> Why assume that more output doesn't come with more input? It doesn't seem presumable to me one way or the other.
It isn't. There is such a thing as too much. Inputs that wash off in the rain is wasted money. Inputs beyond what the plant needs are best sit around (or wash off - see above), but sometimes will make the plant grow taller instead of grow more food - or worse will kill the plant from too much.
There is a lot of room though. Today we can target small areas of land with different amounts of each input. 100 years ago you tried to give each field the same amount of every input since you didn't know what it really needed, and didn't have the ability to do anything about it even if you did.
