I started out the same, although mine had regrettably NOT been left alone for 40 years. It had been corn field, so every resource was thoroughly tapped. My first soil test more or less came back with, "What have you been doing?" written across the top.
I found organics to be the answer, plus judicious use of synthetics to boost specific nutrient levels (like phosphorus, since starter fertilizer is cheaper and more effective than the organic alternatives).