Phytotoxin is an umbrella term that refers to substances (known as phytotoxic substances) that are inhibitory to the growth of or poisonous to plants. Phytotoxic substances may result from human activity, as with herbicides, or they may be produced by plants, by microorganisms, or by naturally occurring chemical reactions. A good soil will protect plants from toxic concentrations of such substances by ventilating gases, decomposing or adsorbing organic toxins, or suppressing toxin-producing organisms.