Crop rotation is one of the oldest, simplest, and most effective non-chemical methods of preventing soil exhaustion or pest or chemical buildup due to agriculture. For this reason it is a popular tool in the ecological and organic farming movements, which attempt to reduce reliance on potentially harmful modern and industrial farming tools such as chemical fertilizers and pesticides.
Essentially, all farming faces a basic problem because it attempts to intensively cultivate a single (or a very small number of) specific plant species rather than allowing a mix to grow and die naturally in the same space. Different crops require different amounts of nutrients in the soil – although all crops require nutrients from the soil. Over time, this means that the soil is gradually depleted of essential nutrients, even as pests specifically attuned to the particular plant species under cultivation gradually build up their local populations. This process continues until it exceeds the capacity of the existing technology to combat it, at which point crop yields decline and eventually it becomes impossible to cost-effectively harvest crops in the area in question.
For thousands of years, the standard method for combating this depletion was to develop a system of so-called crop rotation in which land was subjected to different development (and, ultimately, permitted to lie uncultivated, or fallow) over a multi-year cycle so that it could replenish. This form of crop rotation was developed, either independently or through imitation, by major civilizations throughout Asia, Africa, and Europe. In ancient Rome, for example, farmers typically followed a two-field system, in which half of fields were cultivated in one year, and the other half the next (in the meantime the unfarmed field was permitted to lie fallow and recover).
In medieval Europe, a period with which most people are somewhat more familiar, this system was gradually replaced by a more intensive three-field cycle. Under the three-field crop rotation system, one field is sown with rye or winter wheat, another with peas or beans, and a third lies fallow. Each year the fields progress through the rotation: for example, a given field may be planted with wheat in the first year, peas in the second, and nothing in the third, before returning back to wheat in the fourth year. Barley or other spring crops went on to replace peas and beans in the second-field portion of the cycle. Switching to the three-field system paved the way for considerable population increase, since it effectively increased agricultural yields by one-sixth and also allowed for a much greater variety of crops to be produced on the same land.
Today, intensive industrialized farming allows greater exploitation than ever before. However, organic and ecological farming advocates argue that industrialized farming practices are ultimately unsustainable: sooner or later, human innovation will lose the battle with soil depletion and farms will face ruin. Instead, they argue for a return to non-chemical and other forms of farming which are theoretically less damaging and therefore can continue over a longer period. (Ironically, while this may be true given current ecological knowledge, in the past most major civilizations in the Middle East and elsewhere did cause long-term devastation to their environments thanks to farming, including deforestation, salinization, and desertification.) One of the important ways of doing so, according to these advocates, is a return to modified forms of traditional crop rotation, allowing fields to replenish naturally and therefore reducing our dependence on chemical fertilizers and pesticides to keep the land productive.
