Impact of soil sampling and laboratory analysis on nutrient management.

1.0 NM CEU

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Nutrient management is dependent on the collection and analysis of soil and plant samples for nutrient assessment.  Failure to understand soil testing uncertainty associated with sample collection and laboratory analysis can lead to mis interpretation and nutrient management failures.  When collecting a soil one should consider agronomic production practices and their impact on soil spatial variability and analyte measurement uncertainty.  With decreased tillage and transition to strip till systems there is an increase in nutrient spatial and profile heterogeneity.  Generally, phosphorus and potassium show the highest spatial variability ranging from 7-10% on tilled systems, but averaging 40-50% on long term no till systems.  With regard to laboratory analysis soil tests are subject to bias and laboratory measurement precision which varies by the test method utilized and the performance capability of the testing laboratory.  Generally, in North America soil analysis methods that provide the most reliability, based on intra-lab uncertainty are: pH, buffer pH, electrical conductivity, nitrate, chloride and potassium.  Those with the least reliability are boron, calcium carbonate and soil organic matter.  It is worth noting that lab uncertainty increases for samples in the deficiency range (i.e. low NO₃-N, PO₄-P and micro-nutrients).  Laboratory clients selecting an analytical laboratory should only consider labs which have an effective quality control program, actively participates in an accredited proficiency testing program, and show a sincere interest in providing a high standard of analytical testing.