A system includes a first plurality of pumps connected to draw from a clean fluid supply junction. A second plurality of pumps is operatively connected to a dirty fluid supply. A first valve is connected between the clean fluid supply junction and the dirty fluid supply for supplying clean fluid to the dirty fluid supply. A second valve is connected to feed a dirty fluid to the dirty fluid supply. A controller is operatively connected to the first and second valves and to the first and second pluralities of pumps for controlling downhole concentration and flow rate of proppant from the dirty fluid supply, wherein downhole concentration and flow rate are varied across a continuous spectrum.