The OMAFA-accredited soil testing program provides assurance of laboratory performance and appropriate analyses to support guidelines for nitrogen, phosphate, potash, magnesium, zinc and manganese fertilizer, along with parameters for the amount and type of lime to apply. The analytical methods used were chosen to provide accurate results on the range of soils found in Ontario.
The accreditation program is administered by the Ontario Ministry of Agriculture, Food and Agribusiness in coordination with the North American Proficiency Testing (NAPT) Program. This external assessment ensures accuracy, precision, and repeatability of soil test results for accredited laboratories, and can be used as a method of quality control and continuous improvement of testing services and technology.
Accredited Soil Test Methods
There are currently eight tests accredited by OMAFA for soil testing in Ontario, as shown in Table 1. The testing methods used have been identified through response calibration research as providing consistent, accurate results in terms of crop response using Ontario soils.
Table 1. Soil laboratory testing extractants or methods accredited in Ontario
| Accredited test | Testing extractant/method |
|---|---|
| Soil pH | Saturated paste or 1:1 water |
| Buffer pH | SMP or Sikora |
| Phosphorus | Sodium bicarbonate (Olsen) |
| Potassium | Ammonium acetate |
| Magnesium | Ammonium acetate |
| Zinc index | DTPA, modified by soil pH |
| Manganese index | Phosphoric acid, modified by soil pH |
| Nitrate-N | Potassium chloride |
Soil test results for phosphorus, potassium, and magnesium are reported in units of milligrams per litre of soil (mg/L) or parts per million (ppm), which are approximately equal. Manganese and zinc are reported as indices, due to soil pH having a greater influence on plant availability than the amount of nutrient in the soil.
Soil organic matter is not an accredited test, but most laboratories offer this analysis as a standard measure. Soil organic matter plays an important role in soil fertility.
OMAFA-Accredited Soil Testing Laboratories
Current OMAFA accredited soil testing laboratories are listed here.
Soil Tests from Other Laboratories
Each year, a number of producers ask OMAFA staff to interpret results from laboratories that are not accredited. Provided the laboratory uses the identical test used by the OMAFA-accredited service and expresses its test results in the same units, the OMAFA fertilizer requirements for phosphate and potash can be determined, but there is no assurance of the accuracy of results.
The Olsen extracting solution for P has a pH of 8.5, while the Bray-1, Bray-2 and Mehlich-3 extracting solutions all have a pH of 2.5. This difference can overestimate P availability in alkaline soils due to solubilization of unavailable forms such as some calcium phosphates. The Olsen method is more consistent in predicting P availability for the range of Ontario soils. The Mehlich-3 method extracts comparable amounts of potassium as the ammonium acetate method.
To become OMAFA-accredited, a laboratory must meet OMAFA-approved testing procedures to demonstrate acceptable analytical precision and must provide the OMAFA fertilizer guidelines.
OMAFA-accredited soil tests will provide the most accurate fertilizer guidelines for Ontario field crops.
Soil testing laboratories may provide soil tests such as cation exchange capacity, aluminum, boron, and copper. These tests are not accredited by OMAFA because they have not been found to contribute to better fertilizer guidelines. In some cases, plant tissue analysis may offer a more reliable indicator of nutrient availability. Research has shown that on Ontario soils, use of cation exchange capacity to adjust potash requirements can lead to less reliable guidelines than are currently provided.
For more information on soil test extractions, see OMAFA Publication 611: Soil Fertility Handbook.