Agriculture can deplete the soil, especially when the same crop is planted year after year. Each plant species leeches something from the soil while giving something else back. For instance, soybeans (Glycine max) leave nitrates in the soil while corn (Zea mays) absorbs nitrates. To replenish the chemical composition of soil after annual crops such as these, some farmers and gardeners let their land go fallow -- or unplanted -- so that the soil's natural nutrient balance can be restored.
Crop Rotation and Fallow Land
The core philosophy behind crop rotation is to never allow crops to completely deplete the soil of any one nutrient. Alternating different plants helps keep that balance intact. Letting a field lie fallow, free from any cultivated crop, is often part of a good crop rotation program. By remaining unsown, the ground rests and fertility can be restored. Crop rotation is all the more important in fields where the soil is prone to depletion or where demanding crops have been grown. The same philosophy used by farmers applies to home gardens as well.
Allowing land to stand fallow has been practiced by farmers for centuries. It was commonly done in Medieval times, long before technologies existed to permit persistent, large-scale plantings. With the advent of commercial fertilizers and the increase of agriculture as an industry, it became more and more unpopular to leave land fallow and unproductive in Western societies. Production won out over soil health. Fields supported by chemical fertilizers were left in constant production, but soil health suffered. Home gardeners followed farming examples.
One of the earliest, most primitive forms of crop rotation was what is known as two-field rotation. Farmers divided their fields in half, planting on one half in one year and then the other half the following year. Each year, one half of their land was allowed to rest and be renewed. This cycle was repeated indefinitely. This process had the intended effect of replenishing the soil, but it left half the arable land unused and unproductive. Over history, many cultures continued to nurture the land through leaving it fallow, but others turned to new methods to keep land in constant use.
As time passed, three-field rotation became more popular as an advanced form of crop rotation. As the name suggests, arable fields were divided into threes. In the first field, a farmer sowed annual grains such as barley (Hordeum vulgare) or wheat (Triticum aestivum). In the second field, he planted an annual legume such as lentils (Lens culinaris) or peas (Pisum sativum). The legume helped replenish nitrogen depleted by the grain. The farmer left the third field unplanted, or fallow, so that nutrients could increase in the soil. The same technique filtered down to home gardeners. This allowed fallow land to rest, but kept more land in production. Only one-third of the arable land went unused.
For many years, allowing land to stay fallow was considered a last resort for farmers and gardeners. Fertilizers and intelligent companion planting came first. However, modern farmers and gardeners often turn to the wisdom in ancient farming practices. This philosophy fuels the idea of sustainable farming and gardening, where wise crop rotation -- and allowances for fallow soil -- keep soil naturally sustained rather than depleted. Crop rotations and fallow ground may be seasonal as opposed to annual. Instead of leaving a garden fallow an entire year, seasonal spring and fall crops are alternated. Garden soil nutrients stay high and gardens stay healthy and productive.