Soil Test Interpretations and Recommendations for Environmental Risk Assessment.

Soil testing was originally developed as a guide for managing soil fertility for optimum crop response.  That remains the primary use for soil testing.  However, with increasing concerns with environmental problems related to agricultural nutrient use, soil testing is now also used as a guide for managing nutrients to minimize environmental impact and in some cases as a regulatory tool.  While soil tests can be used for both agronomic and environmental management, it is important to recognize that these are very different uses that may require different sampling, extraction, interpretations and recommendations. Sampling depth may be different between agronomic and environmental testing with agronomics samples taken to assess the nutrient status of the primary rooting zone but the environmental test may only be assessing the nutrient levels at the soil surface where the runoff interacts with the soil. Agronomic soil test extractants are often used successfully for environmental purposes, but there may be situations where the agronomic test is not testing the appropriate soil fraction for the intended purpose, such as when a water soluble test might be more appropriate when assessing the potential for nutrient leaching.  Agronomic soil test interpretations cannot be used directly for environmental purposes.  A common mistake is to assume that a soil test that has a high interpretation for agronomic purposes should also be interpreted as high for the environment.  Soil test calibrations for agronomic interpretation do not usually include any environmental calibration. Separate calibrations are required, even for the same extractant, for agronomic and environmental interpretations.  Soil test recommendations for agronomic management are typically based on recommending whether additional nutrients are required for optimum crop response and if they are how much should be applied.  Environmental recommendations are often to limit or restrict nutrient applications when an environmental soil test threshold is reached. As with agronomic soil testing, environmental soil testing is most effective when it is integrated with other available information to make the best interpretation and recommendation for a given situation. An example is the use of soil testing for managing P to protect the environment.  It has been clearly shown that using the soil test as a component of the phosphorus index provides a much better assessment of the risk of P loss than the soil test alone. Soil testing can be very useful for both agronomic and environmental testing as long as they are used appropriately for the intended purpose. There is a need for research to develop new tests for both agronomic and environmental purposes and especially there is a critical need for appropriate soil test calibrations as soil testing move further into the environmental arena.