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Soil tests for trace elements and toxic metals presently in use are difficult to interpret in terms of bioavailability or potential toxicity. Different methods are used by different laboratories. This has created considerable confusion.<P> To remedy this problem we will: a. Develop web-based educational materials and conduct outreach to ag. advisors, testing labs, farms and gardening communities, elucidating the nature of soil testing for trace elements and toxic metals. We will provide interpretation of the potential soil, animal and human health impacts of these tests and describe inherent limitations or uncertainties. b. Compare quantitatively the various traditional soil tests for trace metals now in use by the NE States to a new soil test, extraction by 0.01 M CaCl2. The relative effectiveness of the traditional and new soil tests will be assessed by measuring their respective abilities to predict trace metal (specifically Cu and Zn) uptake or toxicity in crop plants. <P>Farmers and ag advisors will gain practical information on soil metals, their natural background, deficiency, and toxic levels. Farming practices are presently causing some metals to increase in soils over time, which at some threshold level may have effects that we may not realize until too late. For example, copper, used on dairy farms for hoof health, and in pig feed, accumulates in farm fields. Zinc is also found at high levels in pig and poultry feeds, and is therefore present in manures at concentrations well above soil background concentrations. Homeowners have used pressure treated lumber (CCA) for building, landscaping and playground structures, a wood product that leaches copper as well as arsenic into surrounding soil. Toxic metals are frequently present in soils of inner-city urban areas where children play and gardens are grown. Soil testing allows an assessment of potential human exposure via ingestion or diet. <P>Outcomes include materials helpful to extension educators and farmers and gardeners for assessing the value of soil tests to monitor toxic metal status. Presently, there is confusion about the interpretation of traditional soil tests for toxic metals, and which test would be most appropriate. One expected outcome is a new soil test with an improved ability compared to older tests, to predict uptake of trace metals. Laboratories can use the information obtained in this study to decide if advantages of the new test justify its adoption. <P>An easily interpretable soil test for toxic metals would be a useful tool in more routine monitoring to recognize and minimize environmental impact of practices that contaminate agricultural and urban garden soils, and to protect food crops raised on farms and gardens.
Non-Technical Summary: Soil tests presently used for trace and toxic metal status in the Northeastern states evolved from historical need to diagnose micronutrient (trace element) deficiencies. These tests are chemically aggressive to varying degrees, with recent research in our laboratory showing that some commonly used tests extract a large fraction of the total metals in soils, not necessarily in proportion to the plant-available fraction. Furthermore, there is little field or greenhouse research in the Northeast to validate the accuracy of these soil tests for predicting either trace metal deficiency or toxicity. Recent research has shown that less aggressive reagents, most notably dilute neutral salts such as 0.01 CaCl2, are consistently better at predicting plant availability of excess trace elements in soils than the traditional aggressive tests. This is in large part because neutral salts do not substantially alter the chemical conditions in the soils during extraction, and therefore reflect conditions that plant roots experience. Furthermore, dilute neutral salt extracts are less prone to measurement errors and artifacts arising from interfering elements or matrix effects in the extracts, or re-adsorption phenomena that occur during the soil extraction process. The main obstacle to acceptance and adoption of a soil test such as the 0.01 M CaCl2 extraction is the lack of an adequate database to correlate this new test to the traditional tests used by various States (e.g., Morgan's solution at Cornell Nutrient Analysis Lab) and to trace element uptake by crops. This project will provide that initial database for Cu and Zn, the two trace metals of greatest concern for phytotoxicity in agricultural soils, by establishing relationships between measured uptake of these metals in test crops and extractable fractions of Cu and Zn in lab-contaminated and field-contaminated soils. For metals that have human toxicity but low plant availability, such as lead, the primary concern may be total concentration in soil rather than plant availability. In this case, an aggressive soil test or measurement of total metal may be appropriate in evaluating exposure risk. Nevertheless, soil testing laboratories often do not provide much information in how to interpret test results for toxic metals, and this project will develop interpretative materials for growers that will be used in support of soil testing for metals that may be potentially toxic to crops or to humans. The outcomes of this project will be to simplify and make more understandable the soil test results for metal contaminants in soils. <P> Approach: The first objective, developing web-based and other educational materials and outreach for soil testing, will be achieved by synthesizing existing published research into a logical framework that considers the metal of concern, the soil type and the relevant impact (e.g., microbial toxicity, phytotoxicity, risk to livestock, risk to humans). The nature of the impact being considered dictates which soil test is appropriate (e.g., extractable Zn by CaCl2 is relevant to plant uptake, but a measure of total soil Zn by acid digestion is not). This approach will lead to the establishment of a range of impact scale from deficiency (for Cu and Zn), to normal (no impact) to low toxicity risk to high toxicity risk. The research component will support this educational activity by providing functions that allow approximate translation from one soil test scale to another. The research (second) objective will be achieved using both metal-spiked soils and field-collected soils that have been historically contaminated with metals from various sources. These soils will be selected to span a wide concentration range of copper and zinc, trace metals that affect soil health and crop quality. The metal-spiked soils have already been prepared from two soil series with very different textures (Arkport, Hudson), and have added Zn and Cu concentrations of 50, 100, 200 and 400 mg/kg. Preliminary cropping studies with soybeans after several years of aging show these concentrations to produce yield reductions and toxicity ranging from mild to severe. Some of the metal-contaminated field soils will be collected locally, others will be obtained from project advisors in several Northeastern states. All soils will be subjected to both the traditional trace metal soil tests now used in the Northeast and the new test - CaCl2 extraction. To obtain crop bioavailability data, the metal-spiked and field-contaminated soils will be used in a greenhouse pot study with wheat or corn seedlings, and the measured foliar uptake of metals will allow relationships by statistical regression analyses to be established between bioavailability and the various soil test values for Cu and Zn. Sufficiently large homogenized samples of the field-contaminated soils will be collected to conduct a greenhouse pot study with wheat or corn as well as to measure extractable Zn and Cu by the various soil extractants. The field-contaminated soils will be further characterized for total Cu, Zn, OM content, pH and CEC. The results from the experiments for the metal-spiked soils will allow an evaluation of the relative adequacy of each of the traditional soil tests as well as the CaCl2 extraction method for assessing plant availability and toxicity of Cu and Zn. However, because this initial evaluation uses only a few soil types, and does not address the wide range of soil properties that can affect trace metal bioavailability, results from the field-contaminated soils will provide a further assessment of the ability of any single soil extraction test to estimate plant availability of Cu and Zn across a wide range of soil types and soil properties.