During this harvest season there have been some exciting reports of some corn and soybean fields with pretty high yields.  However, with the low commodity prices, some farmers are considering holding back on fall fertilization. While there is still time, take a step back and think before carrying through with what could potentially be a yield limiting decision.  Consider these facts before moving forward! 

The science of soil fertility teaches us that for every one unit test level we want of phosphorus and potassium it takes 9 lbs/A P2O5 and 4 lb/A K2O respectively.  For example if we want to move our test level from 30 lb/A to a level of 40 lb/A for P2O5 it will require nine times the P2O5 investment to get back to that number.  This can make soil test drawdown an expensive practice.

Big yields this year mean that there was a big draw from soil resident fertility.  Using University of Illinois nutrient removal numbers, which are close to surrounding states, for every bushel of corn harvested, we can expect 0.37lb/A P2O5 and 0.24 lb/A K2O removal.  For soybeans those numbers are .75 lb/A/bu  P2O5 and 1.17 lb/A/bu K2O.  Table 1 illustrates corn removal rates at various yields, while Table 2 does the same for soybeans.

Large yields this year have already relied on the pools of P & K available in the soil. In cases where P & K levels were just into the “sufficient” range, as determined by your individual state, high yields followed by normal yields (next year) could reduce soil test levels significantly. While it may be a year to re-think fertilizer strategy, it rarely pays to forego all fertilization.  Where grid samples have been taken consider added redistribution of fertilizer to meet the customer’s budget.  Consider no application where grid results measured well into the “drawdown or high to very high” range.  To avoid yield loss, treat areas of the field needing build-up application according to where they stand in the build-up sequence since last testing.  Apply removed fertilizer to those parts of the field needed according to last soil test level; those on the low end of sufficiency as defined by your state. Remember to leave a margin of error to account for additional nutrient reductions by this season’s crop yield.   Finally, scrutinize those practices that provide unproven yield results by Land Grant University’s.      

Depleting soil test levels on a complete field basis can be costly to rebuild and may result in incremental yield loss.  Using the technologies at hand for better fertilizer placement according to need and potential budget constraints is a much better approach to fertilization.