Ray,

If you are looking for a variable rate that will increase and decrease with the yields, price etc why don't you just go with a 2/3-1/3 or 3/4-1/4 with the tenant paying all expenses.  That way in a good year you aren't bitchin because the tenant made too much from your ground, but also in the bad year the tenant isn't eating peanut butter and jelly every night because he didn't make any money.

One thing I've noticed with the cash rent thing, He Who Has The Gold Makes The Rules, and if you dont' like the rules he'll get someone else.