Nutrients & sedimentation that formed that specific soil.
Well if you mean what does the controlled part of a scientific investigation, it is the part of the investigation that will be your control or your base, untouched, original, data and such.
When I needed to memorize the states I used this website-http://www.sheppardsoftware.com/USA_Geography/USA_GL_click_1024.html It's basically a game that helps you learn the states. They also have other helpful ones.
I also made some flashcards, but I know that's not for everyone.
Double headed arrows indicate positive or negative effects of one variable on another