The yield of each field has nothing to do with how many fields you plant before harvesting them. How many fields you plant before starting to harvest only affects whether or not you can harvest the first field before it decomposes.
If your goal is just to level up the low crafting levels, you might decide to just plant fields and not harvest them. In that case you don't care about the field vanishing, and can just set the 50, 100, or whatever number of fields and walk away for a bit. This was done by some back when LOTRO first launched and it's much cheaper to do so now. Except instead of walking away they would just chat.
"Everyone's got one. Better to be a smart one than a dumb one." - me
"It's people like you that [I]think[/I] you know everything that give those of us that [I]do[/I] a bad name." - me
"Eat, drink, and be Merry, 'cause Pippin was a fool of a Took."