Purdue University did a study about 8 years back. they found that putting salt out for the rabbits had a significant positive effect on rabbit populations. the first year had only a minor increase, but the second and subsequent years the rabbit population increase was tremendous. the rabbits didnt have more litters, their litters were larger. the study found that indiana habitat is lacking in salt. and by supplying this needed mineral we actual do have a positive effect on rabbit reproduction. the suggested way of doing it is to use salt cubes. they recommended salt cubes with holes in them.( i have no idea where to get something like that) and to run a wire thru a bunch of them to keep them from being carried off. Purdue also mentioned taking a five gallon bucket and cutting it in half length ways. use something to keep the salt off the ground and put the cut bucket on top of it. this keeps the salt from getting wet and melting into the ground. the ideal situation would suspend the salt off the ground , have a cover to keep the rain from melting it, and be protected from the deer. adding salt to the habitat definately increases the carrying capacity of the area , and can easily be accomplished by anyone. This is a lot older than 8 years but you may try and go to do a Purdue University search. I just copied this and pasted to give you more information.
this information came from the indiana department of natural resources and so naturally they pointed out that it is not legal to use salt to bait deer and you must discontinue this practice 10 days before deer hunting in the area.