Soil Analysis: ALP Program Inter and Intra Laboratory Method Performance.

Results from nine years of the ALP program utilizing 135 soils collected across fifty-six US states and Canadian provinces show that specific soil analysis methods such as pH and NO3-N show high stability across 102 commercial soil testing labs in the US.  Phosphorus and potassium soil test methods are inconsistent across labs for specific soils and inter-lab variability averages 15-20%.   Micro-nutrient soil test methods are consistently the most variability and for specific soils may exceeded 40% variation in the lab testing industry.  CEC by displacenment shows high inter-lab precision, whereas apparent CEC by sum of shows poor precision and high bias relative to the former method.  Generally soil organic matter inter-lab varation average 15-20% for soils with 1.0-5.0% SOM.  Results for intra-lab variation show 5-10% of the commercial labs have precision issues for specific methods.