As planters become larger the need to haul large amounts of liquid fertilizer grows in proportion. There are a few ways to accommodate the need for more liquid. The first is to carry it on the tractor in large, bulky tanks that need to be installed when it's time to plant and then removed when the tractor is needed for other functions. The second is to carry liquid on the planter, but the implement just isn't designed to carry that kind of weight.
With a much higher liquid capacity and far easier installation—dropping a hitch pin and coupling a hose—than the first two methods, the best way to haul large amounts of liquid fertilizer when planting is with a fertilizer cart.