Some of the harmful effects of agricultural practices on the environment are as follows:
Climate Change: Farming promotes monoculture because a selected number of species is grown on a large tract of land. This affects the biodiversity of the area. This is not good for the climate.
Deforestation: Large scale farming has been possible because man has cleared vast tracts of forests. Even today, forest is being cleared to make way for agriculture. This has reduced the habitat for most of the wildlife.
Soil Degradation: When farming is done for a long period on a land, its soil is robbed off all the nutrients. This results in soil degradation.
Water shortage: For good harvest, farmers need to use lot of water for irrigation. Most of the crops need plenty of water for proper growth. This has resulted in overexploitation of water resources; especially groundwater. This is the reason; even rural areas are suffering from shortage of drinking water.
Pollution: Farming involves use of synthetic fertilizers and pesticides. These substances cause soil pollution, water pollution and air pollution. Moreover, use of tractors and combine harvesters necessitate burning of fossil fuels. This adds up to air pollution. 

Food chain	Food Web
(a) Food chain is a series of organisms feeding on one another	(a) Food web consists of a number of interlinked food chains.
(b) Members of higher trophic level feed upon a single type of organism of the lower trophic level	(b) Members of higher trophic level can feed upon organisms of the lower trophic levels of other food chain.