At what point is the marginal benefit of applying additional fertilizer no longer greater than the marginal cost of the fertilizer?

When the marginal benefit of applying additional fertilizer will no longer be greater than the marginal cost of the fertilizer, the marginal costs would equal or exceed the marginal benefits from enhanced growth. It will result into shortage of additional fertilizer thus it will only pay for the exact number of fertilizers needed. There are no longer extra fertilizer that can be used for emergencies.

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