The problem with mineral supplementation is that there is no studies that show it has any impact on free ranging deer. The reason is that the diet of free ranging deer is extremely varied. Each plant has different mineral and nutrient scavenging capabilities. There are very few areas where the soils are so deficient in an important trace mineral that it becomes a limiting factor for deer. Clearly the best delivery system for minerals are plants. Not only do deer get the minerals they need from plants, they also get the nutrients and energy.
I think the best way to add minerals to your food plots is to do a soil test for the crop you plan to plant and add specific minerals according to that. There are risks with mineral supplements. Most are delivered with some point source attractant. Point source attractants tend to encourage more face to face than occurs with natural grooming and that can be a vector for the spread of some diseases.
So, if you are going to apply them in hopes that they will benefit your deer without solid evidence, the best way to do it is to distribute them in your food plots.
Thanks,
Jack