SOIL COPPER THRESHOLDS FOR POTATO PRODUCTION

BACKGROUND A rising concern with the application of dairy wastes to agricultural fields is the accumulation of copper (Cu) in the soil. Copper sulfate (CuSO4) from cattle footbaths is washed out of dairy barns and into wastewater lagoons. The addition of CuSO4 baths on dairies can increase Cu concentration significantly in manure slurry. The Cu-enriched dairy waste is then applied to agricultural crops, thus raising concerns about how soils and plants are impacted by these Cu additions. Repeated applications of lagoon water could potentially raise Cu concentrations to toxic levels for agricultural crops, with very limited possibilities for remediation. It has been stated that most crops can only remove 0.1 pound of Cu per acre per year. If a grower waits until Cu plant toxicity symptoms occur (including plant death), they will continue to see Cu toxicities in that field for an indefinite period of time. Most Cu that is added to soil will strongly adsorb to soil surfaces (also referred to as cation exchange sites). As sandy-textured soils have fewer cation exchange sites than silty-textured soils, the Cu will not tend to bind to sandy soils as strongly as silty soils. For this reason, more Cu will be left in soil solution as plant available Cu, therefore potentially increasing the risk for Cu toxicities on sandy soils. Potato growers are concerned about this issue, as many of the predominant dairy producing areas in Southern Idaho are also in Idaho’s established potato production regions. Specifically, these growers are concerned that fields currently suitable for potato production may no longer be productive after excessive lagoon water applications with high concentrations of copper have been applied. The goal of this project was to evaluate potato growth and copper plant uptake for potatoes grown under low, moderate, and excessively high soil copper concentrations. METHODS AND MATERIALS This study was conducted in a greenhouse setting at the USDA ARS Kimberly Research Station. To establish an effective copper response curve, 6 rates of copper sulfate (0, 50, 100, 250, 500, and 1000 mg Cu/ kg soil) were applied to either a Portneuf silt loam or a Quincy sand. Two Russet Burbank potato seeds were planted at a depth of 3 inches in each pots. Plants were thinned down to one surviving plant at the two-leaf stage. Treatments were replicated four times in a complete randomized block design. After 130 days of growth, plants were harvested, separated into shoots, roots, and tubers, and were analyzed for copper concentration. Soils were also analyzed for soil test copper concentration using the commonly used DTPA extractant. Plant uptake of copper Potato plants grown on sandy soil textures absorbed copper more readily than silt loam textured soils, as indicated by increased concentrations of copper in plant roots and shoots (Figure1). This finding illustrates that silty textured soils likely bind more Cu than sandy soils, leaving more copper available for plant uptake on sandy soils. Comparing shoots to roots, we also found between 10 and 17 times greater Cu accumulation in roots than shoots for both soil