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T: Boundary makes the spell a ritual.
The base volume for Terram is 10 cubic paces or 30 cubic feet, so a 30 square foot area gets churned to a depth of 1 foot.
Changing it back to T: Ind and increasing the size modifier a few times increases the volume Size +1 is 300 cubic feet, Size +2 is 3,000 cubic feet, and Size +3 is 30,000 cubic feet. An acre is 43,560 square feet, so one casting per acre is probably sufficient, since most plowing doesn't go a foot deep.
So Base 2, R: Touch, D:Momentary, T:Ind, Size +3 and it's only 10th level.
Walking huge circles can be a (minor) challenge, although adding some low fieldstone walls around the fields would fix that up. It'd be interesting flavour actually, as folk wonder why on earth you use such inefficient and abnormal field shapes.
That looks good. I made it sight to start with because I thought the spell would have to affect every individual at the same time. I can now stick it in a wooden spade as a magic item and make it unlimited uses. Thank you very much for your help.