Most soils are deficient in copper. Some gardeners believe that copper is toxic to plants and should be kept out of the garden. Indeed, too much copper can be toxic to roots and leaves, but a small amount is a necessary component of plant growth. Copper should not be applied, however, before having the soil professionally tested.
Soils with high organic matter form a tight hold on copper and can thus cause copper deficiencies in plants. As a result, soils high in organic content are more likely to respond to copper application.
An early sign of copper deficiency is the uniform, light green color of young leaves. Deficient plants produce small or yellowing leaves and may be particularly susceptible to airborne fungal diseases.
Copper deficiency in watermelon: The vine is stunted, leaves are crinkled.