One of the main problems I see with applying just 5 gpa on a 60'/24 row planter is getting a uniform amount to each row. 5 gpa of anything is not much. Very different back pressure from short hoses on center rows to 30 ft long hoses to outer rows, or thereabouts.

I always felt that equipping a large planter for liquid fertilizer and getting a uniform amount (+/-) is a very elaborate undertaking. Even using a large manifold from end to end of the planter with same length hoses to each row, it is difficult to keep the same manifold pressure all the way out to the end rows.

If you are going to put down liquid with the planter and go to the trouble and expense of doing it right, why not pull a tank or have big saddle tanks on the tractor and put down enough to do some good.  A crop consultant I worked with years ago said the most cost-efficient way to fertilize a corn crop is 1/3 of N (plus some P or micros) down in the row just ahead of the planter (spring strip till) or the same amount on the planter 2x2 or with something like a Totally Tubular TT300. Then side dress the rest of N.  

This way you can raise very good corn with only around 0.8 units of N and less P (& K if strip tilling ahead of the planter) because you are feeding the plant where and when it needs it and not wasting highly water soluble N & S, much of which often goes right into the water table and your well.

And before you say this will never work I will tell you it does and has for many corn growers. 

Often the ones not in prime corn country where, as I used to kid some Illinois friends, "you could throw seed out the window of the truck and get 200 bu corn". They have all sorts of reasons to just keep on applying much more fertilizer than really needed, "just in case", with little thought about maximizing efficiency and profitability, just maximize yield at all cost.

jmho and experience