The version of agriculture practiced in the industrialized world is inseparable from the technologies (and fuels) required to sustain it. While the yields of a modern corn or wheat farm would be unthinkable to a farmer from just a century ago, the requirements in order to sustain that yield are sizable.
A significant fraction of the world’s fresh water is used to irrigate farms; according to a UN report, “two thirds of the water drawn from aquifers and other sources is used for farming". Many of the food crops produced are water-intensive, but few people understand that too much water can leach needed chemicals out of the topsoil. In addition, the tilling process destroys the soil flora and fauna responsible for decomposing organic matter to make it available for plant growth. Establishment of monocultures (which is precisely what a single-crop field is) also reduce their resistance to both pest infestations and disease.
Polycultures are precisely what their name implies, agricultural systems that employ multiple species in order to reap the positive effects of their interactions and mitigate some of the disadvantages of monocultural agriculture. Multiple plant species make the plot less attractive to pests, while the inclusion of nitrogen-fixing species, intact soil structures and communities, and provide biomass to renew soil fertility. Permacultures do this one better to include the usage of perennials rather than annuals- this alleviates the physical labor and environmental damage of having to plow the land to sow new seed.
Beyond just providing food, permaculture and polyculture provide the opportunity to see the interactions of plant and animal species in an environment closer to what you would find in the wild, which holds the promise of increasing both awareness and appreciation for the natural world in which we live.
Learn how to get started with Permaculture Gardening.