Salts can be found in various states in the soil:

1. • Precipitated.

2. • In solution.

3. • Retained in the exchange complex (adsorbed).

The balance between these three states is highly variable and depends on various factors such as:

1. • External factors will determine whether the degree of precipitated salts is greater than that of dissolved salts, or vice versa.

2. • During the dry period the number of salts in soil solution decreases.

3. • However, during the humid period, the number of precipitated salts in the form of crystals or adsorbed decreases and the number of salts in soil solution increases.

As examples found in the literature, we can mention:

1. • In the irrigated areas of India, 3,469,100 hectares suffer from salinization and 2,189,400 hectares from water stagnation.[6][7]​.

2. • A study on 13.6 million hectares of irrigated land in the Indus River Valley in Pakistan showed that more than 5.7 million hectares have salinity problems of which 2.4 million hectares are very serious.[8]​ In more than 3 million hectares, thousands of rupees have been spent installing pumping wells for underground drainage, but only part of the goals were achieved.[9]​ The Asian Development Bank has stated that the 38% of irrigated land is too wet, and 14% too saline for agricultural use.[10]​.

3. • In the Nile River delta of Egypt, underground drainage works are installed on millions of hectares to combat the effects of water stagnation and salinization problems that began to be felt after the construction of the Aswan Dam.[11]​.

4. • In Mexico, 15% of the 300,000 irrigable hectares are affected by salinization problems and 10% by water stagnation.[12]​.

5. • In Peru there are 300,000 of the total 1,050,000 hectares in irrigation districts that are subject to the "twin problems".[13]​.

6. • Approximately one third of the irrigated areas in the most important countries in terms of irrigated agriculture experience serious soil problems with salts, for example: Israel 13%, Australia 20%, Chile: 20%, China 15%, Egypt 30%. The problems manifest themselves in both large and small irrigation projects.[14]​.

7. • The FAO has estimated that in 1990 some 52 million hectares of soil in irrigation projects require improvements in their underground drainage in order to control salinization problems.[3]​.

8. • The following table presents the regional distribution of the 3.23 million km² saline soils, including them in the deserts and along the maritime coasts according to the FAO/UNESCO world soil map.[15]​.

Soil salinity is measured in terms of concentration c in grams of salts dissolved per liter of soil water (g/l), but can also be measured as electrical conductivity σ of the solution in dS/m "Siemens (unit)"). The conversion typically used as an approximation is:.

The ratio varies somewhat, depending on the composition of the soil and salts, but the deviation is usually less than 10%. The salinity of the sea can be 30 g/l or 50 dS/m.

The general reference for salinity is that of the extract of a supersaturated soil whose electrical conductivity is written as ECe. However, salinity is more easily measured in a 2:1 or 5:1 water:soil mixture (in terms of gram of water per gram of dry soil) than in a saturated extract, because this requires centrifugation. The ratio between ECe and EC is approximately 4, so: ECe = 4 EC.[16]​.

Soils are considered saline when ECe > 4. Soils with 4 < ECe < 8 are called slightly saline, with 8 < ECe < 16 (moderately) saline and with ECe > 16 severely saline[17]​.

Sensitive plants lose their vigor in slightly saline soils, most plants are affected by moderate salinity, and only very resistant plants survive in severely saline soils. The University of Wyoming,[18]​ USA, and the Government of Alberta,[19]​ Canada, report data on plant tolerance to soil salinity. There is also tolerance data measured in the farmer's field in tropical countries[20]​.

Soil washing is done with the objective of recovering salinized land or to maintain an acceptable salt content.[21]​[22]​.

The washing process can be illustrated with the washing curves obtained from data obtained from the Chacupe experimental field in Peru.[23]​ The figure shows the salt content in the soil - as electrical conductivity (EC) of the soil moisture as a relation to the initial content (ECi) - as a function of the amount of water that has percolated through the soil profile.[16]​ The top layer of the soil is washed quickly. The salt content of the subsoil is initially rising under the influence of the salts entering from above, but soon the subsoil is also washed away.[24]​.

Since the salts in the soil are unevenly distributed, the washing efficiency (E) can be less than one. Saline soils with low efficiency are difficult to recover. In the clay soils of the Lezíria Grande polder in the Tagus River delta in Portugal, a washing efficiency of only 0.10 to 0.15 was found.[25] The soil could not be easily improved and was used for breeding bulls on natural pastures.

On the other hand, for clay soils in the Nile River delta in Egypt much higher flushing efficiencies of the order of 0.7 to 0.8 were detected (see calibration figure). The figure shows different desalination curves with different washing efficiencies as assumed in the SaltMod hydrological-saline model,[26]​ with data from the Mashtul experimental field. The observed salinity values ​​correspond best to the washing efficiency of 0.75.[27] The figure illustrates the calibration process of the washing efficiency, a parameter that is difficult to measure directly.

The washing rule may refer to:.

• - The total amount of water with which the soil must be washed to reduce the initial high value of salinity to an acceptable value for agricultural cultivation. From Chacupe's figure it can be seen that 800 mm (or 8000 m³/ha) are required to lower the salinity in the 40 to 60 cm deep layer to 60% of the initial value. In the event that the salinity must be decreased even further, the washing curve can be extrapolated visually. For the most accurate estimate, the washing formula can be used (see below), or a hydrological-saline model can be used;[26]​.

• - The annual amount of wash water (i.e. the extra amount above the normal amount necessary for crop growth) required to establish an adequate salt balance in the soil according to the crops' tolerance to salinity.

The relationship:

It is called washing fraction.[21]​.

The descending branch of the washing curve can be described with the desalination formula:[21]​.

In order to maintain a favorable salt balance in the soil in accordance with the tolerance of agricultural crops, the minimum washing fraction should be:[17]​.

The unsaturated zone of the soil, between the surface and the water table (water table) is subject to four important hydrological recharge and discharge factors:

• - Infiltration (Inf) of rainwater (Lluv) and irrigation water (Irr) that cross the surface.

• - Evapotranspiration (Evap) of the liquid in the soil, either transpired by plants or directly into the atmosphere.

• - Percolation (Perc) of water going through the unsaturated zone to the water table and beyond to groundwater.

• - Capillary rise (Cap) of groundwater to the unsaturated zone passing by suction through the phreatic layer.

In the steady state, understood as the amount of water stored in the zone does not change over time, the water balance in the unsaturated zone is written as:.

and then:.

and the salt balance is:.

It has been assumed that there is no runoff of water over the surface, that rainwater does not contain salts, and that the incorporation of salts in plants is so small that this can be neglected.

The concentration Cp can be written as:.

Assuming that the increase in the amount of salts is not desirable (i.e. As=0), and that the concentration Cn reaches the desired value (Cd, Cn=Cd), the salt balance changes by:.

Here, the irrigation term (Irr) can be replaced by:.

and the washing (Perc) by:.

After a re-calculation we obtain:[21]​.

With this equation, the irrigation requirement and the underground drainage necessary to meet the set conditions can also be determined.

The allowed concentration Cd depends on the plants' tolerance limit to salinity.

The value of the washing requirement Lr normally varies between 10 to 20%. In soils with a deep water table, the efficiency of field irrigation in practice is less than 70%.[28]​ In this situation, losses of more than 30% are largely sufficient to maintain a favorable salt balance, although losses can cause other problems such as a rise in the water table, which in turn generates drainage and salinization problems.

Aquifer conditions on irrigated lands and groundwater flow play an important role in soil salinization,[29]​ as illustrated below:.

• - Illustration of the influence of aquifer conditions on soil salinization of irrigated lands.

• - Salinization of the soil in the lower part of the sloping terrain with a significant aquifer.

• - Salinization of the soil in the non-irrigated part of the flat land with a significant aquifer.

• - Soil salinization in land without a significant aquifer and with poor underground drainage.

• - Soil salinization in a coastal delta with groundwater contribution from the aquifer.

underground drainage

Underground drainage is generally carried out using a horizontal drainage system (that is, with trenches or buried pipes), but vertical systems (with pumped wells) are also applicable.

The drainage system designed to control soil salinity also lowers the water table (water table). To reduce costs, the system should be designed so that the drawdown of the water table is as low as possible.

In many cases a seasonal average depth of the water table below the ground surface of 0.6 to 0.8 m is sufficient[30]​ (see crop chart). Since the water table always fluctuates, this means that the water table sometimes reaches the surface of the ground after a rain or a large irrigation application, but also that the water table deepens to more than 1 m. The fluctuation helps the respiration of the soil by expelling the carbon dioxide (CO) produced by the roots as it rises and pulling in the necessary fresh oxygen (O) as it goes down.

Maintaining the minimum water table depth offers the advantage that overly large irrigation water deliveries will be discouraged because the harvest would be reduced at a high water table level. Thus, irrigation water is saved, promoting the most effective use.

The thesis stated above is very general since in some cases even shallower water tables are needed, for example in the cultivation of rice in flooded rice fields, while in other instances deeper levels are required, such as for fruit trees and sugar cane in their maturation period. The determination of the most appropriate depth is subject to "drainage criteria".[31]​.

Cultivation by strips

In irrigated areas with little availability of water and with drainage and salinity problems, strip cultivation is sometimes practiced in which cultivated strips altered with permanent fallow strips are introduced (sacrificial strips).[32] Due to the contribution of water, the level of groundwater in the irrigated strips is higher than in the sacrificed strips, which causes a flow of groundwater towards the non-irrigated strips. This subsurface flow functions as a subsurface drainage of irrigated strips in which the water table is thus controlled to an acceptable level, soil washing is feasible, and soil salinity can be maintained at a sufficiently low level.

In the non-irrigated strips, the soil is dry and due to capillary suction the groundwater rises and evaporates, leaving the salts that were dissolved in the water in the soil. As a consequence, the slaughter belts become salinized. However, non-irrigated strips can have a certain usefulness for agricultural livestock, sowing grasses and edible herbs that have resistance to salinity. In addition, useful trees that are resistant to salinity can be planted (for example Casuarina, Eucalyptus, or Atriplex) taking into account that the salinity of the wet subsoil is not as great as that of the surface soil. In these ways, soil erosion by winds is also controlled. Fallow strips can also be used for salt harvesting.

Most of the mathematical models and software available for soil water and dissolved matter in soil (e.g. SWAP,[33] DrainMod-S,[34] and UnSatChem)[35] are based on the Richards differential equation for flow in unsaturated soil in combination with a differential dispersion equation "Dispersion (pollutants)").

The models need input data of soil characteristics such as the relationship between liquid content in unsaturated soil, capillary suction, permeability and dispersivity. These relationships vary greatly from place to place and are not easy to measure. The models use very small time intervals and require at least daily values ​​of variables that depend on time and hydrological factors. All this leads to the condition that the application on the scale of an irrigation project is an activity of a team of specialists with ample facilities.

Simpler hydro-salinity models such as SaltMod,[36]​ that use seasonal rather than daily data and have an empirical capillary rise function, are also available and useful for long-term predictions that depend on water management such as irrigation and drainage. By varying the handling, the model allows us to simulate the subsequent effects and appreciate the optimal handling.

SaltMod takes into account random spatial variation in salinity (see graph).

Spatial variation due to differences in topographic elevation can be calculated with hydro-salinity models combined with a groundwater flow model in the aquifer, for example SahysMod.[37]​.

• - Alkaline soil.

• - Acid sulphated soil.

• - LeachMod, soil desalination model free download.

• - Wikilibros hosts a book or manual on Environmental impacts/Irrigation and drainage.

Salts can be found in various states in the soil:

1. • Precipitated.

2. • In solution.

3. • Retained in the exchange complex (adsorbed).

The balance between these three states is highly variable and depends on various factors such as:

1. • External factors will determine whether the degree of precipitated salts is greater than that of dissolved salts, or vice versa.

2. • During the dry period the number of salts in soil solution decreases.

3. • However, during the humid period, the number of precipitated salts in the form of crystals or adsorbed decreases and the number of salts in soil solution increases.

As examples found in the literature, we can mention:

1. • In the irrigated areas of India, 3,469,100 hectares suffer from salinization and 2,189,400 hectares from water stagnation.[6][7]​.

2. • A study on 13.6 million hectares of irrigated land in the Indus River Valley in Pakistan showed that more than 5.7 million hectares have salinity problems of which 2.4 million hectares are very serious.[8]​ In more than 3 million hectares, thousands of rupees have been spent installing pumping wells for underground drainage, but only part of the goals were achieved.[9]​ The Asian Development Bank has stated that the 38% of irrigated land is too wet, and 14% too saline for agricultural use.[10]​.

3. • In the Nile River delta of Egypt, underground drainage works are installed on millions of hectares to combat the effects of water stagnation and salinization problems that began to be felt after the construction of the Aswan Dam.[11]​.

4. • In Mexico, 15% of the 300,000 irrigable hectares are affected by salinization problems and 10% by water stagnation.[12]​.

5. • In Peru there are 300,000 of the total 1,050,000 hectares in irrigation districts that are subject to the "twin problems".[13]​.

6. • Approximately one third of the irrigated areas in the most important countries in terms of irrigated agriculture experience serious soil problems with salts, for example: Israel 13%, Australia 20%, Chile: 20%, China 15%, Egypt 30%. The problems manifest themselves in both large and small irrigation projects.[14]​.

7. • The FAO has estimated that in 1990 some 52 million hectares of soil in irrigation projects require improvements in their underground drainage in order to control salinization problems.[3]​.

8. • The following table presents the regional distribution of the 3.23 million km² saline soils, including them in the deserts and along the maritime coasts according to the FAO/UNESCO world soil map.[15]​.

Soil salinity is measured in terms of concentration c in grams of salts dissolved per liter of soil water (g/l), but can also be measured as electrical conductivity σ of the solution in dS/m "Siemens (unit)"). The conversion typically used as an approximation is:.

The ratio varies somewhat, depending on the composition of the soil and salts, but the deviation is usually less than 10%. The salinity of the sea can be 30 g/l or 50 dS/m.

The general reference for salinity is that of the extract of a supersaturated soil whose electrical conductivity is written as ECe. However, salinity is more easily measured in a 2:1 or 5:1 water:soil mixture (in terms of gram of water per gram of dry soil) than in a saturated extract, because this requires centrifugation. The ratio between ECe and EC is approximately 4, so: ECe = 4 EC.[16]​.

Soils are considered saline when ECe > 4. Soils with 4 < ECe < 8 are called slightly saline, with 8 < ECe < 16 (moderately) saline and with ECe > 16 severely saline[17]​.

Sensitive plants lose their vigor in slightly saline soils, most plants are affected by moderate salinity, and only very resistant plants survive in severely saline soils. The University of Wyoming,[18]​ USA, and the Government of Alberta,[19]​ Canada, report data on plant tolerance to soil salinity. There is also tolerance data measured in the farmer's field in tropical countries[20]​.

Soil washing is done with the objective of recovering salinized land or to maintain an acceptable salt content.[21]​[22]​.

The washing process can be illustrated with the washing curves obtained from data obtained from the Chacupe experimental field in Peru.[23]​ The figure shows the salt content in the soil - as electrical conductivity (EC) of the soil moisture as a relation to the initial content (ECi) - as a function of the amount of water that has percolated through the soil profile.[16]​ The top layer of the soil is washed quickly. The salt content of the subsoil is initially rising under the influence of the salts entering from above, but soon the subsoil is also washed away.[24]​.

Since the salts in the soil are unevenly distributed, the washing efficiency (E) can be less than one. Saline soils with low efficiency are difficult to recover. In the clay soils of the Lezíria Grande polder in the Tagus River delta in Portugal, a washing efficiency of only 0.10 to 0.15 was found.[25] The soil could not be easily improved and was used for breeding bulls on natural pastures.

On the other hand, for clay soils in the Nile River delta in Egypt much higher flushing efficiencies of the order of 0.7 to 0.8 were detected (see calibration figure). The figure shows different desalination curves with different washing efficiencies as assumed in the SaltMod hydrological-saline model,[26]​ with data from the Mashtul experimental field. The observed salinity values ​​correspond best to the washing efficiency of 0.75.[27] The figure illustrates the calibration process of the washing efficiency, a parameter that is difficult to measure directly.

The washing rule may refer to:.

• - The total amount of water with which the soil must be washed to reduce the initial high value of salinity to an acceptable value for agricultural cultivation. From Chacupe's figure it can be seen that 800 mm (or 8000 m³/ha) are required to lower the salinity in the 40 to 60 cm deep layer to 60% of the initial value. In the event that the salinity must be decreased even further, the washing curve can be extrapolated visually. For the most accurate estimate, the washing formula can be used (see below), or a hydrological-saline model can be used;[26]​.

• - The annual amount of wash water (i.e. the extra amount above the normal amount necessary for crop growth) required to establish an adequate salt balance in the soil according to the crops' tolerance to salinity.

The relationship:

It is called washing fraction.[21]​.

The descending branch of the washing curve can be described with the desalination formula:[21]​.

In order to maintain a favorable salt balance in the soil in accordance with the tolerance of agricultural crops, the minimum washing fraction should be:[17]​.

The unsaturated zone of the soil, between the surface and the water table (water table) is subject to four important hydrological recharge and discharge factors:

• - Infiltration (Inf) of rainwater (Lluv) and irrigation water (Irr) that cross the surface.

• - Evapotranspiration (Evap) of the liquid in the soil, either transpired by plants or directly into the atmosphere.

• - Percolation (Perc) of water going through the unsaturated zone to the water table and beyond to groundwater.

• - Capillary rise (Cap) of groundwater to the unsaturated zone passing by suction through the phreatic layer.

In the steady state, understood as the amount of water stored in the zone does not change over time, the water balance in the unsaturated zone is written as:.

and then:.

and the salt balance is:.

It has been assumed that there is no runoff of water over the surface, that rainwater does not contain salts, and that the incorporation of salts in plants is so small that this can be neglected.

The concentration Cp can be written as:.

Assuming that the increase in the amount of salts is not desirable (i.e. As=0), and that the concentration Cn reaches the desired value (Cd, Cn=Cd), the salt balance changes by:.

Here, the irrigation term (Irr) can be replaced by:.

and the washing (Perc) by:.

After a re-calculation we obtain:[21]​.

With this equation, the irrigation requirement and the underground drainage necessary to meet the set conditions can also be determined.

The allowed concentration Cd depends on the plants' tolerance limit to salinity.

The value of the washing requirement Lr normally varies between 10 to 20%. In soils with a deep water table, the efficiency of field irrigation in practice is less than 70%.[28]​ In this situation, losses of more than 30% are largely sufficient to maintain a favorable salt balance, although losses can cause other problems such as a rise in the water table, which in turn generates drainage and salinization problems.

Aquifer conditions on irrigated lands and groundwater flow play an important role in soil salinization,[29]​ as illustrated below:.

• - Illustration of the influence of aquifer conditions on soil salinization of irrigated lands.

• - Salinization of the soil in the lower part of the sloping terrain with a significant aquifer.

• - Salinization of the soil in the non-irrigated part of the flat land with a significant aquifer.

• - Soil salinization in land without a significant aquifer and with poor underground drainage.

• - Soil salinization in a coastal delta with groundwater contribution from the aquifer.

underground drainage

Underground drainage is generally carried out using a horizontal drainage system (that is, with trenches or buried pipes), but vertical systems (with pumped wells) are also applicable.

The drainage system designed to control soil salinity also lowers the water table (water table). To reduce costs, the system should be designed so that the drawdown of the water table is as low as possible.

In many cases a seasonal average depth of the water table below the ground surface of 0.6 to 0.8 m is sufficient[30]​ (see crop chart). Since the water table always fluctuates, this means that the water table sometimes reaches the surface of the ground after a rain or a large irrigation application, but also that the water table deepens to more than 1 m. The fluctuation helps the respiration of the soil by expelling the carbon dioxide (CO) produced by the roots as it rises and pulling in the necessary fresh oxygen (O) as it goes down.

Maintaining the minimum water table depth offers the advantage that overly large irrigation water deliveries will be discouraged because the harvest would be reduced at a high water table level. Thus, irrigation water is saved, promoting the most effective use.

The thesis stated above is very general since in some cases even shallower water tables are needed, for example in the cultivation of rice in flooded rice fields, while in other instances deeper levels are required, such as for fruit trees and sugar cane in their maturation period. The determination of the most appropriate depth is subject to "drainage criteria".[31]​.

Cultivation by strips

In irrigated areas with little availability of water and with drainage and salinity problems, strip cultivation is sometimes practiced in which cultivated strips altered with permanent fallow strips are introduced (sacrificial strips).[32] Due to the contribution of water, the level of groundwater in the irrigated strips is higher than in the sacrificed strips, which causes a flow of groundwater towards the non-irrigated strips. This subsurface flow functions as a subsurface drainage of irrigated strips in which the water table is thus controlled to an acceptable level, soil washing is feasible, and soil salinity can be maintained at a sufficiently low level.

In the non-irrigated strips, the soil is dry and due to capillary suction the groundwater rises and evaporates, leaving the salts that were dissolved in the water in the soil. As a consequence, the slaughter belts become salinized. However, non-irrigated strips can have a certain usefulness for agricultural livestock, sowing grasses and edible herbs that have resistance to salinity. In addition, useful trees that are resistant to salinity can be planted (for example Casuarina, Eucalyptus, or Atriplex) taking into account that the salinity of the wet subsoil is not as great as that of the surface soil. In these ways, soil erosion by winds is also controlled. Fallow strips can also be used for salt harvesting.

Most of the mathematical models and software available for soil water and dissolved matter in soil (e.g. SWAP,[33] DrainMod-S,[34] and UnSatChem)[35] are based on the Richards differential equation for flow in unsaturated soil in combination with a differential dispersion equation "Dispersion (pollutants)").

The models need input data of soil characteristics such as the relationship between liquid content in unsaturated soil, capillary suction, permeability and dispersivity. These relationships vary greatly from place to place and are not easy to measure. The models use very small time intervals and require at least daily values ​​of variables that depend on time and hydrological factors. All this leads to the condition that the application on the scale of an irrigation project is an activity of a team of specialists with ample facilities.

Simpler hydro-salinity models such as SaltMod,[36]​ that use seasonal rather than daily data and have an empirical capillary rise function, are also available and useful for long-term predictions that depend on water management such as irrigation and drainage. By varying the handling, the model allows us to simulate the subsequent effects and appreciate the optimal handling.

SaltMod takes into account random spatial variation in salinity (see graph).

Spatial variation due to differences in topographic elevation can be calculated with hydro-salinity models combined with a groundwater flow model in the aquifer, for example SahysMod.[37]​.

• - Alkaline soil.

• - Acid sulphated soil.

• - LeachMod, soil desalination model free download.

• - Wikilibros hosts a book or manual on Environmental impacts/Irrigation and drainage.