Leaching of saline soils

Leaching of saline lands implies removal of excess salts from arable and subsurface soil horizons by flushing water; it is one of the main irrigated land salinity control methods. Before leaching, the field surface is to be leveled, deeply ploughed and dividing into check plots – parcels of 0.2-0.3 ha and more – by borders; then the check plots are flooded by water. Leaching rates (water quantity required for dissolution and displacement of salts from saline soil) are determined depending on salinization degree, composition of salts (sulphates, chlorides, and carbonates), permeability, and groundwater level. Leaching of saline lands is usually carried out in late autumn, when evaporation is minimal and groundwater level is low. Flushing water is diverted through desalting drainage.

Condition of application of soil leaching. If soil is heavily saline and contains more than 0.02…0.03% of chlorine in a one-meter-thick layer, excess of salts is to be removed by washing out, so that by beginning of sowing the quantity of chloride ion should not exceed 0.01% of its mass. To this effect, flooding irrigation is carried out with such water amount that dissolves salts and carries out their excess to lower horizons or more often to a drain.

Leaching of soil is a radical improvement of saline and alkaline soils. Effectiveness of leaching depends on the physical properties of soil and salinity degree, i.e. ratio of soluble salts of Ca and Na ions in the soil.

From alkali soils (where Ca ions prevail), salts can be easily washed out by leaching, if the soils are permeable enough. In alkaline soils (where Na ions prevail), alkalis liberate in the course of leaching, which causes physiological toxicity and deterioration of the physical properties of soil. The more Na ions, the worse soil properties. When the content of Na ions ranges from 20 to 40% of the total absorptive capacity, soil fertility disappears fully. That is why some gypsum should be introduced into alkaline soils prior to leaching; in the result of metabolic reaction, the absorbed Na is replaced by Ca ions and the generated salt is washed out by water. Leaching of alkaline soils without gypsum can be carried out if the content of Na ions does not exceed 10% of the absorptive capacity.

Leaching irrigation is the most efficient with the water application rate that comes to 30…40% of the minimum moisture capacity of the stratum being desalinated. For one-meter-thick stratum of light-textured soils, water application rate of leaching irrigation is 700…900 m3/ha; 900…1100 m3/ha on medium-textured soils; and 1100…1500 m3/ha on heavy-textured soils.

Leaching is to be done on a well-leveled, harrowed land divided into check plots 0.25 ha large at most, with compacted borders, which makes impossible water overflow or breakthrough through them. Leveling should be performed accurate to +5 cm; the height of earth addition at leveling should not exceed 20 cm. Irrigation network is to be cut in such manner that water comes to every check plot by itself.

Leaching is carried out by the blocks of land, and not here and there. When leaching is over and the soil has dried up, it is loosened to reduce evaporation and the borders are made even.

Sometimes, spots of residual salinization remain after leaching irrigation, which reduces the yield of crops. To neutralize these spots it is necessary to introduce gypsum and acid mineral fertilizers. Over the whole area of leaching, it is necessary to make soil structure by different agrotechnical methods: sowing of grass; application of manure, green manure, humus, etc.

Leaching irrigation is applicable without artificial drainage if groundwater has sufficient outflow outside a given irrigated area.

On heavily saline irrigated lands, removal of excessive salts from soil root layer can be completed by leaching irrigation. During a leaching process, water passes through soil layers, dissolves salts, and washes them out to groundwater. Contrasted with drainage, the soil leaching and desalination processes have the highest efficiency.

Leaching is carried out on the soils that contain more than 0.02-0.03% of the chloride mass in one-meter-thick layer. By the beginning of crop sowing, the content of chloride ions should not exceed 0.01%.

Soil leaching without artificial drainage is carried out when groundwater occurs deeply enough with its good natural outflow outside the irrigated area and well-permeable soils (gravel, etc.) lie at a depth of more than 1.5-2 m. If there is occurrence of mineralized groundwater with no natural outflow, artificial drainage must be provided at a depth of less than 2-3 m.

Soil leaching is performed in autumn, when groundwater lies deeply enough. Prior to leaching, the field must be leveled, ploughed up and harrowed; in this case, infiltration of irrigation water into soil will be slower and more uniform. To apply leaching, the field is to be divided into plots of land, i.e. check plots with an area 0.25 ha.

Water and salt regime of soils

Changes in the inter-irrigation, annual or many-year cycle of salt content and its qualitative composition in soil are called the salt regime of soil. It depends on groundwater table, groundwater salinity, salinity of soil solutions and irrigation water, irrigation regime, leaching regime, properties of soils, climatic conditions.

Salt regime is closely related to hydraulic regime. These processes are studied as a single set and are united by a common concept, viz. water-salt regime of soils.

Water regime of soil represents a combination of the processes of absorption, assimilation and exudation of water by soil. The water regime of soil includes such phenomena as: absorption; seepage; capillary rise; surface flow, descending and lateral; physical evaporation; desuction, freezing; defreezing; and water condensation.

Salt regime of soil represents a combination of the processes in soil related to incoming, movement, redistribution, and accumulation of salts, as well as their removal outside the soil profile.

Salt regime implies the history of salt composition and migration in value ecosystem soils. It consists in inwash of salts, particularly by impulverization, dissolving of salts that are in crystalline state, and vice versa in precipitation of salts from solutions, consumption of salts from solutions by plants and partially by soil organisms, their return with abatement, cyclic vertical migrations of salts, carry-over of salts into illuvial horizons during soil formation, carry-over of salts from the system with surface and ground waters, as well as by expulverization. Salt regime can be broken by environmental pollution.

Source: Bykov, B.A. Ecological glossary. Alma-Ata: Publishing House “Nauka”, 1983

Salt regime of soil implies change in the inter-irrigation, annual or many-year cycle of salt content and its qualitative composition in soil. Salt regime of soil is as a rule heavily dependent on irrigation and natural water regime; it (water-salt regime) is usually studied simultaneously.

Source: Soil science thesaurus. Edited by Rode, A.A. Moscow, Publishing House “Nauka”, 1975

Salt regime implies the history of salt composition and migration of salts in soils and water bodies. It is one of the most important environmental factors. It can be broken by erosion of banks, salinization and overwetting of soils, pollution of environment, etc.

Source: Dedyu, I.I. Ecological Encyclopedic Vocabulary. Kishinev: Chief Editorial Board of the Moldavian Soviet Encyclopaedia, 1989

Salt regime of soils implies recurrent movement of simple salts in the soil profile. These salts are typical for soils with non-leaching water regime the profile of which has water-soluble salts (saline soils/solonetzes, alkali soils, chernozems, chestnut soils, etc.). Salt regimes of soil differ in their intensity, i.e. in the mass of moving salts and amplitude of the movement, prevailing direction of salt movement, as well as composition of the moving salts. In alkali soils, upward flows of chlorides and sulphates prevail, which change into downward movement of these salts in wet seasons.

Forecast of the water-salt regime of irrigated lands

During operating period, prevention of repeated salinization and maintenance of optimum content of salt quantity and composition is executed by water supply for irrigation in a quantity exceeding the required by crops by 5-20%. This type of irrigation regime is called leaching regime; it intended for creation of downward movement of water and salts in aeration zone. Leaching regime is implemented by carrying out operating leaching in autumn-winter or spring periods or by raising vegetation irrigation rates.

Operating leaching and vegetation irrigation are made by furrows, border ditches or by sprinkling depending on natural and economic conditions and species of crops.

Necessity of leaching irrigation regime, its intensity and time of additional water supply is to be substantiated by drawing up and forecasting the water-salt regime of soils for a sufficiently long period. To forecast water-salt regime, they use the equation of water and salt balance of surface and subsurface waters and the equations of moisture and salt transfer in soils.

Leaching rate

Leaching rate means the quantity of water to be supplied to the field for removal of excess water-soluble salts that are harmful for cultivated plants from soil. Leaching rate is determined experimentally or calculated by using relevant formulas.

Source: Soil science thesaurus. Edited by Rode, A.A. Moscow, Publishing House “Nauka”, 1975

To calculate the leaching rate for washing out one-meter-thick soil layer, they use the empirical formula of V.R. Volobuev:

M = 10000 * a * lg(S1 — S2)

Leaching depth in clayey soils is four times as much as it is on light-textured soils.

Leaching depth ranges from 1500 to 12500 m3/ha and more and is made up of the water volume needed for saturation of soil layer H down to minimum moisture capacity and of the water volume needed for washing out of dissolved excess salts (S1 — S2) to a drain.

Saline soils are washed out by means of consecutive water application at an interval of no more than eight days.

Net leaching depth is the water quantity needed for dissolving and carry-over of water-soluble salts from estimated soil layer’ it is measured in meters of water sheet or cubic meters per hectare (m3/ha).

Gross leaching rate is equal to the net leaching rate divided by irrigation network efficiency factor η plus the water quantity E0 evaporated from water surface during leaching period and minus atmospheric precipitation Oc for the same period.

Saline soil leaching types

Leaching of saline soils is subdivided into thorough and routine (operating) leaching. Thorough leaching is carried out at average and heavy initial salinization of soils; operating leaching is carried out at weak salinization. The rates of thorough and operating leaching irrigations depend on the composition and chemism of water-soluble salts in soil, thickness of washed out layer, hydro-physical and physical-chemical properties of soils and earth, mineralization of flushing water, and condition of diversion of flushing water.

Such leaching is called thorough (construction, reclamation) that is implemented for reclamation of highly saline soils during construction of new irrigation facilities as well as unused lands in farms with functioning irrigation systems. This leaching is carried out according to designs at year-round kind of works by applying increased leaching rates of over 10,000 m3/ha for desalination of soil root layer.

On old-irrigated soils, thorough leaching is applied in the case of introduction of highly saline lands in agriculture. Since thorough leaching requires enhanced drainage of the area, permanent drainage designed so that to meet the requirements of operating regime of crop irrigation should be strengthened by temporary drainage. At thorough leaching, the main requirements to be met are set for the depth of desalinated soil layer and subsoil, which is to be desalinated to such a degree that ensures normal development of crops.

Thorough leaching rate is determined proceeding from the condition of desalination of soil root layer and subsoil with a glance of the degree and type of salinization, hydro-physical properties of soils, as well as drainage degree of irrigated lands.

With shortage of equipment and water and unsatisfactory condition of drainage systems, such leaching irrigations are carried out more and more rarely and in many cases are not carried out at all. Under current conditions, the radical land reclamation principles should be revised, because salinization problem has become even more pressing than before, and the issues related to reconstruction, water deficit and facilities, equipment, and other resources are becoming more and more problematical.

The results of researches and field experiments proved the possibility to desalinate highly saline soils and alkali soils without thorough leaching by means of leaching irrigation regime. The Fergana Valley, Golodnaya Steppe, and lower reaches of the Amudarya river cane exemplify gradual desalination of the soils in irrigated areas with drainage and rise of crop yields on them.

Materials of numerous researches and estimates indicate that implementation of “thorough leaching”, i.e. leaching irrigation supposedly eliminating resalinization risk forever, can be carried out only with additional temporary deep drainage (the length of which is equal to or more than the length of permanent drainage required during an operating period) as well as temporary irrigation network. When developing large areas with a great part of saline lands, the cost of preparatory and liquidating abandonment works for execution of leaching will be close to the cost of construction of operational irrigation and collector & drainage networks, because on-farm network and sometimes inter-farm network too will need to be duplicated.

Thorough leaching is carried out during the growing season, when water and labour recourses become available and water losses to evaporation are lowest. To increase the effectiveness of soil desalination, leaching rate is supplied to fields by separate cycles of 2-3 ths m3/ha. The intervals between the cycles should be sufficient to ensure full absorption of the flushing water by soil.

Thorough leaching irrigations of saline soils are performed against the background of systematic horizontal or vertical drainage. With deep groundwater table (> 10 m), it is permissible to carry out thorough leaching without drainage for removal of salts from the root layer to the ground of the aeration zone.

With high rates on soils with low filterability, deep tillage of soils is accomplished to shorten leaching period and increase the effectiveness of flushing water use.

To ensure required intensity of flushing water diversion, systematic drainage is supplemented with temporary drainage, if necessary. The most reliable type of temporary drainage is shallow (0.8-1.2 m) open channel drainage.

Temporary drainage is designed taking into consideration a given leaching method. If leaching is carried out by cycles the interval between which is sufficient to ensure absorption of water supplied, then the interval between temporary drains is determined by fitting according to the formula of A.N. Kostyakov.

If leaching is carried out under permanent flooding (for rice), the interval between temporary drains is determined by the formula of V.V. Vedernikov.

Technique of thorough leaching is also very important. In an arid zone where surface irrigation method is applied, leaching irrigation is carried out: in small check plots by separate cycles without dumping of flushing water; in small check plots under permanent flooding and water bypass from one check plot to another by using partial surface method; in large check plots by separate cycles. Leaching by small check plots is one of the most widespread methods on low-permeable soils with installation of temporary drainage. In this case, the sizes of check plots are determined depending on the spaces between temporary drains and slopes of earth surface (the difference between the earth surface elevation at the extreme points of check plots should not exceed 10 cm). Check plot sizes usually range from 20 x 20 m to 50 x 50 m.

The most efficient is “border strip/check” leaching, when supply of flushing water is started from the field’s central part with gradual flooding the other parts. This leaching method is the most labour-intensive and expensive, but it provides fast and uniform desalination of soils over the field edgewise.

On well-permeable soils with slight surface gradient, where there is no need for temporary drainage, leaching is applied by large (1-3 ha) check plots. This leaching method is highly efficient and simplifies flushing water distribution over check plots. In some cases, leaching is implemented simultaneously with rice sowing. Leaching is carried out over small check plots with permanent flooding and partial dumping of flushing water. However, this method does not ensure uniform desalination over the field and, what is more, requires enormous volume of water, because a leaching rate is determined taking into account the needs for soil desalination, but according to the conditions of rice cultivation.

The highest effect of soil desalination is achieved with moisture movement under partial saturation; for this purpose, water is supplied by separate cycles or sprinkling method is applied. In this case, salts are washed out both from large and small pores of soil; with minimum influence upon the organic part of soil, uniformity of desalination over the leached area is achieved and less water is consumed for carry-over of the same quantity of salts as compared to pressure filtration under total water saturation.

The most efficient use of flushing water is ensured under the following seepage rates: in light-texture soils – 0.025-0.05 m/day; in average-texture soils – 0.01-0.03; and in heavy-texture soils – 0.01 m/day. If the seepage rate in heavy soils is less than 0.01 m/day, it is to be increased by tillage, applying manure and other amendments. Intensification of leaching process can be achieved by different agrotechnical methods (slotting, moling, deep tillage, meliorative ploughing) and chemical reclamations that allow creating a structure that will raise the filterability of soil and ground. When leaching by check plots is over, the following is to be carried out: survey of salts; leaching of unleached areas; evening up of temporary irrigation network; leveling of field surface; and deep tillage.

Routine (operational) leaching of saline soils means periodical soil desalination with the view of liquidation of seasonal salinity on poor-drained lands.

The main purpose of operating leaching is to remove salts from the root layer (0-100 cm) to optimum conditions for the rotation crops cultivated on irrigated lands without capital-intensive measures. Preventive and charge watering is a form of operating leaching for washing out of the salts accumulated in summer.

Preventive watering carried out every year or periodically (every 2-3 years) after desalination of active water and salt exchange layer ensures keeping stable salt regime of soil during the growing season.

Charge watering is an agrotechnical method which is used under certain conditions (dry spring, sandy-loam soils) for building up of required deposit of moisture in topsoil and gaining normal sprouting of crops as well as for reducing the consumption of irrigation water during the growing season when water is short. At slightly higher rates, charge watering can serve the purpose of simultaneous removal of the salts accumulated for the previous season from the root layer. In this case, it is called charge-preventive (watering).

Compulsory condition limiting the effectiveness of operating leaching is the drainage degree of irrigated lands and normal functioning of the existing collector & drainage network. Drainage (horizontal, vertical, etc.) creates conditions for descending seepage in a leached soil layer.

Operating leaching rates are fixed proceeding from the need for desalination of the root layer (0-100 cm) down to the toxicity threshold for zoning of crop varieties.

The rates of preventive charge watering are set based on the estimation of the aeration zone water balance for providing descending water flows subject to the depth of groundwater table, quantity of precipitation during autumn-winter and spring periods.

The leaching rates for every irrigated plot, crop-rotation area is fixed depending on the degree and nature of soil salinity, its water and physical properties, depth of groundwater table, technical condition and operation mode of collector & drainage network.