On tha face of it, as a GM, I would say that you have two different intents. Grow and harvest. From what I recall, the spirits don't hang around like that. They help you and then they are done. You would need to roll a second time when the harvest needs to be brought in. It is a separate intent with a gulf of time in between. They probably need different skills. One a skill, the growing the field. Harvest probably requires a Forte test in order to work all day and part of the night to get the harvest in before it goes bad. I would say that the spirit helped in the planting or the sprouting. I would not say that it was present during the whole growing season. I don't think they have such an attention span.
The only way I would let it be a single roll would be if nothing of interest happening in between. It really depends on what is going on in your game.
There are spirits of the market place where you sell it. There are not spirits of knives or cash.
An act or retribution would be to have a crop later wither in the field from blight or plagues of insects. Rain falling at the crucial time of the harvest turning the fields to mud and making the harvest impossible. The retribution is a match and opposite of what the help was. So if you get the spirit to hurt someone, the spirit will later one come back to hurt you.
Inspired by Bobo's Sig
Rule 6 of Kurt Vonnegut's Rules on Short Stories
Be a sadist. No matter how sweet and innocent your leading characters, make awful things happen to them — in order that the reader may see what they are made of.
from Bagombo Snuff Box.