When are secondary nutrients or micronutrients needed for tennessee farm fields?
Savoy, H. J.  2013.  Extension, Extension special publication 645.

Abstract:
This fact sheet provides a listing of secondary and micronutrient soil tests that are available through the University of Tennessee Soil, Plant and Pest Center in Nashville (http://soilplantandpest.utk.edu/planttissue.html). It also summarizes all of the university’s secondary and micronutrient recommendations and guidelines based on current research. Some recommendations are based upon soil test values (Table 1) interpreted as either satisfactory (levels adequate for excellent crop production) or unsatisfactory (levels indicating a need for fertilization). For other micronutrients, such as boron or molybdenum, a general recommendation is made for those crops observed to respond consistently to such fertilization. For copper, the soil test is currently only used to monitor changes in soil copper levels, especially where manures, biosolids or byproduct materials are being utilized. A general discussion is provided for sulfur, as it is often included in fertilizer blends, but seldom increases yield in Tennessee. Soil testing along with a plant analysis will give a better diagnosis of the need for sulfur application. A weak acid extractant called Mehlich 1 is used by the University of Tennessee to test soils for nutrient levels. Critical secondary and micronutrient soil test values (the soil test value below which a recommendation for use of the nutrient is warranted) listed in this publication are only valid when the laboratory testing your soil uses the Mehlich 1 soil test for phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), zinc (Zn), manganese (Mn), iron (Fe), copper (Cu), and boron (B) or the ammonium acetate extractable sulfate-sulfur (SO4) soil test.