The terminology associated with soil fertilization can be confusing at times if you don’t work with it on a regular basis.  Midwest Land Grant University’s such as University of Illinois, Iowa State University and University of Wisconsin all employ the sufficiently level theory of fertilization which establishes ideal soil test levels utilizing fertilizer rate trials conducted within each given State (Graphic 1).   

For nutrients like phosphorus (P) and potassium (K), the “Critical or Build-up level” and the “Maintenance level” are determined through field research across multiple years, soil types, and growing environments.  The maintenance range is that at which additional P and K fertilizer applications do not result in any additional yield gain.  At soil test levels less than this point, fertilizer is added to the soil to build the test level past the Critical level or build-up range.  Below the Build-up level yield suffers.   When soils test at levels above the Maintenance level, no fertilizer needs to be applied and soils are in the “Drawdown Range”.  Regardless of what State’s fertility program you follow, it takes a certain number of pounds of applied fertilizer to build soil test levels by 1 unit, whether measurements are in part per million (ppm) or lbs. per acre.  Those numbers for phosphorus as P2O5 or potassium as K2O are respectively 9 lbs./A P2O5 and 4.5 lbs./A K2O.  The dynamics of needing to apply more to build a soil 1 unit of test level makes it an expensive proposition to forgo the minimum application of removal rate fertilization (maintenance). This is due to fertilizer purchased in the past, most of the time, being lower in cost than the replacement price today. Maintaining soil test levels to the sufficiency level assures the necessary balance of plant available fertility in the soil solution to maximize yield potential. 

While fall fertilization budgets are tight, the focus should still be on Return On Investment (ROI) by putting resources where necessary with the goals of maximizing yield.  If financial need exists, consider drawing down areas of the field where soil test levels far exceed the sufficiency level, while still building areas where below sufficiency test levels are measured.  Apply crop removal (maintenance) levels of fertilizer to all areas of the field based on previous yield using yield maps normalized across several years.  Budget by the fertilizer dollars for the whole operation versus fixing on maximum dollar amount that you can spend per acre.  Following these practices will allow you to shift fertilizers dollars to where they are needed while maximizing yield potential. For more information on fall fertilizer strategies or how to maximize your yield potential while staying on budget contact your FS Crop Specialist.