How Much Fertilizer Does it Take to Build Your Soil Test Levels?

We all strive to have soil test levels for each nutrient at the optimum level so that nutrients are not a yield  limiting factor, however this is a difficult and continuous project.  The optimum soil test level or “Critical Level” in the chart below is also where it is possible to achieve 100 % of maximum yield.

Chart source: Dupont Pioneer

There is a different Critical Level or Optimum Soil Test Value for each nutrient and these optimums also vary by soil type. Above the critical level a yield response to applying more fertilizer is unlikely. Below the critical level yield loss is occurring and applying fertilizer containing the lacking nutrient will increase yield. 

30% Yield Loss - fertilizer response likely

At Optimum Level

50% Yield Loss - fertilizer response very likely

30% Yield Loss - fertilizer response likely

The Phosphorus (Bray) soil test levels listed above show that the majority of this field is below the optimum range of 44-83 ppm for this sandy loam farm. When your nutrients are below optimum and into the Low or Very Low rating it is equivalent to leaving out every other or every third seed when planting because due to nutrient levels your yield potential is reduced 20 to 50% from the start before weather or pest factors can further affect yield.  The obvious goal is getting to the optimum level for each nutrient, but how much fertilizer does it take to increase soil nutrient levels?  The answer is a lot, and it usually cannot be done all in one season.

To build 1 ppm (part per million)

Phosphorus requires 35 lbs P₂O₅ (actual) which is approximately 67 lbs/ac of MAP (product).

To build 1 ppm Potassium (K) requires

20 lbs K₂O (actual) which is approximately 33 lbs/ac of Potash (product).

These build values are required above crop removal rates in order to see the soil test value of the nutrient  increase 1 ppm.

Building soil test levels can be a long process but definitely pays for itself through yield         response as a result of the added nutrients.

Build values based on OMAFRA research.