Abstract: This invention provides apparatus and methods for stabilization of ultrasonically generated nutrient fog. To eliminate the sensitivity to the humidity and temperature of the intake air, the fog is recirculated from the growth chamber to the fog-generating reservoir. The chamber and reservoir are connected by a fog supply conduit and a fog return conduit which form a fog circulation circuit. The recirculation eliminates the fog density dependence on intake air humidity and stabilizes the amount of nutrients used. To monitor the fog density, this invention provides an optical fog density detector that operates by measuring the absorbance of the fog. The density detector can be coupled with the fog generator so that the rate of fog generation is responsive to the measured fog density. The detector can be coupled with an alarm to signal fog generator malfunction. This invention also provides fog concentration stabilization by using a large ballast reservoir connected to a fog-generating reservoir.