Planting the same crop year after year on the same piece of land strains soil resources. 

Different crops demand more soil nutrients while adding other nutrients. For example, legumes use more phosphorus and potassium than nitrogen. Soybean monoculture will result in excess nitrogen in the soil and less of other nutrients. Such imbalances mess with soil health.

Various bacteria and microorganisms replenish the soil and keep it fertile. Different microbes mean many kinds of nutrients and soil health services. Research has established that plant diversity influences soil health and the type of microbial community in an area2. With monocropping, the variety of microbes significantly decreases.

Nutrient pollution 

Since monoculture depletes soil nutrients, farmers must maximize production by using chemical fertilizers. However, there are severe environmental costs associated with using artificial fertilizers. Chemicals from fertilizers enter rivers, lakes, streams, and reservoirs through rainfall runoff or erosion, and they can also seep into groundwater. 

Oversaturation of soil nutrients in aquatic habitats can cause harmful algae blooms, depleting the water's oxygen. Swimming in or accidentally swallowing water contaminated by such algae can cause serious health issues. Soil nitrate can also find its way into our drinking water. Water with high enough nitrates can be fatal to infants.

Excess fertilizer use also contributes to climate change. Nitrogen-based chemical fertilizers produce nitrogen oxide (N2O), a greenhouse gas with 265 times more warming power than carbon dioxide3.