The lack of diversity in a monoculture system eliminates all the functions that nature provides to plants and the soil.
Planting the same crop in the same place each year zaps nutrients from the earth and leaves soil weak and unable to support healthy plant growth. Because of the problem that soil structure and quality become so poor, farmers are forced to use chemical fertilizers to encourage plant growth and fruit production. These fertilizers, in turn, disrupt the natural makeup of the soil and contribute further to nutrient depletion.
Monocropping also creates the spread of pests and diseases, which must be treated with yet more chemicals. The effects of monocropping on the environment are severe when pesticides and fertilizers make their way into ground water or become airborne, creating pollution.
With no ground cover plants to help improve moisture retention in the soil, and the tendency for land planted with a monoculture to lack topsoil, which serves to increase rain runoff, modern monoculture agriculture requires huge amounts of water to irrigate the crops.