Part 1, Section 2: Soil Fertility Management
Soil Fertility Management
SOIL TESTING
Analysis
The soil testing procedures currently used by the Penn State soil testing program are listed below and on the soil test report.
| Test | Method |
|---|---|
| Soil pH | Water |
| Acidity | Mehlich Buffer |
| Phosphorus | Mehlich 3 |
| Potassium | Mehlich 3 |
| Magnesium | Mehlich 3 |
| Calcium | Mehlich 3 |
This information is useful if you compare analytical results from different labs. Direct comparisons can be made only between labs using exactly the same procedures. There are many different methods in use around the country, each containing both strong and weak points. Which test will be used in a given area is based on research to determine how well the test works under local conditions. The tests used by the Agricultural Analytical Services Lab at Penn State have been determined through extensive research to work best for Pennsylvania conditions. The Bray 1 extractable phosphorus and neutral 1 N ammonium acetate extractable potassium, calcium, and magnesium tests used at Penn State before 1991 are similar to the Mehlich 3 extractable amounts of these nutrients. Note that some state regulations, such as the Pennsylvania Nutrient Management Law, require soil tests to follow the recommended procedures in the list above. While most soil testing laboratories doing business in Pennsylvania use the recommended test methods listed above, it is important to determine if the lab you use is following these recommended procedures for your soil test results to be valid.
Soil nutrient levels on the Penn State soil test report are given as parts per million (ppm) elemental P, K, and Mg. However, while other labs doing soil testing in Pennsylvania may use the recommended methods above, they may report the results in different units. Below are simple mathematical conversions that can be used to change soil test results from labs using the recommended procedures but reporting results in different units than units used on the Penn State soil test report.
| lb P2O5/A ÷ 4.6 = ppm P |
| ppm P2O5 ÷ 2.3 = ppm P |
| lb P/A ÷ 2 = ppm P |
| lb K2O/A ÷ 2.4 = ppm K |
| ppm K2O ÷ 1.2 = ppm K |
| lb K/A ÷ 2 = ppm K |
| lb MgO/A ÷ 3.2 = ppm Mg |
| ppm MgO ÷ 1.6 = ppm Mg |
| lb Mg/A ÷2 = ppm Mg |
For example if a soil test reports the Mehlich 3 P level as 140 lb P2O5 /A you would need to divide by 4.6 to convert this to 30 ppm P. If the result were reported as 60 lb P/A you would simply divide by 2 to convert this to 30 ppm P. Finally, if the result were reported as 70 ppm P2O5 you would divide by 2.3 to convert this to 30 ppm P. Note that in this example, 140 lb P2O5/A, 60 lb P/A, and 70 ppm P2O5 are all the same as 30 ppm P, just reported in different units. This conversion is important if you want to compare results from different labs because they must all be in the same units. Also, if you want to look up a Penn State recommendation using a result from another lab, the results must be in ppm P, K, and Mg.