After 20 years of adding fertilizer upon fertilizer, I wouldn't be surprised if your soils had some mineral excesses and deficiencies.
Haddii dhirtaagu sii korodho ciidda isla muddada afartan sanno, haddana maaha mid la yaab leh in dhirta caleenta caleemaha ay noqoto jaale iyo qalalan, iyo kuwa cusubi ma koraan.