Chucking this one out there for examination:
Ploughing the Entire Field
Rego Terram level 25
Base 2, Range: Sight (+3), Duration: Momentary, Target: Boundary (+4)
Upon casting this spell, you turn over and plough all the earth in an entire field, marked by a boundary fence or dry-stone wall. Useful to put in an item for more food production for a covenant.
If there's somewhere else I should be submitting spells before putting them in the grimoire, please let me know.
Why not just make it Touch range (+2), that way you will get it down to level 10.
Welcome to the forum!
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.
I think it has to be T:Part as all the other ReTe that affect the ground.
Only 15th level, then. 
Or for an easier spell, make it T:Circle.
Makes for a funny arrangement to the fields, but hey, that makes it fun. 
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.
Well, make sure you pick up the correction Tugdual made.
Still, only need a ReTe of 30 for a lesser device, which isn't that hard to get to.
Crest of the oxens tred. Using crest of the earth wave as a base you could create a faster messier version
A