The degree of control required from nematocidal treatments, application methods and rates vary from crop to crop. Less than 10 nematocides are available commercially, and only three or four of those were introduced within the past 15 years. Some nematocides are distributed in the soil by diffusion in the gas or vapor state, by water solutions or emulsions, or by mechanically mixing the chemical into the soil. Whatever the method, the chemical must be dispersed through the soil to kill nematodes to the desired depth, without leaving a phytotoxic residue.