You know, jake, I don't know if there is a minimum depth to plow to take out an alfalfa sod.  I think the real question is... how good are you at plowing?

My gut feeling is that you want to get that plant completely turned over and buried - and you need to plow the depth that it takes to make that happen.  I think when alfalfa is not killed by plowing is when it is not completely inverted. 

That is my gut perception.  What does your experience tell you ?