Phytotoxicity is a toxic effect by a compound on plant growth. Such damage may be caused by a wide variety of compounds, including trace metals, salinity, pesticides, phytotoxins or allelochemicals.
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Inorganic compounds
High concentrations of mineral salts in solution within the growing medium can have phytotoxic effects. Sources of excessive mineral salts include infiltration of seawater and excessive application of fertilizers. For example, urea is used in agriculture as a nitrogenous fertilizer, but if too much is applied, phytotoxic effects can result, either by urea toxicity or by the "ammonia produced through hydrolysis of urea by soil urease". Acid soils may contain high concentrations of aluminium (as Al3+) and manganese (as Mn2+) which can be phytotoxic.
Herbicides
Herbicides are designed to kill plants, and are used to control unwanted plants such as agricultural weeds. However herbicides can also cause phytotoxic effects in plants that are not within the area over which the herbicide is applied, for example as a result of wind-blown spray drift or from the use of herbicide-contaminated material (such as straw or manure) being applied to the soil. The phytotoxic effects of herbicides are an important subject of study in the field of ecotoxicology.