Among agricultural malpractices, the improper use of chemicals such as fungicides, insecticides and herbicides is the most common cause of plant injury and disease. The misapplication of pesticides to soils and plants may lead to plant injury or growth retardation. Many pesticides are toxic to some plant species, and a few, such as the fumigants, are toxic to most plants. Crop plants will be injured or killed if these chemicals have not decomposed or volatilized. Certain pesticides decompose to simple inorganic substances which may injure sensitive plants. Toxic decomposition residues of pesticides include compounds of arsenic, bromide, chlorine, copper, iodine and mercury. Several plant species are sensitive to inorganic bromides or chlorides: onions and citrus fruit may be damaged by bromide-containing pesticides; avocado by chloride-containing pesticides. Pesticides which decompose very slowly may, if used continuously, build up to such concentration in the soil that they seriously retard crop growth.
Treatment of the soil with any pesticide which kills soil microbes may result in plant injury. The killing of bacteria which oxidize ammonia results in the accumulation of toxic concentrations of ammonia from decomposing organic matter. If large numbers of microbes are killed, a temporary phytotoxicity may occur, manifested by reduced absorption of phosphorus, zinc and copper. The toxicity may last from a few weeks to a year; young citrus, peach, and certain other tree seedlings, are especially sensitive to this toxicity.
Chemicals used for seed treatment are frequently toxic, especially to some species of plants. For example, plants of the cabbage family are stunted by copper-containing seed treatment materials. Vegetative organs, such as potato tubers, are very susceptible to chemical injury, and strong poisons, such as mercuric chloride, can cause considerable damage. Materials applied to the soil to control fungi, bacteria and nematodes may injure plants grown in the soil too soon after treatment.