I wish I know. I've been wondering the same thing.

I think how much depends a lot on the product being used. There are usually instructions on how much to use per square foot. But, yes, a soil drench is just basically pouring the mixture on the soil, usually around the soil just inside the drip line of the plant, to feed the root zone. Foliar is usually sprayed but I suppose could be poured or drenched onto the leaves.

I didn't see anything about per square foot other than 1.5 ounces per gallon.

It says it can be applied foliarly or as a soil drench.

Here is the stuff I am using.

http://www.ladybugbrand.com/products/Johns-Recipe.asp

I've never seen that product before so I have no idea how much per plant. Maybe you could email them and ask?

From the last paragraph of the page linked below (it sounds like they recommend using a foliar application for the John's Recipe product):

Fertilizer
Roses will bloom quite nicely on their own with lots of sunshine, but if you want more (and what gardener doesnâÂÂt?), you can fertilize. Fertilizing is for the active growing season and while roses never really go dormant in Central Texas, you donâÂÂt want to create a lot of new growth when it might freeze off or stress out a plant trying to cope with a brutal summer. March to May and then October to December are the best time slots for fertilizing. Lady Bug Brand Flower Power is a good choice if you want a dry, sprinkle-on option. Each feeding lasts 4 to 6 weeks and needs only to be watered in. Lady Bug JohnâÂÂs Recipe is also an excellent choice for those who prefer a more regular weekly or bimonthly feeding for the whole garden and are not afraid to wield a hose-end sprayer.

Here is a link that might be useful: Ladybug Brand How To Articles: Roses

Soil drench commonly means applying the solution to the soil in the same way and with the same volume as when watering the plant.
Nik