Herbicides are chemicals used to destroy weeds without harming the useful crops. This is possible because of the different rates at which herbicides are absorbed by the weeds and crop plant.
Some crops have a dormancy period during which there is very little physiological activity. Herbicides can be safely used during this time. Some weeds have chemicals called enzymes that break down apparently harmless compounds into toxic products. Peas and other pod bearing plants, which do not contain this enzyme, remain unaffected. Such compounds are used as herbicides in legume fields. On the other hand, some crops contain compounds which convert the toxic herbicides into harmless compounds; most weeds lack this compound and are therefore killed. Factors like soil type, leaf shape, and method of application also affect the action of herbicides.