Improved soil testing for toxic metals
CALS Impact Statement
This project is developing simple soil tests that can determine the thresholds for toxicity of heavy metals to plants and microbial processes in soils. It will provide a key measurement for the evaluation of soil health as affected by metal pollutants. The tests are designed to improve upon existing soil tests that are chemically aggressive and do not measure the biologically available or soluble fraction of metals in soils.
At present, the most widely used soil tests in North America for heavy metals and other trace elements are relatively poor predictors of the bioavailability or mobility of these elements in soils because the chemically aggressive nature of these tests changes soil pH and elemental extractability. As a consequence, soil test data for metals such as copper, zinc, molybdenum, and cadmium have limited value to agronomists and regulators for evaluating potential for trace element or toxic metal uptake into crops or leaching to groundwater when a wide range of soil types is being evaluated. In fact, there has been little field validation done in support of the interpretation of traditional soil tests by soil testing labs, and interpretations that are done are not straightforward because individual soil properties such as pH and organic matter content affect interpretation.
A simple and non-aggressive soil extraction method, using 0.01 M CaCl2 solution at 90O C, was shown in our laboratory and greenhouse research to be superior to the most commonly used soil extraction method in the United States. This conclusion was based on the strength of correlation between the amount of trace metal extracted from a number of soils and the concentration of the trace element in test crops grown in these same soils. Further studies in our laboratory have shown that the same simple soil test is able to assess thresholds of metal availability (copper and zinc) that impact soil health; specifically, enzyme activity and pesticide decomposition rates in soils. We are preparing a bulletin that describes the value of this soil test within the context of soil testing principles, and the importance of validating trace metal soil tests. This bulletin will be made available to growers and agricultural advisors on the websites of the Cornell Waste Management Institute and the Department of Crop and Soil Sciences.
The Cornell Nutrient Analysis Laboratory is now offering this soil test as an alternative to traditional methods, and several research groups at Cornell and Penn State are adopting the soil test for elements of concern, including cadmium, copper, molybdenum, and phosphorus. Our assessment of impacts on soil health by copper and zinc using this soil test has provided strong confirmatory evidence for our recommended limits for soil copper and zinc concentrations. It is interesting that both the United Kingdom and Australia are now adopting similar concentration limits for these metals in their agricultural soils. We believe that our research and publications using this soil test have influenced the development of regulations for protecting soils from metal contamination in other countries.