In production terms, sustainable agriculture refers to the ability of a farm to continue producing indefinitely, with a minimum of outside inputs. In order to grow, the crops and livestock draw energy from the soil, air, water, and sunlight. As crops are harvested, the energy they used must be replaced in order to continue the production cycle. Some of that energy comes from the larger environment, from sun, air, and rain. Some can be recycled: crop residues and manure from livestock retain nutrients that can be returned to the soil. Using animals (including the farmers!) that are fed by the farm to perform farm labor is another form of energy recycling. Anything that has to be imported, like fertilizer for plants, or petroleum products to run machinery, reduces sustainability. The less the farm needs outside inputs to maintain production levels, the greater its level of sustainability.
In environmental terms, given the finite supply of natural resources, agriculture that is very inefficient - low on the sustainability scale - will eventually run out of resources, or the ability afford scarce resources, and cease to be viable. And agriculture that relies mainly on outside inputs contributes to the depletion and degradation of natural resources.
In an economic context, the farm must generate revenue in order to acquire things that cannot be produced directly. The way that crops are sold then becomes part of the sustainability equation. Fresh food sold from a farm stand requires little additional energy, beyond growing and harvest. Food that is packaged and sold at a remote location, like a farmers' market, incurs a greater energy cost, for materials, labor, transportation, and so forth. The more complex the food system in which the farm participates, the greater the farm's costs, including energy consumption, and the more it relies economically on externals, notably, the price of oil.
In a social context, the approaches required for higher sustainability profoundly affect business methods and our way of life. Current large-scale agricultural practices are not conducive to sustainability. In order to increase sustainability, significant changes in agribusiness would be required.
In practice, there is no single approach to sustainable agriculture, as the precise goals and methods must be adapted to each individual case.
