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Of the earth's 148,000,000 km² (57 million square miles) of land, approximately 31,000,000 km² (12 million square miles) are arable; however, arable land is currently being lost at the rate of over 100,000 km² (38,610 square miles) per year. A major element of arable land loss comes from deforestation (starting in the Middle Ages in Europe as well as Asia). Such deforestation continues to the present day primarily in tropical countries by commercial over-exploitation of tropical forest. At times, deforestation can be so extreme that it leads to desertification, or the total loss of arable land, as has occurred in portions of the central highland plateau of Madagascar following extensive slash-and-burn activity.
A smaller, but important loss of arable land arises from the lack of renewal of rich flooding sediment due to flood control works. A large part of the arable land on earth is around the largest rivers on earth; for example, the Nile River, the Mississippi River, the Tigris and Euphrates Rivers, the Yellow River, the Amazon River, the Ganges and the Rhine River.
The most productive portion of arable land is that from sediments left by those rivers and the sea in geological times. In modern times, the rivers do not generally flood as much agricultural land, due to the demands of flood control to support intensive agriculture required of a heavily-populated Earth.
The Nile continues to flood regularly, overspilling its banks. When the flood is over, the waters recede, leaving behind rich silt. This silt provides excellent fertilizer for crops. Even if the land is over-farmed and all the nutrients are depleted from the soil, the land renews its fertility when new deposits of silt arrive following the next flood. Flood-control projects in the region, such as levees, may increase human comfort but cause substantial adverse impact to the quantity and quality of arable land.
Land which is unsuitable for arable farming usually has at least one of the following deficiencies: no source of fresh water; too hot (desert); too cold (Arctic); too rocky; too mountainous; too salty; too rainy; too snowy; too polluted; or too nutrient poor. Clouds may block the sunlight plants need for photosynthesis (making sunlight into food), reducing productivity. Plants can starve without light. Starvation and nomadism often exists on marginally arable land. Non-arable land is sometimes called wasteland, badlands, worthless or no man's land.
However, non-arable land can be converted into arable land. New arable land makes more food, and can reduce starvation. This outcome also makes a country more self-sufficient and politically independent, because food importation is reduced. Making non-arable land arable often involves digging new irrigation canals and new wells, aqueducts, desalination plants, planting trees for shade in the desert, hydroponics, fertilizer, nitrogen fertilizer, pesticides, reverse osmosis water processors, PET film insulation or other insulation against heat and cold, digging ditches and hills for protection against the wind, and greenhouses with internal light and heat for protection against the cold outside and to provide light in cloudy areas. This process is often extremely expensive.
Some examples of infertile non-arable land being turned into fertile arable land are:
- Aran Islands: This island off the west coast of Ireland, (not to be confused with the Isle of Arran in Scotland's Firth of Clyde), was unsuitable for arable farming because it was too rocky. The people covered the island with a shallow layer of seaweed and sand from the ocean. This made it arable. Today, crops are grown there.
- Israel: Israel's land primarily consisted of desert until the construction of desalination plants along the country's coast. The desalination plants, which remove the salt from ocean water, have created a new source of water for farming, drinking, and washing.
- Slash and burn agriculture uses nutrients in wood ash, but these expire within a few years.
Some examples of fertile arable land being turned into infertile land are:
- Droughts like the ' dust bowl' of the Great Depression in the U.S. turned farmland into desert.
- Rainforest Deforestation: The fertile tropical forests turn into infertile desert land. For example, Madagascar's central highland plateau has become virtually totally barren (about ten percent of the country), as a result of slash-and-burn deforestation, an element of shifting cultivation practiced by many natives.
- Romans' destruction of Carthage: At the end of the Punic Wars, legend has it that the victorious Romans sowed the earth with salt, to symbolize total victory. The Roman symbol meant that Carthage would never grow back - their civilization ended. (Whether this actually happened is debatable due to the logistics involved. Salt was very valuable and was used as money at the time, and it would have taken a lot of salt to ruin the whole area. See Carthage for details.) Most crops do not grow in highly saline soil. Consequently, salt water cannot be used to water crops.
- Each year, arable land is lost to desertification and erosion from human industrial activities. Improper irrigation of farm land can wick the sodium, calcium, and magnesium from the soil and water to the surface. This process steadily concentrates salt in the root zone, decreasing productivity for crops that are not salt-tolerant.
- Urban sprawl: In the United States, 8,900 km² (about 2.2 million acres) of land was added to urban areas between 1992 and 2002.