How to calculate NPK & Calcium, magnesium and Sulfur in fertilizer

Calcium (Ca), magnesium (Mg), and sulfur (S) are the three secondary nutrients required by plants, but less likely to be added as fertilizer as N, P or K. A standard soil test measures the relative availability of Ca and Mg in soils. There is no accurate soil test for S at this time. A plant analysis is the best diagnostic tool for confirming S availability.

   Excessive use of K fertilizers can greatly reduce the uptake of Ca and Mg. Forage with high K or low Mg concentrations can cause grass tetany, milk fever, hypocalcemia, and other health problems for ruminant animals. 

   Potassium recommendations above the critical level are less than crop removal so as to discourage excessive uptake (i.e., luxury consumption) of K and improve Mg uptake. This can sometimes result in fields rotating out of alfalfa to have depleted soil test K levels.