Conclusion

Even though it took me years to figure it out, the approach is fairly simple:
  1. Explode concentrations to balances of concentrations
  2. Identify a healthy zone and map a model to it
  3. Run the model to predict the yield class of new value
  4. If unhealthy, identify the target zone
The tricky part is the one after step 4. Correcting a plant ionome will need precise fertilization with the right chemicals and, above all, the right professional agronomist. The next article shows how to do these computations in the R language.
Oh, yes, this concept could be also applied to your blood test. Just saying.