The water-fertilizer mixing process with an open-tank in the fertigation system was analyzed by a simplified dynamic mixing process in this paper. Then a variable universe fuzzy-PI control strategy was proposed which aimed at the characteristics of nonlinearity,pure hysteresis,time degeneration and uncertainty in the control system. A set of stretching factors with proportional exponential function was used to modify the universe of the fuzzy controller on line for improving its adjustability. Considering t...