How does agriculture affect the environment?
Agriculture affects the environment in multiple ways.
One of the primary environmental concerns with modern agriculture has to do with the chemicals we put on crops and what happens when those crops end up in the watershed. Pesticides and other chemicals are often used to produce more crops, and these pesticides don't just fall on the crops and stay there: they are transported via wind and water and affect the surrounding ecosystems.
Fertilizers are also used in agriculture, and they also end up in the surrounding ecosystems. Fertilizers, while a nutrient for some plants, end up in streams and lakes in amounts higher than would occur naturally.
The result may be eutrophication, an excessive amount of nutrients in a body of water resulting in a substantial growth of plants, such as algae, and a lack of oxygen in the water because of this increase in plants.
There is a finite amount of land on the planet, and agricultural practices take up a lot of land. Forests, grasslands, and other ecosystems are converted for farmland.
When we clear land for agriculture, we often lose some soil. The plant species originally on this land are gone, and what we replace those with may be plants that aren't as effective and retaining the soil and its nutrients. Thus, the soil degrades over time. If we're in areas with heavy rainfall, replacing forests with cropland often results in an increase in flooding.
Another problem has to do with the amount of water some crops require. Agriculture is the industry with the largest water consumption. It takes a tremendous amount of freshwater to grow crops and feed for other animals that we consume.
Sometimes we use too much water and sometimes we try to grow plants in environments that aren't suited for them, using more water than we would if we had grown them in an environment more like what that crop would typically be found in.