Double cropping refers to growing two crops subsequently on the same piece of land during a single year. It is not the same as intercropping or companion planting; both of these involve planting more than one type of plant on the same land at the same time. Nor is it quite the same as crop rotation, although there is some overlap between the concepts. Double cropping takes place within a single year, while crop rotation usually takes place over multiple years. All of these various practices are types of polyculture.
Double cropping is technically a subset of multiple cropping, but most parts of the world cannot manage three crops a year. Rice is one of the few crops that is sometimes farmed to produce three crops from one field in one year, but that is only possible in warm, wet climates. Most double and multiple croppings rely on newer strains of common grains that mature more quickly than traditional strains, so we may see more three- or even four-crop yields in the future. This sort of extreme productivity might also require fancy hydroponic or aeroponic techniques. In the meantime, most agriculturists still think double cropping is pretty cool.
The most obvious benefit of double cropping is increased yield, but there are others pluses. A second crop growing on the land means that the field has greater resistance to erosion, reducing soil loss and harmful runoff into nearby bodies of water. There is also the possibility that double cropping can help break pest cycles that are encouraged by monocultures, and thus reduce pesticide use.
The best known type of double cropping is the growing of winter wheat. After the summer crop (often soybeans or corn/maize) is harvested, a second crop of wheat can be planted. If things are planned out correctly, the wheat will have germinated before the first snow, and the seedlings will remain insulated underneath the snow for the winter, neither growing nor dying, until the snow melts in the spring. The wheat then grows to maturity, and is harvested just in time for the next summer planting. Canola is also a good winter crop, hibernating through the winter in the same way as wheat. Since canola stubble is smaller and lighter than wheat stubble it is also a good choice for farmers who are using no-till farming methods. The biggest downside to canola is that it needs to be planted earlier than wheat, before many summer crops are ready to be harvested.
Because summer crops need to be planted and harvested in the window between the harvest and planting of the winter crops, they often have a lower yield than full-season crops. The advantages a second harvest generally make up for the reduced yield of the primary crop, but farmers must choose their crops carefully. Soybeans are a very popular choice for the summer crop, as they fix nitrogen, renewing the soil for the next crop. Winter cropping can greatly increase the food yield from a piece of land, although if the snow melts early or there is a very severe freeze you can loose the whole winter crop.
In some cases one of the crops can be planted before the other has been harvested, a technique known as relay cropping. This allows a farmer to plant crops that he otherwise could not plant because of clashing planting/harvesting schedules, but it also takes extra planing. The planting/harvesting of one crop can hamper the growth and food production of the other, and many relay cropping techniques are still in the developmental stage.
Some farmers have the option of simply planting two crops during the summer season. Farmers in the American mid-west will sometimes have as much as another three months of potential growing season remaining after the wheat harvest. If they are very quick, there is just enough time to squeeze in a quick crop of soybeans before winter sets in. This requires conditions to be just right; the wheat has to be harvested as quickly as possible, and soybeans will not germinate unless there is rain. Even with optimal conditions, the late-summer soybean harvest will be significantly smaller than a normal soybean harvest.
But many parts of the world are warm enough to grow crops year-round. In many cases the primary block to double cropping is the water supply, which may dry up after the rainy season. If water is not a problem, farmers will often alternate corn or wheat with soybeans. If water is a problem, double cropping may not be wise (the water used for irrigation may cost more than the second crop is worth), but rice farmers can sometimes profitably plant a second crop during the time that the rice field is not flooded, usually a bean (mung or soy) to fix nitrogen.
Double cropping is good for farmers (more produce means more money) and good for the environment (producing more on a unit of land means less land will need to be devoted to farming). If done intelligently double cropping has a great potential to increase the world's food supplies. India and China have already started using double cropping to their benefit. America and Europe have long restricted crops to avoid surpluses, but they have continued to research double cropping methods even so. While shortages of water and healthy soil continue to limit food production in many areas, double cropping has the potential to raise food production in some of the world's most crowded countries.