Yield Point Phosphorus Crop Removal  (lbs actual P2O5)

Potassium

Crop Removal (lbs actual K20)

133 bu/ac 80 173
188 bu/ac 113 244
260 bu/ac 156 338
312 bu/ac 187 406

Often when we are reviewing soil test results with farmers our recommended actions include variable rate application of fertilizer. Those recommendations often lead to the question: Is variable rate really worth it?  We say if your field has variability, yes.

Let’s take a look at our corn yield map from last fall and some notes about this field’s characteristics to use a reference for the discussion.

The four yield points selected across this field have a 179 bu range, from the low at 133 bu/ac to the high at 312 bu/ac.  The total crop removal rate by corn for Phosphorus (P2O5) is 0.6 lbs/bu and Potassium (K20) is 1.3 lbs/bu.  Therefore, across this field there is a significant difference in the amounts of Phosphorus and Potassium removed in each of the yield zones.

This field variability in yield is very common and can result from many factors  including those indicated on the example map: soil changes (eroded side hills vs  loam), management practices (starter only strip vs in-season Nitrogen), and  abundance or lack of field tile.

 If the amount of nutrient removed by each crop is variable it would make sense to  variable rate apply the nutrients you replace on that field instead of using a straight  rate across a field with variability because a straight rate is only going to exaggerate  the nutrient variability over time.  With a straight rate application on a variable  yielding  farm, over time the nutrient levels will fall in the high yielding areas—  eventually  causing yields to become stagnant or even decrease, and nutrient levels will increase  in the low yielding areas where there is not as much removed by the crop, thereby  increasing the risk of nutrient loss through erosion or leaching.  

The debate then moves to the capabilities of the application equipment.  Yes, there is still a long way to go to be able to variable rate spread fertilizer in less than 40 to 60 feet boom widths and have rate transitions occur instantaneously instead of taking a few seconds and 100 feet to change the rate.  But given the huge advances in machinery automation in the last 5 years, we truly believe that these equipment shortcomings will be overcome in the not too distant future. 

The variable rate debate comes down to making a decision on each farm: straight rate or variable rate?  Based on each farm’s characteristics, you must weigh the risk of applying a single rate and not applying enough in the high yielding areas and too much in the low yielding areas vs variable rate applying and the equipment limitations resulting in over or under application in transition zones.