ESTA® Kieserit FINE is a sulfatic magnesium and sulfur fertilizer. About K+S. Our Business & Products . Sustainability. Investor Relations. Newsroom. ... Contains only sulfatic sulfur and magnesium of natural origin (kieserite). ... ESTA® Kieserit 26 fine is magnesium sulfate for further processing or direct application. 26%. MgO. 53%. SO 3.