Afarmer who has grown corn for the past five years notices in the sixth year that his harvest is significantly smaller, despite an increase in labor on the farm. the following year, the harvest continues its decline. which of the following explanations most plausibly accounts for the reduction in corn yield?

This is most probably because of the lowering of the quality of the soil on which the corn is planted. The soil's mineral base and fertility year by year declines, thus giving the plants much less food for them, which contributes to the plants to become less developed and produce lest corn. If the farmer was taking care properly of the soil and to keep it's fertility at a high level, then this wouldn't have happened.