Procedures and Cautions

The samples were collected in the fall of 2014 or spring of 2015 and therefore reflect fertility status for the 2015 crop year.

Many soil test procedures are used for P and K determination in North America, although just a few are dominant. In order for data to be pooled among laboratories using different procedures, ranges of agronomic equivalency for each test were defined. These ranges were either taken from the literature or estimated by soil fertility specialists in consultation with IPNI NA or Nutrient Program Directors. In the summary, all soil test data are reported in terms of well known soil test procedures. Procedures used for reporting purposes are Bray and Kurtz P1, ammonium acetate extractable K and Mg, 1:1 soil:water pH, calcium phosphate extractable S, DTPA extractable Zn, and water extractable Cl

A challenge in pooling data from many laboratories over a period of years is to accurately account for changes in extractants, how the extractants are employed in a specific procedure, how the elements are detected in the extracted solution, and finally, how the results are reported to clients. Changes in or miscommunication about any of these steps can result in serious errors in the summary process. The IPNI staff and cooperating laboratories were diligent in maintaining the accuracy of these factors.

Though IPNI attempts to be comprehensive and consistent in conducting the summary and avoid distorting the contributed data in any way, weaknesses exist in the summary process due to the diversity and dynamic nature of soil testing services:

  • Quantity of sample results is low in several states and provinces.

  • Not all sample results could be definitively associated with a particular state.
  • It is likely that the better managers regularly test their soil and that their results may not be representative of those that do not soil test.

  • Due to the requirement of nutrient management plans for many livestock operations, the percent of samples in the summary from manured fields could be higher than in the past for some regions and inflate soil test levels, especially for P. Summary protocol included separation of samples into manured and non-manured fields, but these categorizations were left to individual laboratories to define and very few laboratories had those metadata.

  • Although an attempt was made to define calibration equivalency for each of the soil test categories among the various testing procedures, it is likely that error was introduced in this process.

  • Some laboratory data were submitted using categories other than those specified in the sampling protocol, and interpolation routines were created and used to translate between the two systems.

These weaknesses need to be considered in interpreting and using the results of the summary.