Answer:
Particular plants use particular nutrients from the soil. If you were to consistently plant a large number of plants, of the same species, in the same area. The plant will use all of the nutrients it needs, before the nutrients are replenished. This can cause a decrease in the farmer's yield. By doing this you also create a habbitat for pests. Some pests live off to certain plants, so if your not planting other crops, the pests will thrive. Plating a variety of crops is good because some plants will repel pests from other crops that the pests may be attracted too.