Tips for Fall Application of Phosphorus and Potassium

September 9, 2009 at 8:15 am

University of Illinois’ the Bulletin published an excellent article on September 4, 2009 regarding phosphorus and potassium recommendations. I’ll distill that article into key points for you here.  The past three years have seen incredible price jumps in phosphorus and potassium, and even though prices have retreated there is still very high interest in phosphorus and potassium fertilization. In fact, I cannot recall a time when farmers and agronomists have placed so much interest in phosphorus (P) and potassium (K) fertilization.

Over the past few years, there are some familiar questions that I hear over and over;

Do I need to apply P and K?

How much P and K should I be applying?

Which is better, one application every two years or should I apply every year?

Is it better to apply P and K in fall or spring?

The Tri-State Fertilizer Guide tells us that the absolute first place to begin with regards to answering these questions is with a soil test. A soil test is the only tool currently available to let us know the P and K status of our soils as it relates to a crop.

When soil tests are below the critical level (Figure 1), the soil is not able to supply the P and K requirements of the crop. Soil tests below the critical level should be considered as indicating a soil that is nutrient deficient for crop growth and recommended rates of fertilizer should be applied annually. Placement techniques to enhance nutrient availability, such as fall strip tilling, may also be beneficial on nutrient-deficient soils.

Above the critical soil test level, the soil is capable of supplying the nutrients required by the crop and no response to fertilizer would be expected. Recommendations for soil test values on the maintenance plateau are designed to replace the nutrients lost each year through crop removal. Because the purpose of fertilizer applications in the maintenance plateau range is to maintain fertility, no response to fertilizer in the year of application would be expected. Therefore, farmers may choose to make multiple year applications.

When soil test levels exceed the maintenance plateau level, the objective of the fertilizer recommendation is to utilize residual soil nutrients. Fertilizer recommendations are rapidly reduced from maintenance levels to zero. There is no agronomic reason to apply fertilizer when soil tests are above the maintenance plateau level.

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