Derived from the Greek words allelo (other) and pathy (causing injury to), allelopathy is a form of competition among plants. One plant produces and releases a chemical into the surrounding soil that inhibits the germination or growth of other species in the immediate area. These chemical substances are both acids and bases and are called secondary compounds. For example, black walnut (Jugans nigra) trees release a chemical called juglone that prevents other plants such as tomatoes from growing in the immediate area around each tree. In this way, plants such as black walnut reduce competition for space, nutrients, water, and sunlight.