Potassium Are You Really Losing?

eKonomics News Team

Figuring out removal rates for phosphorus and potassium in grain might not be as cut and dried as you think. Usually, one doesn’t need to replace the exact amount of nutrients that yield calculations show were removed. University of Minnesota’s Dr. Dan Kaiser recently discussed the relationship of crop phosphorus and potassium removal to soil tests and crop response. Here are some principles to help understand the basics.

Phosphorus and potassium in your grain can vary

• One must take multiple grain samples each year to know exactly what nutrient quantities are removed from the field.
• Studies and tests can give you a range, but keep in mind those numbers are estimated ranges and not exact.

Your soil test only measures a portion of the phosphorus and potassium in the soil

• Soil tests measure a portion of soil phosphorus and potassium described as “available,” but this is only a portion of the total soil nutrients and an index that relates to what is available.
• Nutrients in the soil are found in different forms, or “pools.”  Some pools provide nutrients in the short term, while others are medium and long-term mineral or organic reserves.

The rate soil tests change isn’t always linear

• The rate soil tests change with crop nutrient removal can vary based on soil minerals, organic matter, biological activity, crop rotation, tillage, and fertilizer application history.
• Soils may only need a portion of what is removed to be reapplied, so one must continue to monitor soil levels with soil testing.

Adding phosphorus past a certain point has diminishing returns

• As soil-test values increase, the ability of the soil to give crops what they need increases until applied fertilizer is only replacing what is taken up by the crop.
• The higher the soil test value, the lower the chances of an economic return on added fertilizer.

Soil sampling is essential

• Excess nutrients incur unnecessary costs and can lead to environmental risks.
• Regular soil testing should help guide fertilizer applications to apply what is needed for good yields while avoiding the risk of economic and environmental losses from excess application.
• Your soil can be used to “bank” certain plant nutrients, but the bank does not benefit from having more nutrients than what is needed by your crops.  

What does it all mean?

Soil-test levels are not likely to decline if phosphorus and potassium applications are within 10 to 15 pounds of calculated removals, but don’t stress over exact numbers too much. And if anything, don’t overdo it, as that would likely just be wasting nutrients that can wash away. Follow the 4R Nutrient Stewardship tenet of applying the right rate, which will prevent excess applied fertilizer while maximizing the return on that investment.    

Dig deeper into the science and studies behind phosphorus and potassium removal by checking out the original source for the above information:

5 Things to Know About Removal-Based P and K Strategies. Published by University of Minnesota.