Based on the FAO/Unesco Soil Map of the World, Table 1 shows the regional distribution of salt-affected soils. It should be borne in mind that areas given in the table are not necessarily arable but cover all salt-affected lands. The table indicates that the total area of saline soils is 397 million ha and of sodic soils 434 million ha at global level. Of the current 230 million ha of irrigated land, 45 million ha are salt-affected soils (19.5 percent) and of the almost 1 500 million ha of dryland agriculture, 32 million are salt-affected soils (2.1 percent) to varying degrees by human-induced processes.

Saline Soils of the World

Salt-affected lands are reflected as saline seeps in dryland agriculture and secondarily salinized irrigated lands (Tanji 1995). Table 2 shows that globally more than 77 million ha of land is salt-affected by human-induced salinization (Oldeman et al. 1991). The authors have not distinguished between the extent of salt-affected land in irrigated and non-irrigated areas.

Sodic Soils of the World

Dregne et al (1991) estimated that about 43 million ha of irrigated land in the world's dry areas are affected by various processes of degradation, mainly waterlogging, salinization and sodication.

Salinity also poses a major management problem in many non-irrigated areas where cropping relies on limited rainfall. Dryland salinity has been a threat to land and water resources in several parts of the world although only in recent years has the seriousness of the problem become widely known. If it is accepted that 77 million ha of land is affected by human-induced salinization (Table 2), a global total of more than 33 million ha can be attributed to secondary salinization of non-irrigated lands. In rainfed agriculture intrusion of saline seawater to areas lying near the sea can cause land salinization during dry periods.

Based on the FAO/Unesco Soil Map of the World, Table 3 shows the distribution of salt-affected soils in participating counties. It should be borne in mind that areas given in Table 3 are not necessarily arable but cover all salt-affected land. The percentage of saline soils in participating countries amount to 49.2 percent of total saline soils of the world and that of sodic soils to 45.8 percent of total sodic soils of the world.

Table 4 shows the extent of human-induced salinization (saline + sodic soils) in participating countries in irrigated, as well as rainfed, agricultural lands. The table shows that the percentage of human-induced salt-affected soils in participating countries (57.9 million ha) is about 78 percent of the global human-induced salt-affected soils (77 million ha).

The causes and origin of salinity and sodicity must be identified so that they, and not the symptoms, can be controlled.

In semi-arid and arid areas of the world, the scarcity, variability and unreliability of rainfall and high potential evapotranspiration affect the water and salt balance of the soil. Low atmospheric humidity, high temperature and wind velocity promote the upward movement of the soil solution and the precipitation and concentration of the salts in the surface horizons. In arid regions, various types of Na, Mg and Ca salts are concentrated, mainly chloride and sulphate: salinization.

In less arid climates, salts are less concentrated and Na dominates in carbonate and bicarbonate forms which enhance the formation of sodic soils. The sodication process involves the presence of soluble sodium salts in the soil solution and their adsorption on the exchange complex. The process of sodication includes desalinization in the absence of enough divalent cations and with insufficient drainage, evaporation of groundwater rich in NaHCO3 and Na2CO3, decomposition of sodium alumino silicates, denitrification and sulphate reduction under anaerobic conditions; use of water with low salinity but with dominant HCO3 anions for irrigation and migration and accumulation of sodic salts in arid climates. Desalinization in the absence of enough divalent cations and with insufficient drainage is the main cause of sodication in participating countries.

Two main salt accumulation cycles can be distinguished, i.e. natural and anthropogenic cycles. Natural cycles include marine cycles, continental cycles, development of saline seeps in rainfed agriculture and artesian cycles. Of these natural cycles, marine cycles have been considered the main natural cause of salinization in the ongoing collaborative projects in the countries participating in the Network. Marine cycles are connected with the accumulation of marine salts in areas lying near the sea or saline lakes. The seawater directly or indirectly influences soils and groundwater of these areas, giving rise to saline soils and groundwater with salt concentrations ranging between 25 and 100 g/l.

Extent and Causes of Salt-affected Soils in Member Countries

Countries with FAO Collaborative Projects

Ghana: Total of 788 000 ha of salt-affected land have been mapped and classified mainly as Arenosols (70 000 ha), Solonetzs (600 000 ha) and Solonchaks (118 000 ha).The greatest extent of these salt-affected soils occur within coastal scrub and savannah and mangrove swamps of the Lower Volta Basin in the Greater Accra and Volta Regions. With the exception of sandy soils of the bar between the Kata and the sea, the rest of the affected soils are heavy textured exhibiting halomorphic properties.

Most of the salt-affected soils of the country occur within the Accra-Ho-Keta Plains which cover the southeastern corner of the country, extending eastwards along the coast from Accra to the border with Togo. They occur within the Coastal Savanna Agro-ecological zone. The major problems associated with these soils are their high salt content intolerable by crops, hydromorphic nature, heavy textures and poor nutrient contents. Impenetrable sodium-saturated pan occurs in some of these soils on the uplands (Agawtaw series). Elsewhere along the coasts of Western and Central Regions, outside the Plains, patches of these salt-affected soils occur.

Natural cycles are mostly responsible for salt accumulation in soils of the Plains. The semi-arid nature of the Plains, with only 760 mm of unreliable annual rainfall and high potential evapotranspiration, greatly affect the water and salt balance. The high and constant temperatures, low relative humidities, and constant high wind verosity of the Plains are important factors in promoting upward salt movement into the exploitable volume of the soil. The soils have been salt-affected mostly as a result of intrusion and inundation by seawater on low-lying tidal flats and flood Plains. Overland soils sprays blown from the lagoons and the sea (Gulf of Guinea of Atlantic Ocean) running along the coast of Ghana, are important causes. Elsewhere within the Plains, the problem is attributed to natural cycles during the weathering processes of salt-bearing rocks (Agawtaw series). In the upland, salinity is due to poor soil and water management and poor drainage

Kenya: Sodic and saline soils in Kenya cover approximately 8.2 million ha and occur mostly in North-Eastern, Eastern, Coast and parts of the Rift Valley Provinces. Most salt-affected soils are unfavourable for crop production unless special management techniques are used or salt-tolerant crops are grown. Total irrigated land in Kenya is about 66 000 ha, of which more than 20 000 ha is estimated to be mainly saline. The Kimoriga scheme, for example, has been out of operation since 1966, due to salinity problems although some drainage systems were constructed. Part of the 400 ha Komleza scheme that used to grow rice is also out of operation since 1966. The land was found to be sodic or saline sodic.

Most of salt-affected soils resulted from natural geological cycles such as migration and redistribution of salts accumulated formerly in sedimentary salt-bearing rocks. Also deposition of salts during the process of weathering and soil formation by surface and groundwater results in salinization. Secondary salinization is caused by soil and water mismanagement through improper irrigation and poor drainage.

Nigeria: Sodic and saline soils in Kenya cover approximately 5.6 million ha and occur mostly in the Sudan savanna in the northern states of the country. The main annual rainfall is about 850 mm with 524 mm, or over 60 percent occurring in the months of July and August with potential evapotranspiration exceeding 1400 mm per annum. The rainfall is, therefore, unable to satisfy all year round crop production and hardly provides leaching, particularly on lands where subsoils are slowly permeable and internal drainage is lacking. Total irrigated area in Nigeria is about 300 000 ha, however, the cropped irrigated land is estimated at 150 000 which falls far short of the planned area that should be in production. About 20 percent of these lands, mostly in semi-arid northern Guinea and Sudan Savanna have been degraded as a result of salt accumulation in the soil. Examples of irrigation projects seriously salinized can be found in Hadejia, Kano and Sokoto-Rina Valleys and Turgon Tudu and Kalmalo Lake projects in the northwest. In the northeast there are such projects as Yau and Abadon irrigation schemes on the northern shores of Lake Chad which have similarly been affected by salinity.

Problems of salt-affected soils are attributed to natural cycles during the process of weathering in salt bearing rocks; mismanagement of soil and water, the use of low quality water for irrigation without provision of leaching and subsurface drainage.

Tanzania: Based on the FAO/Unesco Soil Map of the World, the actual extent of the salt-affected soils in Tanzania is estimated as 1.7 million ha saline soils (local estimations give an area of 2.9 million ha) and sodic soils as 300 000 ha (local estimation gives an area of 700 000ha). One of the most affected sectors with respect to salt-affected soils in Tanzania, is irrigated agriculture. Estimates suggest that 5 million ha are potentially suitable for irrigation in Tanzania. So far only 190 000 of this land is actually being irrigated in different schemes. In regions like Kilimanjaro, some of these schemes have already been abandoned because of the hostile soil conditions on the staple crops in the area. The frequent development of salt-affected soils in irrigation schemes is often related to poor soil management, which plays a big role in their degradation.

In Mlingano on 10 estates, with the exception of Mufindi Tea Estate, it is suggested that they are already encountering major salt related problems in their soils. Besides these, there are many smaller irrigation schemes classified as traditional irrigation schemes, which are community managed. Three of such schemes were all found to have developed saline and sodic soils largely because of mismanagement of the soils and irrigation and drainage principles. A further expansion into irrigated areas is expected in the regions of Kilimanjaro/Arusha (4 000 ha), Mbeya Region - Usangu Plains, Kimani Village (5 000 ha), Tanga (2 000 ha) and Tabora, Shinyanga, Dodoma, Mwanza regions (4 600 ha) as part of the Mbuga Development Project. As long as soil management may be a problem in these lands, it may be proper to regard them as potential salt-affected soils.

The occurrence of salt-affected soils across the country has been associated with various factors. The causes of salt-affected-soils can be classified into three groups: (a) climate and inherent soil properties: in some parts of Tanzania soils become salted either because of the aridity of the surrounding environment or the soils have developed from a parent material which is more prone to development of saline soils such as in Pangani valley and the Rift valley areas in Arusha region, and the areas surrounding Bahi swamps in Dodoma which are in the northern and central parts of Tanzania, respectively; (b) landform/topography such as in Dodoma region in central Tanzania, around the Bahi swamps, Mahomanyika and Hombolo areas and Mtwara region in the southern end of the country; and (c) man-made causes including poor designing of the irrigation schemes, seepage from irrigation canals and drainage systems which are not incorporated in the initial layout such as at Kileo and Kigonigoni in Mwanga district.

Bangladesh: The salt-affected zone of Bangladesh is essentially the coastal zone. The total area in the coastal regions of Bangladesh affected by varying degrees of salinity was about 0.83 million ha in 1966-75. Over the last two decades the area affected by salinity has expanded markedly from 0.83 million ha to 3.1 million ha. Presently, slight to strong soil salinity problems exist in 20 districts. The distribution of the saline areas are situated in four of the thirty AEZs of the country [the Ganges Tidal Floodplains (AEZ-13), the New Meghna Estuarine Floodplains (AEZ-18), the Chittagong Coastal Plains (AEZ-23) and St Martin’s Coral Island (AEZ-24)].

The causes of soil salinity include: (a) natural causes such as tidal flooding (the freshly deposited alluvium from upstream becomes saline as it comes in contact with seawater through the rivers, canals and creeks); tidal surges due to cyclones or exceptionally high tides push the salinity front further inland and into the groundwater; and (b) anthropogenic causes such as the reduced availability of fresh water, drying up of rivers and saline water intrusion from the sea into the Ganges basin area due to the withdrawal of the Ganges river water upstream outside the boundaries of Bangladesh; upland shrimp culture causing seepage of saline water from the shrimp ponds into adjacent agricultural lands increasing soil salinity; sinking of shallow tube wells; and irrigation from poor quality irrigation waters.

The soil and water salinity levels vary widely from place to place and with seasons in the year. Irrespective of location, the common trend is an increase in salinity with time from November-December to March-April until the onset of the monsoon rains. Usually July-August is the period of minimum salinity, January-February of intermediate salinity and March-April of maximum salinity corresponding with the peak dry season.

China: In China 38.5 million ha are salt-affected in various degrees. The main types of salt-affected soils include:

a) Coastal salt-affected soils, spreading along the coastal area of China, mainly caused by saline parent material and seawater intrusion. Large-scale water projects have also caused salinity problems.

b) Fluvo-aquic and meadow salt-affected soils are widely distributed in the North China Plain, and other northern regions of China. These types of soils are formed under the influences of groundwater, surface water and cultivation. Irrational irrigation and poor drainage management play significant roles in formation and evolution of salt-affected soils.

c) Solonetz and alkaline soils, mainly distributed in Inner-Mongolia and Northeast China regions and intervening in the North China Plain, are formed both under natural and anthropogenic processes. Cultivation and poor irrigation management has certain influences on the formation of sodic soils.

d) Haplic solonchaks, mainly spreading in the inland region, such as Northwest China, are caused by strong evaporation of groundwater and surface salt accumulation.

e) Potential salt-affected soils, mainly distributed in arid and semi-arid regions of China, are neither saline nor sodic at present but may result in severe salinization or sodication under human intervention, especially by inappropriate irrigation systems. Improper exploitation of land and irrational management (especially irrigation) in some areas has enhanced salinity problems or even induced secondary salinization or sodication.

In general, irrigation mismanagement including shortage of fresh water resources, irrigation at low efficiency, seepage from canals with insufficient drainage system and generally poor soil and water management are the main causes, particularly in Xinjiang Region. Soil salinity in coastal areas (Jiangiu Province) is due to accumulation of salt in soil from parent materials, seawater intrusion in both surface and groundwater and also due to poor soil and water management.

Indonesia: Indonesia consists of more than 13 000 islands, with a total area of 192 million ha. Swamp land covers vast areas estimated at 33.4 million ha, which are divided into three zones: (i) brackish/saline tidal lands, (ii) fresh water tidal lands and (iii) non-tidal lands. The coastal lands are in the brackish/saline tidal land category and are estimated at 8.09 million ha. The coastal saline lands have been differentiated into four types: (i) beach ridges/dunes, (ii) potential lands, (iii) acid sulphate lands and (iv) peat lands. Parts of each type are saline because of brackish/saline intrusion. Causes of coastal saline tidal land include seawater intrusion and soil and water mismanagement, and also agricultural activities extension in coastal problem soils (acid sulphate and peat soils) without appropriate soil management measures.

Pakistan: Of the 20.2 million ha being cultivated in Pakistan, about 16 million ha are being irrigated through canal and tubewell networks. Of the irrigated land, 26 percent is affected by salinity to different degrees. About 70 million ha land in Pakistan can be classified as falling in the semi-arid zone, including 11 million ha of deserts, e.g. Cholistan, Thar, Thal and Kharan. The desert areas have peculiar types of problems different from irrigated regions, i.e. high rate of evaporation and strong summer winds. Water is the most critical and limiting factor that prohibits using the area for agricultural production. The underground water is mostly saline.

Salt-affected soils in Pakistan (total of 11.5 million ha) are classified as slightly saline-sodic or saline gypsiferous (3.5 million ha), porous saline-sodic or saline gypsiferous (1.9 million ha), severely saline-sodic and saline gypsiferous (1.1 million ha), and soils affected due to use of sodic tubewells water (2.3 million ha). Over 2.5 million ha of irrigated areas is affected with severe surface salinity with 18 percent affecting lands in Sind, 3 percent in the Punjab and 2 percent in NWFP. The moderately affected areas are 10 percent in Sind, 4 percent in the Punjab and 2 percent in NWFP.

Accumulation of rain and floodwaters in natural depressions is the major cause of primary salinity. Secondary salinity due to man-made causes is related to seepage from canals, lack of drainage, use of poor quality groundwater for irrigation, and insufficient leaching and poor land management practices.

The Philippines: The extent of salinity-prone coastal land which is an integral component of the agricultural area has been estimated to range from 0.5 to 0.6 million ha, of which 0.2 million ha are considered seriously salt-affected soils. The distribution of these soils, mostly associated with coastal landscapes, is geographically situated as follows: Luzan 180 000 ha, Visayas 220 000 and Mindanao 160 000. Although these are traditionally rice- producing areas, soil productivity in terms of rice yield is marginal since rice production ranged only from 30 to 50 cavans /ha in severe cases of salinity. The salinity-prone areas in the country are small compared to other countries in the south and south-east Asia, but they are potential and an important resource base for production of rice and other staple food.

The land resources study indicated that salt-affected areas are generally associated with tidal flats and flood-prone alluvial plains.

The degree of salinity in the coastal soils of the Philippines is generally related to typhoons, tidal river flooding, low elevation (generally 5 meters asl or lower), impedance of natural drainage by tidal fluctuation, subterranean saltwater intrusion, overland salt sprays, and use of brackish water for irrigation of rice crops. The productive period of these areas covers only five to six months every year because of inadequate irrigation facilities. February to April are months of low rainfall and high salinity problems. On the other hand, an average of five to six strong typhoons visit the country every year that cause strong tidal surges, widespread flooding, and significant damage to crops and livestock.

Saline seawater intrusion into fresh groundwater or perched aquifer below the farming areas, surface saline seawater intrusion during high tidal movement, natural cycles during the process of weathering of salt bearing rocks, soil and water mismanagement including irrigation with slightly saline water from contaminated groundwater, rivers or irrigation canals being near or constructed along the sea are the main causes of salt-affected soils in the Philippines.

Thailand: Salt-affected soils in Thailand cover an area of approximately 3.4 million ha. Salinity can be caused by factors such as seawater encroachment; shallow and saline watertable; inadequate drainage due to landscape characteristics. Salt-affected soils are mostly found along the coastal area and in the Northeast region of the country as follows:

1. Inland saline soils, covering an area of 2.9 million ha, exist in 18 provinces (Nakornratchasima, Roi-Et, Chaiyapum, Khonkaen, Loei, Mahasarakam, Ubonratchatani, Srisakhet, Udornthani, Nongkai, Sakonakorn, Burirum, Surin, Yasothorn, Nakornphanom, Kalasin, Mookdaharn and Amnartchareon) of the Northeast region.
2. Coastal saline soils occur along 2 600 km of the coastal belt covering an area of 0.58 million ha in 23 provinces (Bangkok, Samutsakorn, Samutsongkram, Samutprakarn, Petchaburi, Prachuabkeereekhan, Chumporn, Suratthani, Nakornsrithammarat, Songkla, Pattani, Narathiwat, Pattalung, Krabi, Phuket, Trang, Pang-nga, Satul, Ranong, Chacheongsao, Chonburi, Chantaburi, Rayong and Trad).
   Salt-affected soils in Thailand are caused both by natural phenomenon and anthropogenic soil salinization as follows:

1. Coastal saline soils are formed from marine and brackish water deposits (active tidal flat areas and former tidal flat areas) and characterized by salt-tolerant mangrove vegetation.
2. Deforestation and planting of shallow rooted crops (e.g., rice, field crops and pastures) are resulting in less consumption of rain water and more excess flowing water down to the watertable increasing the watertable level and causing salinity.
3. Reservoir construction near the salt source or in area close to shallow saline groundwater resulting in increasing shallow saline groundwater and transporting towards the soil surface.
4. Salt making as in the Northeast region of the country. This activity results in seepage of salt water to adjacent areas.
5. Shrimp farming. Brackish shrimp farming in Thailand has been operated along the coastal areas for the last 70 years. The number of farms has rapidly proliferated in recent years. However, because of improper farming practices, soils and water pollution and shrimp disease outbreaks occurring in many cultured areas, brackish water shrimp farming has then been introduced into fresh water areas of arable land in the central plain. At present, shrimp culture is practised in approximately 72 000 ha of the arable lands in the central plain. The increasing number of farms has resulted in large areas of mangrove forests and agricultural lands being converted into shrimp ponds. Moreover, discharged saline water from shrimp ponds into irrigation canals or seepage of saline water from the ponds to adjacent rice farms has caused soil salinization in those areas.

Viet Nam: The salt-affected areas which consist of saline and acid sulphate soils are 4 million ha. The coastal saline soils excluding acid sulphate soils (2.0 million ha) occupy about 2 million ha along the coastal regions with seawater intrusion through river estuaries and creeks as the main causes of salinization. In Viet Nam salt-affected soils are concentrated in the two large deltas of the Red River and Mekong River. The effect of seawater intrusion is only 15 km in-land in the Red River delta, but can reach 40-50 km in the Mekong River delta. According to FAO–Unesco classification, saline soils can be divided into: (a) Gleyi-salic Fluvisols, mainly used for aquaculture (shrimp culture); (b) Hapli-salic Fluvisols, commonly used for cultivating only one rice crop in the rainy season. The main causes of salinity are intrusion of seawater and salt-rich groundwater. As management practices, the farmers construct dykes and leach salts during rice cultivation or make beds for other crops; and (c) Molli-salic Fluvisols due to salt-rich groundwater are used for two rice crops. The management practices include proper water management and drainage systems.

Coastal saline soils in Vietnam can be classified in the following four groups according to the degree of salinity:

• Mangrove saline soils, alternatively inundation of very salty tidal water (potential acid sulphate soils - 716 000 ha)
• Strong saline soils with a total soluble salt of more than 1 percent (400 000 ha)
• Moderate saline soils with a total soluble salt from 0.5-1.0 percent (200 000 ha), and
• Slight saline soil with a total of soluble salt from 0.25-0.5 percent (600 000 ha).

Hungary: The total area of salt-affected soils in Hungary exceed 2.3 million ha, which is more than 24 percent of the territory of the country. Salt-affected soils include both existing and potentially saline and sodic soils which have developed mainly in the Hungarian Plain and only in small spots in other regions of the country. More than 95 percent of the existing salt-affected soils are situated in the valleys of the Danube and Tisza rivers. The salt-affected soils of the Trans-Tisza region belong to the group of solonetz type (alkaline soils with structural B-horizon).

Hydro-geological properties cause the formation of salt-affected soils in lowland areas. Mineralized groundwater, poor physical properties of the soil, high watertable, former saline swamps and bogs, poor natural drainage or no drainage system and soil and water mismanagement cause the formation of salt-affected soils in other areas.

In the Hungarian Plain, extension of salt-affected soils exceeds 25 percent of the total area and there is a potential hazard of salinization/sodication/alkalization in the major part of this depressed lowland. The climate is characterized by 500-600 mm mean annual precipitation against 750-800 mm yearly potential evapotranspiration. This water deficit is equilibrated by surface runoff, seepage in the unsaturated zone and groundwater flow from the neighbouring territories, particularly from the whole extensive Carpathian Basin surface and subsurface water catchment area to the deepest parts of the Basin with poor natural drainage. It leads to the accumulation of soluble weathering products of a large watershed within a relatively small territory of the Basin, because the water evaporates but the transported soluble products remain in the affected region.

Romania: Distribution of salt-affected soils in Romania is as follows:

• Solonchaks occupy (64 000 ha) small areas especially in the driest part of the country (450 mm precipitation and 600 mm evapotranspiration) and are connected to low-lying or depressionary landforms with very shallow (1.5 m depth) and strongly saline (15 g/l) groundwater. They are widespread in the Danube Delta (where some intrusions of marine water occur) and in the eastern part of the Lower Romanian Dunube Plain (Clamatzui Valle and Low Plain of Sireth). They are predominantly medium texture, their salt content is higher than one percent with pH 8.3 to 8.5 and low biological activity
• Solonetz (144 000 ha): are more widespread than Solonchaks. They occur mainly in the sub-humid zone (400-600 mm rainfall and evapotranspiration 600 mm), and occupy dispersed areas corresponding to low lying and depressionary landforms with groundwater deeper (1,5-2,5 m) and less saline (15 g/l) than Solonchaks. They are distributed in the Romanian Lower Danube Plain, Western (Tisa) Plain, in the flood plains of rivers and in the Plain of Moldova. Unlike Solonchaks, the Solonetz show a textural differentiated profile, have a high percentage of exchangeable sodium ((15 percent of CEC) in natric horizon with columnar structure and a pH 8.5-10.
• Salinized and/or sodicized soils (400 000 ha): This group includes a large range of soil types, e.g. Alluvial soil, Gley Soils, Vertisols and Chernozems with shallow (3 m depth) and mineralized groundwater. They are widespread mainly in the eastern part of the Lower Romanian Danube Plain and in the Western (Tisa) Plain. The salt content and/or the exchangeable sodium is less than 1 percent, and less than 15 percent, respectively, in the upper layers.
• Soils with potential salinity (1 021 000 ha): This group includes almost all soils of the sub-arid zone and zone with salinized ground water table at less than 5 m depth. They are found mainly in Lower Romanian Danube Plain and Western (Tisa) Plain and are represented mostly by the phreatic phase of Chernozems developed on loess-like and alluvial deposits. Of this area human-induced salinity (by irrigation water and/or drainage works) cannot be accurately calculated but is estimated as 500 000 ha.

In general, causes of salt-affected soils include poor drainage system in low plains with salt rich shallow groundwater in sub-arid and sub-humid zones, and the geological substratum of old marine intrusion of marine water and unfavourable soil physical properties, and human- induced salinity through irrigation mismanagement and lack of drainage system.

Turkey: Major salt-affected soils are located in Turkey as follows: Konya-Eregli, Aksaray and Malya Plains of the central Anatolian and alluvial plains of lower Seyhan, Igdir, Menemen, Bafra, Soke, Acipayam and Salihli of all the major river systems. The last soil survey indicated that 1.5 million ha of land have some degree of salinity and sodicity problems and 2.8 ha of land have both salinity and waterlogging problems (this survey gives estimated salt-affected soils as almost twice the figure given according to FAO/Unesco Soil Map of the World (2.5 million ha). This soil survey indicated that the mentioned 1.5 million ha which have some salinity and sodicity problems include 60 percent slightly saline, 19.6 percent moderately saline, 0.4 percent moderately alkali, 12 percent slightly saline-alkaline, and 8 percent moderately saline–alkaline, respectively. Although sodium salts are the main components of the salt-affected soils, magnesium soils in Denizli-Aclpayam, potassium-alkali soils in Nide-Bor, Kayseri and gypsiferous soils in Central Anatolian regions are also common in Turkey.

Climatic, geochemical and hydrological conditions of the country often promote salt accumulation in the groundwater and the soil profile, particularly in arid and semi-arid lowlands where groundwater contains considerable amounts of soluble salts. Saline soils often develop in the Konya Basin in association with marshes. In general, poor water management and agronomic practices without sufficient drainage systems are the major causes of salinity.

Argentina: Of the 12 million ha irrigated land in Latin America, 1.7 million ha are in Argentina, of which 600 000 ha are salt-affected soils. The Mendoza River in Argentina carries about 1.3 million tons of salt per year which are applied to the land by irrigation. Also, the Rio Colorado has a considerable salt load reaching about 179 000 ha of the potentially irrigated land receiving river water. Salt-affected soils are widespread in the Lower Rio Colorado Valley, covering an area of 33 000 ha of the 80 000 ha irrigated area. Irrigation with saline water and intrusion of seawater through estuaries and rivers, mismanagement, natural fluctuation of watertable close to surface, waterlogging, insufficient drainage system and poor maintenance of the collector and main drains are the principal causes of salt-affected soils in irrigated areas of Argentina.

Brazil: Although the information about the saline areas is not well defined, it is estimated that 20-25 percent of the irrigated areas near-by rivers and intermittent streams, mainly in alluvial soils, face salinity and/or drainage problems. The source of water for irrigation are surface reservoirs (dams) where the water is stored during the rainy season, rivers and groundwater in sedimentary areas having in general good quality level.

Besides the saline soils of irrigated areas, salt-affected areas by natural causes exist in large areas (2.4 percent of the total land area of Brazil). According to a soil survey made for the States of Bahia, Sergipe, Alagoas, Pernambuco, Paraiba, Rio Grande do Norte and Ceará, comprising of 110 million ha at a scale of 1:500,000, the salt-affected areas were estimated at 9.1 million ha corresponding to 9 percent of the surveyed area, including Soladic Planosol, Solodized Solonetz, Solonetzic Solonchack and Halomorphic soils.

Over-irrigation, poor soil and water management, seawater intrusion and insufficient drainage system with consequent rise in watertable and waterlogging, shallow soils with low fertility and irrigation with saline water without proper management are the major causes of salt-affected soils in Brazil.

Cuba: Salt-affected soils cover 14 percent of the total land (about 1 000 000 ha), distributed in the provinces: Granma 250 000 ha; Holguin 170 000 ha; Camaguey 140 000 ha; Sandi Spiritus 100 000 ha; Villa Clara 80 000 ha; Ciego de Avila 75 000 ha and the rest in smaller areas in Isle de la Juventud, Pinar del Rio, La Habana, Matanzas, Cienfuegos; Las Pinas; Santiago de Cuba and Guantanamo Provinces.

In Cuba natural saline soils cover a small area associated with the muddy zones nearby the sea. Salinization is due to seawater intrusion during extraordinary tides, typically caused by hurricanes. The peat and calcareous sediments that constitute most of the Cuban coastal marshes generally have intermediate salt concentration. In the southeastern part of the Guantánamo Valley, as well as in small coastal valleys of that province, the average yearly precipitation is less than 700 mm. Therefore, it is possible to find fully developed solonchak soil of primary origin. However, in 90 percent of salt-affected soils in Cuba, the salinity is secondary due to:

1. Highlands deforestation: As a consequence of the intensive deforestation, the soil water regime was altered due to the frequency of floods, the watertable of the low zone increased, therefore the territory of the swamped zones increased, and the watertable became saline and shallow and soils became salt-affected.
2. Coastal marsh deforestation: As a consequence of seawater intrusion, secondary coastal solonchak soils have formed. The coastal zones remained unprotected and in many areas the sea has destroyed them.
3. Use of saline groundwater for irrigation: The intensive exploitation of groundwater in the karstic zone close to the sea has caused salinization of the groundwater. This problem first occurred in the 1950s in the Pinar del Rio Southern Plain, in relation to rice production.
4. Rise of the watertable: Due to the construction of a great number of dams and irrigation systems without drainage systems, raised the level of watertable to reach the soil surface and causes salinization.
5. Use of poor quality irrigation water: The river waters that are formed in saline sedimentary rocks are frequently used for irrigation. In the eastern provinces, such names as "Salty River" and "Bitter Stream", indicating poor quality water (e.g. EC > 2 dS/cm) are frequent. In some cases the good quality water is carried to the fields through channels that run through saline soil zones and is therefore salinized. The increasing frequent use of industrial waters for irrigation further aggravates the problem, due to insufficient drainage of most of the salt-affected soils in Cuba.

Mexico: Soil degradation due to salinity is strongly associated with irrigation schemes as well as water mismanagement in all the irrigated areas of the country. All the information concerning the soil salinity status in Mexico corresponds to the irrigation districts (ID), without considering the small and numerous irrigation units where information is usually limited and vague, if any. The most extensive saline areas in Mexico are found in cultivated lands. On average, 25 percent of the area of the top ten ID in Mexico is salt-affected and in some cases this area goes up to 70 percent like in the Ciudad Juarez Valley ID (northern Mexico).

It is roughly estimated that 20 to 30 percent of the 5.5 million ha of irrigated land in Mexico is salt-affected in some degree, considering an EC greater than 4 dS/cm as the basic indicator of excess of soluble salts in the soil saturation extract.

Surface irrigation (furrow or border irrigation) is practically the only irrigation system used in the ID in Mexico. The lack of appropriate devices to measure delivered volumes at individual fields, along with the erroneous belief that the more water applied the better for the crop, result in low application efficiencies in all ID in Mexico. Field managers also contribute to this problem by allowing much more water than necessary to be served, in order to prevent personal conflicts with the landowners. More efficient irrigation systems and adequate soil management (land shaping and leveling, soil reclamation practices, if necessary) are additional factors that should be considered in order to increase the efficiency of the entire system. The main source of soil salinity in Mexican ID is basically poor water management, which includes excess of applied water, low water quality and inappropriate or non-existent drainage systems.

In coastal areas, salinity has three main origins: underlying ancient marine sediments, cyclic salts and saline intrusion. Marine sediments are the most serious problem in coastal agricultural lands. Both wind and tides promote salty breeze to move up to 10 km inland. Once the salts are deposited on the soil surface most of them finally runoff back to the sea through natural streams (rivers and creeks). Typical cases in Mexico are located in the Bajo Rio Bravo and Bajo Rio San Juan ID, and San Fernando and Soto La Marina regions, all of them located in northeastern Mexico.

Saline intrusion is derived from aquifer overdrafts mainly in coastal plains, where salty water from deep layers is increasingly pumped out. This is a major problem in the Santo Domingo, Hermosillo and Guaymas Valleys in northwestern Mexico.

Inland salinity has two principal sources: undrained closed basins, and subsurface saline layers naturally deposited in time. Evaporation and salt accumulation in the surface layers are the main processes of salinity build-up in closed basins. Such is the case of the artificially drained Texcoco basin, near Mexico City, and the Del Carmen Lagoon in southeastern Mexico.

Egypt: The majority of salt-affected soils in Egypt are located in the Northern-Central part of the Nile Delta and on its Eastern and Western sides. Other areas are found in Wadi El- Natroun, El- Kebeir, the Oases, many parts of the Nile Delta and Valley and El-Fayoum province. Nine hundred thousand ha suffer from salinization problems in cultivated irrigated areas. Sixty percent of the cultivated lands of Northern Delta region are salt-affected, twenty percent of the Southern Delta and Middle Egyptian region and twenty five percent of the Upper Egypt region are salt-affected soils.

In some coastal areas the extraction of groundwater has proceeded to the point where intrusion of saline seawater into aquifers has degraded the quality of these resources. Continued irrigation with such low quality groundwater has contributed to the expansion of land salinization. Saline soil distribution is closely related to environmental factors such as climatic, geological, geochemical and hydrological conditions.

Inundation of the soil by sea and lake salty water over a long time is the major cause of salinization in the mentioned soils, besides the tidal effect and the salty soil solution such as in Shalma and El-Hamoul South Burullus lake. In Mariut and Tal-El-Kabeer areas, the main factor responsible for the deterioration of their soils is seepage from irrigation canals in Mariut and from Ismailia canals in Tal-El-Kabeer.

In general, poor soil and water management and intrusion of seawater are the main causes of salinization in addition to the use of slightly saline water (drainage or mixed water) for irrigation without proper management and agronomic practices.

Iran: Several studies indicated that the soils having various degrees of salinity, alkalinity, and/or waterlogging cover an area of about 27 million ha, including the irrigated lands, dry farming areas and rangelands. However, recent studies revealed that the magnitude of the salt-affected areas is much larger than that originally estimated. According to the data extracted from 1:1,000,000 soil map of Iran (in digital format), slightly to moderately salt-affected soils cover about 25.5 million ha and soils having severe salinity occupy some 8.5 million ha. The slightly to moderately salt-affected soils are mostly formed on the piedmonts (normally, on the distal parts of the alluvial fans) at the foot of the Zagros and Alborz mountains. Lands having severe to extreme salinity are mostly located in the Central Plateau, the Khuzestan and Southern Coastal Plains, and the Caspian Coastal Plain.

Causes of salt-affected soils include

1. Natural occurring salinity

• Geological composition of the parent material of the soils. Iran is rich in evaporate deposits, which were deposited episodically throughout the Phanerozoic. The best known evaporates are the "Infracambrian", Hormuz salts of the southern Zagros and Persian Gulf region and their equivalent in Central Iran, the Permo-Triassic anhydrites of the Gulf and coastal Zagros, the Upper Jurassic evaporates in the Gulf region and eastern Central Iran, the lower Fars (Miocene) salt of the Zagros, and the Tertiary salts of the Great Kavir basin in Central Iran. Stream salinity. Salinization of the surface water resources, mainly due to natural conditions, is one of the main causes of salt accumulation in the soils of the Central Plateau.
• Wind-borne salinity. Strong winds blowing most of the times during the year in the Central Plateau, contribute to the expansion of soil salinity in the region by distributing the accumulated salts at the soil surface to a wider area.
• Seawater intrusion. This phenomenon occurs mostly in coastal areas where saline seawater enters the inland channels or inundates coastal lowlands by tidal waves. Seawater intrusion into the shallow groundwater in the coastal areas has also led to salinization of soils in the Caspian Coastal Plain, and in the Khuzestan and Southern Coastal Plains in southwestern Iran.

2. Human-induced salinity

The human-induced soil salinity occurs mostly in numerous closed basins formed by geotectonic forces in the Zagros mountain chains. The relatively large intermountain basins have been under irrigated agriculture for centuries. Therefore, the soils have a long history of human intervention. In these areas, salt accumulation in soils is mostly because of mismanagement of soil and water resources. Poor soil and water management, over-exploitation of groundwater and grazing are amongst the main causes of human-induced soil salinity. In recent decades, use of groundwater for irrigation purposes has increased considerably. Presently, it is estimated that the groundwater balance is negative by 4 billion cubic meters. Continuous use of saline water in these areas has undoubtedly increased soil salinity but there are no data available on the extent of the problem.

Syria: Salinity and waterlogging are extensively present in relation to irrigated agriculture in Syria. It is estimated that 532 000 ha or about 40 percent of the present total irrigated areas are salt-affected soil by varying degrees. At present 60 000 ha of previously fertile soils have been excluded from production and 100 000 ha have only 50 percent of their potential production. Examples of salt-affected soils are 125 000 ha salt-affected soils in the Euphrates Valley extending as a strip from Helebia-Zalabia in the west, down to the Iraqi border in the east; 150 000 ha salt-affected strip along the Khabour river from Ras el Ain in the north, down to Sowar town near Deirezzor city in the south; 13 800 ha in the Ghab Valley and 21 000 ha in the Jabool area, 25 km south of Aleppo city.

There is a strong relation between the secondary salinization and gypsum availability in the soil surface and subsurface. The gypsic lands are estimated to reach 20 percent of the total country area. Random pumping of groundwater for irrigation in marginal land resulted in salinity increment that has led to land salinization. The same can be said in the coastal area of the Mediterranean Sea. Over pumping to irrigate fruit trees and other crops and vegetables, water wells that have become salinized due to seawater intrusion to the groundwater have subsequently resulted in soil salinity and decreasing productivity.

Over-irrigation without measures being taken to control salinity and proper soil and water management and insufficient drainage systems are the main causes of salinity. The dominance of gypsiferous soils along the water resources in the Euphrates, Balikh and Khabour rivers cause pollution and salinization of fresh waters. The scarcity of water resources in some areas calls also for the use of available drainage, saline water or treated wastewater for irrigation (in Ghab Valley) without proper soil and water management practices causing expansion of salt-affected soils.

Tunisia: Salt-affected soils in Tunisia represent 10 percent of the total area, i.e. 1.8 million ha (1.3 million due to natural causes and 0.4 because of anthropogenic reasons), mainly in the central and southern parts of the country. Salt-affected soils occur to a considerable extent in irrigated areas - as an example, in the Kairouan Governorate 50 percent of its irrigated land is salt-affected. Seventy percent of the total water resources and 55 percent of groundwaters are being used for irrigation, of which 30 percent contain over 3 g/l salt (10 percent in the northern part of Tunisia, 20 percent in the centre and 50 percent in the south).

The aridity of the climate and high potential evapotranspiration, saline groundwater and seawater intrusion, poor water management/agronomic practices, use of low quality water for irrigation without proper management are the major causes of salt-affected soils commonly formed in depressions and low lying parts of the landscape.

Associated members

Australia: Salt-affected soils in Australia are classified under two major categories:

• Natural or primary salinity; land that has been salt-affected before European settlement.
• Induced or secondary salinity; land that has become saline because of the activities of man since European settlement – mostly related to agriculture.

The total area of salt-affected land in Australia is estimated to be approximately 32 million ha in arable and permanent cropland. Most of this is naturally saline occurring in arid and semi-arid rangeland areas. These soils have low productive potential and if cropped suffer structural problems due to sodicity (e.g. crusting, slaking), and are prone to surface erosion. It is estimated that natural salt-affected soils occupy approximately 29.5 million ha. Approximately 2.5 million ha are affected by secondary salinization processes. Of this total area, more than 2 million ha is dryland, i.e. non-irrigated soils and there is a potential for this to increase to approximately 12 million ha over the next 20 years. Secondary salinity, resulting mostly from agricultural activities is predominantly caused by changes in groundwater patterns that result from increases in water accessions to aquifers. A change in the water balance underlies the expression of salinity problems in both dryland and irrigated soils. Depending on the soil structure, surface salinity problems occur when the watertable rises to within 1 to 2 m from the surface. Salinity problems are usually associated with waterlogging. In dryland soils, clearing of deep-rooted (perennial) native vegetation and replacement with shallow rooted (usually annual) crops has changed the patterns of water use. Because annual crops usually use less rainfall, excess water will move to deeper aquifers causing rise in the watertable level. In irrigated soils, rise in groundwater, due to increased applications of water for irrigation affect large areas of the Murray Darling Basin (Victoria, New South Wales and small area in Tasmania). At least 250 000 ha of irrigated land are presently affected. There is a potential for serious effects in a further 615 000 ha over the next 20 to 40 years.

Canada: In Canada dryland salinity falls into one of two categories: visible or invisible. Visible salinity observed as tell-tale white crusts on soil surfaces, not only creates poor rooting environment for plants but often pollutes water supplies, generates less crop residues lowering protection against soil erosion, delays the phenological development of crops, promotes weed growth, and accumulates excess soil water causing difficulty with farm machinery. Invisible salinity, on the other hand, is generally less severe. On the Canadian Prairies visible salinity affects some 2.2 million ha, with another 10 million ha estimated to contain enough root-zone salts to rate as invisibly salinity.

Over 79 percent of agricultural land in Canada are located on the Prairies, 36.3 million ha as cultivated and tame land and 17.2 million ha as permanent pastures. A figure of 2.3 million ha has been estimated as severely saline land where non-irrigated crop yields would be lowered by 25 percent or more. An estimate of the Prairie agricultural land showing slight to moderate salinity, indicated that low level salinity across the Canadian Prairie covers about 7 million ha of arable land and 3 million ha of permanent pasture.

Summer fallow and agricultural activities aggravate natural weather-dependent movement of dissolved salts into crop root zones. Marine sediments underlying the glacial deposits supply the salts now giving rise to the root zone salinity and associated dryland salinity found throughout Canada's semi-arid lands of the Northern Great Plain and Prairies (provinces of Alberta, British Colombia, Saskatchewan and Manitoba of Canada). Poor soil and water management are also contributory causes of the development of salinity in these areas.

Colombia: Total of 750 000 ha are actually equipped with irrigation or drainage facilities at present, with no available quantification of the salt-affected areas nor identification of the type and extent of the saline problem. Some approaches to the extent of the affected area calculate it to be 3 percent of the total area of the country; this means that more than 3 500 000 ha are saline soils.

Salinity has several manifestations and there is no defined distribution of the problem in each region. Saline areas are located in the north coast between Panama and Venezuela borders and there are smaller areas in the Andean Valleys – Magdalena and Cauca valleys, and in the flat upper savannas as the Savanna of Bogotá (2 600 masl). Salinity and sodicity are common but magnesium affected soils are becoming more and more important as research assesses the problem and gives new tools to identify it. Physical damage to the soil structure and soil hydraulic conductivity, especially low structural stability and very poor soil permeability are main causes of salt-affected soils development.

India: Salt-affected soils have been estimated to occur in 8.6 million ha, of which about 3.0 million ha are coastal saline soils which have been reclaimed from the sea or have developed due to seawater intrusion (Punjab has 0.48 million ha almost in all districts; in Haryana, salt-affected soils are present in more than 0.55 million ha; in Uttar Pradesh an area of 0.35 million ha is suffering from waterlogging and salinity; Bihar where 1.8 million ha cultivated area suffer from surface accumulation of water and salinity; in Rajasthan total salt-affected area occupies 0.10 million ha, in addition to an estimated area of 0.75 million ha, out of the 1.2 million ha well irrigated area in the state is salt-affected; in Mahdya Pradesh salt-affected soils exist in 23 districts; Gujarat has salt-affected soils both in the inland and in the coastal zone; in Maharashtra, the salt-affected soils exist in the coastal districts to an extent of 0.06 million ha ; in Karnataka the two coastal districts of Uttar Kanada and Dakshin Kanada have about 0.04 million salt-affected soils; Andhra Pradesh has salinity to the extent of 0.03 and 0.02 million ha, whereas the spread of alkalinity is 0.10 and 0.12 million ha, respectively; in Tamil Nadu about 0.2 million ha saline and alkali soils exist in the coastal districts; in Orissa salt-affected area is mainly confined to coastal zone and West Bengal has 0.82 million ha).

A number of factors which may be geological, climatic and hydrological in nature, are involved in the formation of salt-affected soils. The main causes include (i) weathering of rocks and salt brought down from upstream areas to the plains and their subsequent deposition along with alluvial material, which is responsible for primary salinity; (ii) capillary rise from subsoil salt beds or shallow brackish groundwater; (iii) impeded drainage; (iv) intrusion of seawater along the coast; and (v) salt laden sand blown by sea winds. Secondary salinization which is related to anthropogenic cycles of salt accumulation, results from human activities such as introduction of irrigation without proper drainage system, removal of natural plant cover and flooding with salt rich waters, high watertable and the use of groundwaters of poor quality which occur quite extensively in the states of Haryana, Rajasthan, Gujara and Andhra Pradesh, for irrigation.

Italy: Information on the exact extent, distribution and degree of degradation is not available for all soil affected by salinity in Italy. However, most salt-affected soils are spotted about in semi-arid regions particularly in Sicily and in general the southern part of Italy, and amounted to 450 000 ha, including potentially salt-affected soils. Irrigation with saline sodic waters is practised in Sicily in many areas where these waters represent the only source of available water for irrigation (only 300 million m³ of good quality water is available against the need of 1600 million m³), causing secondary salinization and sodication. In semi-arid and dry subhumid areas of southern Italy salinity is a result of seawater intrusion as well as poor soil and water management practices in these areas. 

Spain: Several types of salt-affected soil have been recognized in Spain in the semi-arid region, particularly coastal areas of Communidad Valenciana. At the same time expansion of irrigated agriculture has caused the increase of areas affected by salinity in Spain. Now total affected area amounts to about 2.4 million ha. Soils may show primary salinity (histosols), some of them potentially acid sulphate soils with other mineral soils as sulfaquents containing sulfidic materials, aridsols (salids gypsids) or in other types of soils (inceptisols, alfisols) in which salinity is due to secondary salinization process. The over exploitation of groundwater along the coastal area for irrigation (Comunidad Valenciana), has led to marine intrusion of the groundwater and the deterioration of its quality and more salinity development. In the Comunidad Valenciana several types of salt-affected soils can be found:

• Saline sodic soils, showing high values of EC and accumulation of very soluble salts, mainly chlorides and sulphates of sodium and magnesium, in depressed areas and in cultivated areas irrigated with water with high TDS, generally due to marine intrusion of over-pumped aquifers
• Gypsiferous soils in inland areas where Triassic (Keuper) gypsum is present. They include extensive areas at the South
• Potentially acid sulphate soils concentrated mainly in the lagoons with peat formation, accumulating big amounts of sulphidic materials. Some areas lack carbonates, developing acidity and aluminium toxicity, whereas in other the lithogenic or biogenic carbonates can partially counteract the developed acidity. Alkali soils are not present within the territory although a tendency to sodication is observed in some soils irrigated with saline water developing crusting and swelling features.

Sudan: Total salt-affected soils in Sudan is 4.8 million ha, of which 2.1 million ha is saline and 2.7 million ha is sodic/solonetz soils. The majority of salt-affected areas are located in the low rainfall regions in Northern Sudan in the higher terraces along the Nile River, South Khartoum, North Gezira and the White Nile Scheme, north of Kosti due to climate conditions (desert, semi-desert and semi-arid), natural causes of weathering of salt bearing rocks, poor soil and water management in irrigated areas including insufficient drainage system.

Uzbekistan: Total land area in Uzbekistan, with potential for irrigation and the area currently under irrigation, are 11.8 million ha, of which about 69 percent are located in the desert and semi-desert zones with poor natural drainage, often suffering from presence of mineralized groundwater and requiring artificial drainage; the remainder is in the sierozem belt. As is typical of undrained lands in arid zones, the irrigated areas suffer from salt accumulation and secondary salinization.

The total area of salt-affected soils in Uzbekistan is 10.9 million ha, of which 1.8 million ha is characterized with a high degree of soil salinization. In the Syr-Darya basin salt-affected soils cover 3 million ha, of which 44 percent is moderate and highly saline with high gypsum content. These soils are located mainly in the Hunger steppe and Central Ferghana. Salt-affected soils in the Amu-Darya basin cover an area of 7.8 million ha. These soils are dominantly located in mean stream (Kashkadarya, Bukhara and Navoiy) and downstream area (Karakalpakstan and Khorezm).

The two major land degradation problems in Uzbekistan are the secondary salinization and waterlogging caused by high groundwater levels. Up to 54 percent of irrigated lands of mean and low streams of the Amu-Darya and Syr-Darya basins are classified as highly saline, while downstream (especially in Karakalpakstan and Khorezm) about 95 percent is saline, highly saline and very highly saline. In the zone of new irrigation of the Karshi steppe early and intermediate stages of salinization are observed. Salinity is closely related to drainage conditions. Moreover, since 1990, a reduction in the quantity of water allocated to each farm, lower water quality, and the decay of companies responsible for maintaining the drainage network have resulted in increased salinization.

Climate aridity, relief, hydromorphology and geological formations of the Pamir and Gissaro - Alay Mountains and the development history of Turan province is the natural cause of salt accumulation and river salinization. Main source of soil salinization is the salt bearing in parent materials, mineralized groundwaters and surface water quality.

Anthropogenic causes of formation of irrigated salt-affected Soils include; irrigation and drainage mismanagement; discharge of drainage water to river network has caused deterioration of river water quality, increasing secondary salinization and reduction of crop yields and deterioration of ecological situation in upper reaches; poor on-farm drainage and land leveling; low water application efficiency; fertility mismanagement and the decay of companies responsible for maintaining the drainage network have resulted in increased salt salinization.