How would you control variables such as plant location or amount or type of soil?

If you want to eliminate these variables, plant everything in the same location, using the same type and amount of soil. Buy some potting soil from a store, and put all of your plants in similar containers with the same amount of soil. And, to control location, just keep all of the plants in the same area.





Of course, if your talking about planting things outside, you can't really control these variables.