Why do fertilizer recommendations differ from lab to lab?

Here’s an article that helps explain why different soil labs offer different fertilizer recommendations: "Fertilizer Recommendation Philosophies."

Basically, there are two key reasons that account for the differing recommendations: "Farmers receive varying fertilizer recommendations depending on which lab they consult because labs (1) employ different chemical methods and procedures to analyze the samples and (2) subscribe to different fertilizer recommendation philosophies."

The article then goes on to compare and contrast three of the major philosophies:

1. Sufficiency level of available nutrients (SLAN) or crop nutrient requirement (CNR).
2. Build-up and maintenance.
3. Basic cation saturation ratio (BCSR). 

In the studies mentioned, the SLAN approach is what optimizes profitability while minimizing negative environmental impacts.