So only 1/3 of fourth graders were able to find the experimental setup to find the best fertilizer level out of nine, when you are only allowed to try six out of them.
The correct strategy consists in going in two steps, first trying out interspaced levels e.g. 2-4-6-8 then "refining" with the two remaining tries around the approximate minimum. This necessitates to model implicitly/intuitively the plant growth as a unimodal (increasing, then decreasing) function of the fertilizer level, thinking ahead with the limited tries constraint, and mentally planning different outcomes of the two steps.
I'd go contrary to the flow and say that 33% of 4rth graders solving an assignment of this difficulty is pretty darn awesome.