Modern weed killers are put in two categories: selective (affecting specific plant species) and nonselective (affecting plants generally). These, in turn, are classified as foliage-applied and soil herbicides. Contact herbicides (e.g., sulfuric acid, diquat, paraquat) kill only the plant organs with which they are in contact. Translocated herbicides (e.g., amitrole, picloram, and 2,4-D) are effective against roots or other organs to which they are transported from aboveground treated surfaces (i.e., soil). With respect to planting time, herbicides are also classified as preplant, preemergence, or postemergence weed killers. Preplant herbicides may be applied to the soil or to weeds before crop planting.Many millions of hectares are treated from the air each year, particularly cereal croplands, pastures, rangelands, forests, and other situations where an airplane can be used. Modern equipment for treating row cropland with herbicides has made weed control increasingly convenient. Sprayers, soil incorporation equipment, and spreaders for pelleted herbicides have all added to the convenience of, and removed uncertainty from, herbicide application