Agriculture can be draining to the soil, especially when farmers plant the same crop year after year. The reason is simple: Each species of plant leeches something from the soil while giving something else back. For instance, soybeans leave nitrates in the soil while corn absorbs nitrates. To replenish the chemical composition of soil after a crop, some farmers let their land go fallow so that the native plants can naturally restore the soil's balance.
The core philosophy behind crop rotation is that letting a field lie fallow enables it to restore minerals depleted by crops. Crop rotation is all the more important in fields where the soil is prone to depletion or where demanding crops have been grown.
Allowing land to stand fallow was commonly done in Medieval times when technologies had yet to develop that permitted persistent planting in a field. Since the advent of commercial fertilisers and the increasing understanding of common planting, it has become more and more unpopular to leave land fallow in Western societies. Leaving the land fallow reduces yields considerably.
The most primitive form of crop rotation was known as two-field rotation. Farmers would divide their fields in half, planting on one half in one year and then the other half the following year. This cycle would then repeat itself indefinitely. Although this process had the intended effect of replenishing the soil, it left half of the arable land unused.
Three-field rotation was a more advanced form of crop rotation. As the name suggests, arable fields were divided into threes. In the first field, a farmer sowed grain such as barley or wheat. In the second field, he planted a legume such as lentils or peas. The legume helped replenish nitrogen depleted by the grain. The farmer left the third field fallow so that other nutrients could return to the soil. Although this technique was considerably more efficient than two-field rotation, one-third of the arable land went unused.
In modern practice, fallowing is generally considered a last resort, with fertilisers and intelligent companion planting coming first. When it is used, it's often seasonal as opposed to annual. Instead of leaving a field fallow for an entire year, a farmer might engage in "summer fallowing," harvesting one crop in the late spring and planting the following in early fall.