Abstract: An apparatus for adding fertilizer to water flowing in an underground sprinkler system includes a pressure chamber with a bladder therein which holds liquid fertilizer. Water under line pressure surrounds the bladder in the chamber to pressurize the bladder and fertilizer therein. The pressurized liquid fertilizer flows from the bladder through a valve such as a needle valve which controls flow rate and through a valve such as a butterfly valve which opens to allow fertilizer flow when water flows through the line to which the fertilizer is to be added. With no flow of water, the valve is closed to prevent flow of fertilizer. The lid to the opening to the inside of the bladder used to fill the bladder with fertilizer is arranged to operate two valves. One controls flow of pressurized line water into the chamber to pressurize the bladder in the chamber when the lid is opened and the other opens a drain to depressurize the chamber when the lid is opened to allow fertilizer to be added to the bladder.