Oldcastle Rampart Wall Does A Retaining Wall Need To Be Set On Top Of A Base Of Crushed Stone?

Does a retaining wall need to be set on top of a base of crushed stone? - oldcastle rampart wall

I am building a retaining wall 2-1/2 feet tall, 64 kilos of concrete blocks Oldcastle Rampart. Is it necessary to put on a base of gravel or I can put it directly on the earth? How many inches of crushed stone or gravel, which must be directly behind the wall? It's good to be gravel or crushed stone?