While my method is geared more for water soluble, I purchased my tank sprayer that easily holds 15 gallons of material from Rural King for about $65 if I remember correctly. Primary use is as a sprayer on the back of my carryall cart. BUT, when I want to use a soil drench, I fill it with my material including water soluble fertilizers, power up the cheap boat bilge pump to keep the material agitated and attach a 15 ft hose to the drain outlet. At the end of the hose I have an inexpensive water meter with a shutoff valve and let gravity do its thing. It works reasonably quickly and when the material starts to get low, I just prop up the opposite side of the tank to keep the material at the drain end. With the meter attached, I can monitor just how much material has come out.