The invention provides an apparatus and method for enabling at least a minimum pressure and flow rate of stored NH3 from a storage tank to knives for injection into soil, regardless of ambient temperature. A pressure sensor and control valve monitors the pressure of the NH3 flow. If the sensed pressure drops below a desired minimum, e.g., 0.5 psi, a controller drives the control valve to open and also signals ramping up of a boost pump. The boost pump pumps NH3 from the storage tank to ensure maintenance of a minimum flow rate and pressure of NH3 from the tank to the soil. The pump runs on-demand only as needed, thereby reducing power requirements when the pump is not needed to supplement NH3 tank pressure.