Herbicide, an agent, usually chemical, for killing or inhibiting the growth of unwanted plants, such as residential or agricultural weeds and invasive species. A great advantage of chemical herbicides over mechanical weed control is the ease of application, which often saves on the cost of labour. Most herbicides are considered nontoxic to animals and humans, but they can cause substantial mortality of nontarget plants and the insects that depend on them, especially when applied aerially. Chemical weed control has been used for a very long time: sea salt, industrial by-products, and oils were first employed. Selective control of broad-leaved weeds in fields of cereal crops was discovered in France in the late 1800s, and this practice soon spread throughout Europe. Sulfates and nitrates of copper and iron were used, and sulfuric acid proved even more effective. Application was by spraying. Soon sodium arsenite became popular both as a spray and as a soil sterilant. On thousands of kilometres of railroad right-of-way, and in sugarcane and rubber plantations in the tropics, the hazardous material was used in tremendous quantities, often resulting in the poisoning of animals and occasionally humans