For each of the years represented, laboratories were asked for all soil samples for farm advisory purposes for the crop to be grown in that year. In most instances, this included the fall of the preceding year and the spring of the current year. Soil sampling opportunities each fall and spring depend on weather conditions, and affects the total number of samples obtained to represent each region in each of the samplings.
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 or TFI agronomic scientists. 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. Soil organic matter was requested for the first time in 2020, and was generally determined by loss on ignition (LOI).
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.
Despite efforts to conduct the summary comprehensively and consistently, weaknesses exist in the summary process due to the diversity and dynamic nature of soil testing services:
- 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.
- The percent of samples in the summary from manured fields could vary over the years owing to nutrient management regulations that mandate soil testing. Few labs were able to separate data from manured and non-manured fields.
- Quantity of sample results is low in several states and provinces.
- Not all sample results could be definitively associated with a particular state.
- 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 protocol, and interpolation routines were created and used to translate between the two systems.
- Data on precise depth of sampling is not given. We assume the samples were taken at the depth recommended for typical soil fertility interpretations. In most states and provinces, this is usually the top 6 to 8 inches of soil.
- These weaknesses need to be considered in interpreting and using the results of the summary.