Probably you'd need the metalworks as a whole... Eight cores. As for the circle for transportation to the core - it's a simple levitation enchantment, and doesn't have to be particularly smart with me to check it. If you did it as a single task, bring fruit from grove to core, that would probably only require one core to power; if you built something more elaborate to give me, effectively, a pair of "hands", that would take correspondingly more power but be more flexible. 

Hailey hums. "Can you generate a reduced amount of fertilizer with fewer cores?"

Yes, but it's more efficient the more cores you dedicate because of economies of scale.

"How much fertilizer can you do with four or five rather than six? We have enough liftfruit to run enough shipyard to build the new circle, run agriculture, run the island, and run living quarters, plus either four or five for fertilizer, depending on whether Maya wants to tie her core up here."

"Speaking of which, how much does letting her core contribute to running things here, or being the core of a ship, drain her in any lasting way?"

With four I can do about a third as much fertilizer, with five about two thirds as much as with six. The hearts of the Sweetheart tribe are like common liftfruit - and subject to the blight - but never run dry of mana so long as they are appropriately and moderately tapped, which I am presently doing. But if you tried to have her leave the island she'd be unable to with her core in my core. 

Maya flushes. "We will keep my core in the altar for now, then, only taking it out when it is time to fire up the ship. This will get us more exponentiation without taking additional time. Does the fertilizer need manual application? How is the focus, speed versus yield, specified?"

Manual application is not necessary, the systems are set up to handle that. There are two different formulations of the fertilizer but they're mutually incompatible.

Maya gives the others a contemplative look.

"Yield, then."