The most commonly used technique for creating more land for agricultural uses was called

To irrigate is to water crops by bringing in water from pipes, canals, sprinklers, or other man-made means, rather than relying on rainfall alone.

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Cultivation can benefit agriculture by controlling weeds and pests, and by creating suitably sized soil aggregates for a good seed bed. However, cultivation also disrupts microbiological activity and causes oxidation of organic matter. Its effect on soil organisms and organic matter has been likened to a fire through ploughed soil: cultivation causes a decline in organic matter, which can lead to a general loss of fertility, unless counteracted by actions such as using fertilisers and rotating crops or pastures to restore organic matter levels. Loss of organic matter often leads to soil structural problems, such as surface sealing and hard-setting. Excessive cultivation was widespread during the first half of the 20th century, and still remains a problem in some locations.

Conservation agriculture is a set of soil management practices that minimise the disruption of the soil’s structure, composition and biodiversity. During recent decades, techniques of conservation agriculture have been developed that emphasise retention of crop residues, appropriate rotations with legumes and reduced tillage, or even no tillage. In these systems, seed is drilled directly into the soil, minimising disturbance of soil structure and biota, oxidation of organic matter and the threat of erosion. Maintaining soil cover on sloping land is especially important to protect against erosive rainfall. These changes have a major influence on soil condition and trend. Although declines in uptake of conservation agriculture (e.g. direct drilling) have been reported (Darbas et al. 2013), in some jurisdictions the proportion of cropping land sown using no-till methods increased from 16 per cent in 1999 to 67 per cent in 2013.

Continuous dryland cropping increases run-off and causes erosion, and long-fallowing dryland cropping contributes to rising watertables. Irrigated agriculture also contributes to rising salinity levels, with run-off of sediments, nutrients and pesticides. Maintaining vegetation in riparian zones helps to reduce nutrient run-off, trap sediments and reduce erosion, particularly erosion due to summer rainfall (Darbas et al. 2013). There is also some data to suggest that wetlands have a role in sequestering nutrients from water, although this may involve accumulation in wetland soils, with the potential for remobilisation during flood events (McJannet et al. 2012).

Conservation agriculture practices have the potential to improve crop yields while maintaining soil ecological health. The minimum-tillage and direct-drilling practices of conservation agriculture, which reduce water erosion through minimal disruption of soil, are widely practised across central and southern New South Wales, south-eastern South Australia, Victoria and Tasmania (Peterson et al. 2014). Nationally in 2014–15, the most common land cultivation practice for crops and pasture was ‘zero or minimum till’ (i.e. no cultivation apart from sowing). Of the pasture land cultivated, 2.3 million hectares received no cultivation apart from sowing, and, of the crop land cultivated, 12.4 million hectares received no cultivation apart from sowing. The use of 3 or more cultivations was the least reported cultivation practice in 2014–15 and had the largest decrease of all cultivation practices, falling by 31 per cent to 660,000 hectares since 2013–14 (ABS 2016c).

The most common crop residue management practice reported in 2014–15 was for standing residue to be retained, which was undertaken on 7.4 million hectares of crops. This practice was followed by residue retained on the ground and residue grazed off, with each reported to be used on 4.8 million hectares of crops. There was a 16.7 per cent decrease in stubble being incorporated into the soil and a 3.5 per cent decrease in stubble being removed by hot burn in 2014–15 compared with 2013–14.

Rates of adoption of conservation agriculture have decreased in the Queensland Murray–Darling Basin catchments, in part as a result of reduced soil extension services, unclear profitability and, possibly, costs of practices. Similarly, although understanding of the farming practices that result in soil erosion and salinity is high in the New South Wales Murray–Darling Basin, adoption of conservation agriculture is low and sometimes decreasing (Darbas et al. 2013).

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land reclamation, the process of improving lands to make them suitable for a more intensive use. Reclamation efforts may be concerned with the improvement of rainfall-deficient areas by irrigation, the removal of detrimental constituents from salty or alkali lands, the diking and draining of tidal marshes, the smoothing and revegetation of strip-mine spoil areas, and similar activities.

In the United States, the most extensive reclamation activities are those concerned with irrigation, particularly in the western United States. The United States Bureau of Reclamation, an agency of the Department of the Interior, has as its central purpose the construction and maintenance of dams, canals, and pumping systems to bring irrigation water to arid, semiarid, or subhumid lands in the 17 western states. Among the more than 600 dams constructed by the Bureau of Reclamation are Grand Coulee Dam on the Columbia River and Hoover Dam on the Colorado River.

Explore traditional Egyptian irrigation methods and tools including shaduf, water screw, and waterwheel

Traditional irrigation methods in rural Egypt.

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Other extensive areas reclaimed from a desert or near-desert status by irrigation are found in Israel, Egypt and other countries of the Middle East, India, Mexico, Peru, Russia, and China. Irrigation of arid lands has been practised in the Middle East for thousands of years and extensive irrigation projects were developed in what is now southwestern United States by American Indians long before the arrival of Europeans. The real impetus to reclaim the arid western United States by irrigation, however, came about the middle of the 19th century when the Mormons established settlements in the Great Salt Lake basin, and settlers drawn to California by the discovery of gold turned to irrigation in order to supply themselves with food.

The extent to which land is improved by irrigation is related to the aridity of the land in its natural state. The deserts of the Middle East, the alluvial valleys of the Central Asia, the Imperial Valley of California, and the Columbia basin are examples of areas reclaimed from a near-desert state to a highly productive intensive agriculture. Land reclamation by irrigation is, however, not limited to desert regions. Much of the land irrigated in humid areas throughout the world is used for the production of paddy rice, although irrigation of vegetable crops, usually with overhead sprinkler systems, also is practised on a considerable acreage.

Imperial Valley

Water is diverted at the Imperial Dam before flowing into the All-American Canal (centre-right foreground). The canal carries water from the Colorado River to the farms of the Imperial Valley.

Andy Pernick/U.S. Bureau of Reclamation

Many irrigation projects have failed because of salt accumulations in the irrigated soils or the silting in of reservoirs and irrigation canals. It is essential, therefore, that provision for drainage of the irrigated lands be considered in the planning of all irrigation reclamation projects and that water relatively free from dissolved salts be used.

Discover how water projects begun under Soviet rule led to the rapid evaporation of the Aral Sea

An overview of the shrinkage of the Aral Sea.

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Reclamation of land by irrigation was extensively developed by the Soviet Union. By the late 1950s the Soviets reported a total of about 27 million acres (11 million hectares) under irrigation, about one-half of this being in the Central Asian republics. The Syr Darya and Amu Darya rivers were the principal sources of irrigation water in this area. These rivers discharged into the Aral Sea, and diversion of these rivers from their natural course caused an ecological disaster in the region. With its primary sources of inflowing water gone, between 1960 and 2010, the Aral Sea shrank to a tiny fraction of its former size. The collapse of the Soviet Union left the now-independent Central Asian republics bordering the Aral Sea struggling to coordinate a response to the crisis. The loss of the world’s fourth largest body of inland water led to more-extreme temperature swings in the region, and wind storms carried contaminated dust from the exposed seabed across the region. Populations living in the region subsequently experienced a range of diseases and disorders at markedly high rates.