There are a number of benefits to applying liquid fertiliser over granulated fertiliser apart from the lower cost. Foliar feeding and liquid injection at sowing are highly efficient ways of introducing nutrience to plants. 

There are a couple of basic systems that can be used to apply liquid fert at sowing.  These include "pressure systems" that rely on the pressure of the fluid being injected and the ground speed of the sowing unit to give the correct application rate. A single pump can be used to adjitate and deliver the liquid with the use of orifice discs to assist in even distribution. This system can have its problems though. Orifice discs are prone to blockages and a variation in ground requires adjustment of pressure to maintain the correct application rate. 

 The second type of system uses squeeze pumps (peristaltic pump)                                            to accurately meter fluid to each row. Metering can occur by either regulating the speed of the squeeze pump or by regulating the flow to the squeeze pump. A centrifugal pump is often the best way to provide the squeeze pump with charge pressure and adjitation of the liquid fert which is prone to settling.