Potassium magnesium fertilizer

A method for producing a fertilizing double sulfate of potassium and magnesium, comprising producing potassium sulfate by reacting sulfuric acid with potassium chloride followed by reaction with wet methanol yielding precipitated potassium sulfate and dilute sulfuric acid of a concentration of from 20 to 30% w/w from said reaction, producing magnesium sulfate using the dilute sulfuric acid by cavitation-assisted sulfatation of calcined serpentinic silicate, and dry mechanical attrition of the potassium sulfate and the magnesium sulfate, wherein said producing the magnesium sulfate comprises calcining serpentinic silicate, reacting a resulting anhydrous magnesium silicate with sulfuric acid of a concentration of about 20-30% w/w acid under cavitation at a temperature in a range from 30 to 50° C., at a pressure in a range from 20 to 25 psi, neutralizing, filtration and distillation.