Most coastal plains are rich in plant nutrients and are, therefore, suitable for agriculture. But a combination of human activities, overgrazing, and erosion can reduce this potential. With enough water for irrigation, shores and even deserts can also be used for cultivation, although the loose composition of sandy soil (and its large grain size) causes rain water to soak through it rapidly, leaching out the nutrients. Such terrain is also particularly vulnerable to wind erosion, which tends to strip away the topsoil. For thousands of years people have tried to reclaim coastal and desert land, to stabilize the areas, and eventually cultivate them.
Shores and dunes
Estuaries and shorelines are reclaimed initially to protect the land from tidal flooding and the seepage of salt water.
Coastal dune systems are also formed to protect the hinterland. They usually comprise a frontal dune, which forms a buffer zone, and a hind dune, which protects the sheltered areas from salt spray or sand-drift damage. Dune formation can be encouraged by placing an obstacle in the path of the prevailing wind, which causes it to slow down and deposit some of the sand that it is carrying. The obstacles may be fences but most often consist of plants. Marram grass (Ammophila breviligu-lata), for example, has a rapid rate of upward growth, is perennial, and can reproduce vegetatively. It therefore increases the rate of sand accumulation while keeping above the rising levels. Dunes produced in this way typically reach a width of 100 feet (30 meters) or more and a height of 33 feet (10 meters) in about 10 years. Other dune-forming methods include the spreading of a film of rubber compound on the mobile sand, which stabilizes it enough to allow the germination and growth of such grasses as couch grass (Agropyron junceiforme).
Once the initial stabilization of the dune has been achieved, tree lupines (Lupinus arboreus) are frequently planted to further stability, together with the spiny sea buckthorn (Hip-pophae rhamnoides), which discourages animals and people from trampling on the dune surface, causing it to erode.
Dunes are made usable in several ways after stabilization. Nitrogen fertilizer is usually added, and shrubs such as Scotch broom (Cys-tisus scoparius) are planted as an intermediate step before trees are established. Crops of lucerne or alfalfa (Medicago sativa) are planted in temperate areas to introduce nitrogen into the soil. Two to four years after planting marram grass, the dunes are sufficiently stabilized to introduce tree species that can tolerate salty winds, such as Monterey pines (Pinus radiata), gum trees (Eucalyptus spp.), and acacias (Acacia sppl These plants are deep-rooted and can resist drought. They also grow rapidly and can soon form a stand of mature trees. Acacias help to increase the nitrogen content of the soil by means of bacteria contained in their root-nodules.
In Argentina, crops of rye and sorghum, with the addition of millet, have successfully stabilized dunes covering 15 acres (6 hectares) in as little as 18 months. This has then facilitated the spread of natural vegetation.
The main cause of desertification is inappropriate methods of land use on the fragile semi-arid marginal lands.
Desertification can be reduced by the rational use of land resources from the individual level upward, by the introduction and improvement of proper irrigation methods, increased soil moisture storage, the restoration of degraded pastures, and the improvement of strategies of pasture rotation.
Some deserts have underground water deposits that have been tapped by wells to enable the irrigation of crops. This practice is used in north Africa, the Middle East, and the southwestern deserts in the United States. In the American deserts, water is being used more rapidly than it can be replaced. The result is the need to drill deeper wells for the limited water.
One of the problems of irrigation is that, depending on the mineral composition of the water, it can lead to the salinization or alkalin-ization of the soil, and vegetation cannot be established. To counteract salinization, flushing irrigation is used to keep the soil constantly moist; the careful application of chemicals to the soil, such as gypsum, prevents alkalinization.
Pastures that have been overgrazed have been improved in some desert areas, as in Kazakhstan, by the introduction of forage plants. These include prostrate summer cypress (Kochia prostrata), oriental saltwort (Salsola orientalist, and Mexican mesquite (Prosopis sp.). The value of these shrubs is that when grown in stands they reduce wind speed and protect the soil from drying out and contracting. In addition, they survive the parching summer heat (whereas many other plants die down) and so provide a food source in sum
mer. They also have a richer protein content than many grasses do.
The use of sand covers, such as oil resins or polyethylene sheeting, helps to prevent some evaporation from the sand surface and stabilizes it so that plants may be established. Fuel-oil mulches have also been used to stabilize dunes but preclude the establishment of natural vegetation. On many stabilized areas, vegetables have been cultivated on experimental production plots, such as the Negev Desert in Israel. These experiments have been successful particularly with the application of nitrogen, phosphorus, potassium, and, occasionally, manure. Increasing effort is being put into land reclamation with these methods, particularly because of the land and food shortages resulting from overpopulation.