The water-soluble phosphorous and part of the structure-soluble phosphorous are extracted from ordinary calcium superphosphate by aq. soln. of potassium sulphate acidified by phosphoric acid. After filtering, the extract is neutralized to PH 4-5 by adding aq. soln. of potassium hydroxide or ammonium hydroxide, then concd., and dewatered to obtain said solid compound base fertilizer. The total content of N-P2O5-K2O of the present base fertilizer exceeds 68%, so it is a high concn. fertilizer, and can be easily formulated with other industrial products to make a series of commercial multifunctional N-P2O5-K2O compound fertilizer. |