Copper is required for many enzymatic activities in plants and for chlorophyll and seed production. Most Minnesota soils supply adequate amounts of copper for crop production. However, copper deficiency can occur in high organic matter and sandy soils. The amount of copper available to plants varies widely among soils. Copper in the soil is held with clay minerals as a cation and in association with organic matter. Copper deficiencies often occur in soils with peaty soils and high concentrations of organic matter. Sandy-textured soils are more likely to be copper deficient than loams and clays. Soils that contain greater amounts of oxides and carbonates tend to have low available copper. Soils with a pH of 7.5 or greater should be monitored when crops sensitive to copper are grown. In corn, copper deficiency first appears on new leaves as they come out of the whorl and develop a bluish green tint. New leaves may emerge from the whorl as spiraled. Necrosis may occur on older leaf-tips and edges and may die. Corn response to copper has not been documented in Minnesota. Corn is only moderately sensitive to copper deficiency. Copper deficiency is rare in soybeans. Soybean response to copper has not been verified in Minnesota. Use soil and plant tissue tests on organic soils to determine deficiencies and need for fertilizer. Copper can be broadcast or incorporated before planting and can also be applied as a mixture with other fertilizers. Copper is not recommended for mineral soils in Minnesota, due to lack of research. Copper sulfate is the preferred source of copper fertilizer because of low cost compared to chelated sources. Copper use efficiency is improved if the fertilizer is water soluble and the particle size of the fertilizer is small. A single application of copper can last for many years. There is a narrow range between copper deficiency and toxicity. Copper toxicity can persist for an extended period of time and is difficult to correct because of copper’s low solubility in water. Toxic concentration of copper in soil affects seed germination, root system development and plant vigor.