PO 19. Recognize how the following affect soil test interpretation.

As a first requirement, a good soil testing laboratory needs to have a good quality control system in place.  However, even high-quality laboratories can give different results if you were to split samples and send subsamples to different laboratories.  This is because soil testing laboratories can differ in:

1. Nature of the extract used (i.e. soil tests for P - Morgan, modified Morgan, Mehlich-3, Bray-1, Olsen)
• These different methods were developed for specific purposes (Bray for low-pH soils, Olsen for calcareous soils, etc.) and results from methods can only be equated to each other if reliable conversion equations exist.  CAFO plans in New York limit lab choice to those that have developed conversion equations to Morgan equivalents
2. Shaking time
3. Solution-to-soil ratio
4. Analytical procedure and instruments used
5. Way of reporting results (ppm or lbs/acre, P or P₂O₅)

The table below shows the soil test P results made by different laboratories on identical soil samples, and compares the different soil test methods.  Again, be sure to always use the same laboratory to avoid improperly fertilizing your soil!