Research and understand what fertility each crop needs to produce X number of bushels. Grid sample your fields and run full analyst not just N, P, and K. Also understand what nutrients are mobile in your soils and what nutrients are immobile in your soils. Nitrogen is mobile in soil thus you shouldn't try to build residual levels. Phosphorus is immobile in soil and residual levels can be built. These are factors you can control but there is a big you can't, Mother Nature. Control what you can and set yourself up for success.