 Acid soils may contain high concentrations of aluminium (as Al3+) and manganese (as Mn2+) which can be phytotoxic.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.