can I add soy and corn to my feed

You will be changing the amount of vitamins and minerals in their diet if you do this. If their only food is their feed, that is if they don't forage for bugs and grass and the like, I'd be concerned about changing the makeup of their feed much at all. I don't know how soy compares to feed but I do know corn has a good deal less protein than layer feed, also.

 

You'd have to calculate how much protein the soy was adding. Corn is basically bulk calories, just like for humans. Lots of starch, but not much nutrition overall. One thing to do is just see how your birds do on any given diet. If production drops, or you start having illness or parasite issues, you need to adjust your feed.

 

In my experience you won't save much by adding corn, soy or scratch feeds... Alone they are basically treats and not a balanced diet so although you might be stretching the food you are no longer providing a balanced diet egg production, growth and health will likely suffer...

Also I made the mistake once of trying to cut my feed with a little scratch grain as a treat, never again... The birds emptied out the entire feed container onto the ground and picked out the scratch grains leaving the rest to waste...

If you want to save money on feed may I suggest you take a walk around town and locate the ma/pa non-corporate food stores in your area, and then attempt to cut a deal with the owner to remove their day old bread and produce every few days... Offer to sign a "not for human consumption waiver" to sweeten the deal... Even a small corner grocery store tosses enough day old produce out to supplement dozens of chickens and you will be getting a variety of items that offers a more balanced supplemental feed...