How to Manage Salt Affected Soils? | Soil Science

In order to generate, maintain or increase the agricultural productivity of salt-affected soils and to prevent at least 20% of the irrigated land from an annual global income loss of about US$ 12 billion for being salt-affected soils, as in many arid and semiarid regions of the world, various ways of amelioration efforts and improved management practices have been proposed depending upon the targeted crop and severity and rapidity of salinity/ sodicity build up.

These are again controlled by a number of interacting factors such as amount of dissolved salt and exchangeable sodium in the soil, soil and hydrological conditions, quality of irrigation water, local climate, land physiography and pattern of land utilisation. Assurance of sustainability in crop production out of salt-affected solum may be obtainable only with wise implementation of proper management alternatives including reclamation techniques, wherever necessary.

With the backdrop of partial or entire restriction of the plant growth caused due to the adverse level of salinity and alkalinity in these degraded soils, bipartite options have been conceptualised; tailoring of the soil resource according to the need of the target plant and tailoring the plant/introducing new species according to the characteristics of soil.

The former technique demands the management of the soils using suitable package of techniques for successful crop growth while the later requires the development or adoption of plant types for acclimatisation in the unfavourable salt affected soil conditions.

Systematic planning to uplift the productivity of salt affected soils starts with the proper examination of the problem: its characteristics, extent, root cause, possible way out and probable outcome. Each of the specific problems, thereafter, calls for the most appropriate management practices.

All the management options are aimed at obtaining economically viable yield, follow the two basic principles:

(i) Use sufficient quantities of good quality irrigation water to leach the excess soluble salts from the soil profile- especially for saline soil.

(ii) Leaching after addition of a direct or indirect source of calcium to replace the exchangeable Na⁺ in soil- especially for sodicity affected soil.

Kovda et al. (1973) critically explained the most common techniques to address the problems in the salt affected land including saline and sodic soils under the four heads:

1. Physical Amelioration

2. Hydro-Technical Amelioration

3. Chemical Amelioration

4. Biological Amelioration

Reclamation of Dryland Salt Affected Soils:

By nature, it is not possible to reclaim saline soils using any chemical amendments, conditioners or fertilisers; removing salts from the plant root zone is only the option, whereas, adoption of salt tolerant crops may be done in solo or in combination with other practices.

Physical removal of accumulated salt, wherever necessary, followed by one of the three practices can serve the purpose:

(i) Moving the salt below the root zone by applying more water than the plant needs. This method is called the leaching requirement method.

(ii) Where soil moisture conditions dictate, combines the leaching requirement method with artificial drainage.

(iii) Salts can be moved away from the root zone to locations in the soil, other than below the root zone, where they are not harmful. This third method is called managed accumulation.

The basis of the reclamation of the sodic soils lies in the replacement of the exchangeable sodium by a direct or indirect source of calcium followed by leaching. Soils dominated with exchangeable sodium in the exchange complex, results in high pH due to hydrolysis of the soils and cause dispersion of the soil and hence subsequent aeration of the soil is needed for better productivity.

Common strategies employed in the purpose of desalinization of the dryland salinity and reclamations of sodic soils are mentioned herein.

Strategy # 1. Physical Amelioration:

a. Scrapping of Salts:

Accumulated salts from the surface of the land, wherever it is severe, can be removed directly through mechanical means which may improve the crop growth temporarily but the disposal of salts still a major problem. This practice has limited application although many farmers have resorted to it.

b. Profile Inversion:

When the surface soil is somewhat good but the adjacent subsoil has undesirable salt deposition, profile inversion is followed to retain the surface soil while inverting the subsoil and substratum. After replacing the surface soil, the profile is ploughed deeply so that the subsoil goes down and the soil below the subsoil comes up.

Then finally the surface soil is replaced in its position. Soils can be fully improved if profile inversion is used when the surface soil is good but the upper subsoil has undesirable characteristics. This condition frequently occurs in solpnetz soils having a favourable surface soil.

c. Deep Ploughing, Sub-Soiling and Sand Filling:

Mechanical treatments of the soil, commonly adopted to overcome the problems of soil alkalinity are deep ploughing, sub-soiling, sanding, and profile inversion. The purpose of the first three treatments is to increase soil permeability directly by mixing fine and coarse textured layers and to obtain a more uniform soil (deep ploughing), by breaking up impermeable layers (subsoiling), and by incorporating sand into a fine textured soil (sanding) ultimately for enhancing the infiltration and transportation of water soluble salts in the deep soil layer.

These physical measures are usually employed with the prime objective of improving the physical geometry of soils for facilitating assured drainage and removal of soluble salts. Deep ploughing is most beneficial on stratified soils having sodium-affected surface or subsoils underlain by soil containing considerable gypsum impermeable layers lying between permeable layers.

This serves to break up and bury the sodium-affected soil while supplying soluble calcium to bring about reclamation. The benefits continue for several years. Sanding gives more permeable bed; thus relatively permanent change in surface soil texture and improved water permeability is obtained, which facilitates leaching of salts in fields where surface soil permeability limits water penetration.

d. Land Leveling and Construction of Bunds:

Lands should be properly leveled to ensure uniform leaching. Constructions of bunds are necessary in order to avoid the runoff of most of the rain water after gypsum application. At the same time field should be provided with 0.1% slope to drain excess water. A heavy irrigation is recommended to flush out free salts once before amendments are applied.

Strategy # 2. Hydro-Technical Amelioration:

Washing out of excess soluble salts from the root zone is only the most popular practical meant to manage dry land salinity. Leaching in solo or in combination with drainage, where shallow water table prevail, is practiced for obtaining favourable distribution of soluble salts in soil profile. Ponding of fresh water on the soil surface and allowing it to infiltrate is the basic procedure, efficiency of which depends on mode of water application, quantity of soluble salts distributed in profile, quantity and quality of applied water, leaching methods, water table depth and initial salinity and chemical composition of salts.

Leaching, preferably being followed when soil moisture content is low and the underlying geological material may be deep and permeable making artificial drainage meaningless, otherwise required. Surface irrigation can usefully facilitate leaching in saline soils and deep percolation losses of nutrients are almost inevitable, but, it may be followed periodically providing that these losses are not excessive with respect to crop nutrition or economics.

A practical estimation of the quantity of water (leaching requirement), based on the initial salt content of the soil, desired level of soil salinity after leaching, depth to which reclamation is desired and soil characteristics, is required to be performed for salt leaching programme. As a thumb rule a unit depth of water will remove nearly 80 percent of salts from a unit soil depth. For example 30 cm water passing through the soil will remove approximately 80 percent of the salts present in the upper 30 cm of soil.

U.S. Salinity Laboratory defined leaching requirement (LR) as the ratio of equivalent depth of drainage water to the depth of irrigation water, that it is required to maintain a given soil solution concentration at the bottom of the root zone. It is expressed as a fraction or as percent. It is also equal to the inverse ratio of the corresponding electrical conductivities.

Where,

LR= Leaching requirement expressed in percentage

D_dw = Depth of drainage water in inches

D_iw = Depth of irrigation water in inches

EC_iw = Electrical conductivity of the irrigation water in dSm^-1

EC_dw = Electrical conductivity of the drainage water in dSm^-1

Amount of water required to leach down the salts depends upon the amount and type of salts in the soil, soil texture, level of salts desired in the root zone and depth of reclamation. Leaching requirement and water required for leaching under different textured soils are presented below (Table 3.7).

Ponding on the surface of the soil, continuously or intermittently, with sufficient amount of good quality water to leach the soluble salts down the root zone is the basic procedure of leaching. For the accomplishment of effective leaching, the land must be leveled and the application of water must be uniform. Verma and Gupta (1989) studied the leaching behaviour of black soils under continuous and intermittent application of water and concluded that intermittent application of water did not show any advantage over continuous application (Table 3.8) under that condition, though, generally it is thought that intermittent submergence is more effective than continuous ponding.

i. Drainage System:

To regulate the water and salt balance on the surface of saline soils the subsurface drainage system of soil must be designed accordingly in the soils where geogenic materials are of shallow depth, or water table is shallow. Accumulation and stagnation of rainfall or excess irrigation water may also create the problem to be managed with appropriate drainage assistance.

Drainage is also important in order to prevent evaporation from the groundwater by keeping the groundwater table below the critical depth that will not cause soil salinisation through drainage. Provision of adequate drainage measures is the only way to control the groundwater table. Proper land shaping and provision of surface drains are needed to solve the problems of surface water stagnation.

Horizontal sub-surface drainage improves aeration in the root zone by leaching of salts and lowering water table and control salt concentration through the removal of excess subsurface water. Depth of the water table in an irrigated area is governed by the schedule and amount of deep percolation, the physical characteristics of the substrata; i.e., the depth, stratification, pore space, permeability and continuity, and the topography, including the intensity, location and depth of natural outlet channels into which groundwater will discharge. The permissible groundwater depth and salinity level of the soil for securing good crop growth are the determining factors of drainage amount and design.

Boumans (1963) developed a quantitative assessment technique to estimate the required leaching quantities and the quantities to be drained (natural or artificial) for salinity control and that was further extended by van der Molen and Boumans (1963).

The moisture balance of the root zone of an irrigated field may be written as:

Where,

I_r = field irrigation supply minus surface losses, dm month ^-1

N = precipitation less interception and surface runoff, dm month^-1

ET = evapotranspiration, dm month^-1

P = deep percolation below root zone or capillary water supply from below (P negative), dm month^-1

ΔV = variation of the quantity (V) of moisture stored in the root zone, dm month^-1

For the groundwater balance below the root zone:

S_p = underground water supply, dm month^-1

D_n = natural drainage, dm month^-1

D_a = artificial drainage, dm month^-1

D_t = total drainage, dm month^-1

ΔW = variation of moisture storage below root zone, dm month^-1

Salts supplied by precipitation or assimilated by the crops are negligible.

Now the salt balance of the root zone becomes:

C_ir = salt concentration in irrigation water, me L^-1

C_p = salt concentration in percolation water, me L^-1

ΔZ = variation of quantity of dissolved salts (Z) in root zone, me L^-1

P is negative, when it represents capillary rise. Both positive and negative percolation occurs, if during the program period, P is considered as the algebraic sum of the both.

The efficiency of the leaching operation and the required drainage for salinity control is determined by the leaching coefficient (k), an empirical quantity smaller than 1, mainly controlled by soil structure, pore size distribution, presence of cracks, soil texture and soil permeability. If the share of the effective water passage to the total percolation is k and the ineffective part due to cracks and others is 1- k, the following relationship becomes valid-

Where, C_sm is the salt concentration of the soil moisture in the root zone at field capacity and k, the leaching coefficient.

Though the actually applicable value may be obtained from in- situ or laboratory tests on undisturbed soils, the value of k may be approximated as below:

The limitation of the calculations is that the salt and water balances presumes all salts in the soil are soluble and present in the soil solution and valid for soluble salts as all chlorides, all sodium and potassium salts and magnesium sulphates, but not for the low soluble salts as magnesium and calcium carbonate and calcium sulphate.

ii. Magnetized Water in Leaching:

The salt-leaching efficiency of the irrigation water may be increased with the application of magnetized water. The structure and some physical characteristics of any liquid such as density, salt solution capacity, and deposition ratio of solid particles are likely to be changed when it is allowed to pass through a magnetized field. In water sample containing CaCO₃, the calcium and carbonate ions on entry into the area are influenced by the magnets and pushed in opposite directions, due to their opposite charges.

As all of the calcium ions are pushed in one direction and all of the carbonate anions-arc pushed in the opposite direction, they tend to collide. The electrical charge of suspended particles decreases and stay in the form of snowball phenomenon and suspend in water along with many other changes in the characteristics of water. Thus changed water has the higher leachability of salts.

iii. Flushing:

Especially in the soils with shallow water tables, where high concentration of soluble salts are accumulated there at or near the surface of the soil, flushing may be followed to desalinise the surface as it is desirable to remove the accumulation instead of leaching into the profile. In ploughed field with clay substratum, alternate row leaching technique can improve the efficiency over physical removal of salt crusts by mechanical alternatives.

iv. Method of Water Application:

Efficient water management leads to increased crop yield under saline conditions. Drip, sprinkler and pitcher irrigation have been found to be more efficient than conventional flood irrigation method, since relatively lesser amount of water is required under the improved methods. Drip and pitcher methods are very useful for saline soils as they add water directly in to root zone at controlled rates and even saline waters can be used under these methods without any detrimental effects on crop growth owing to dilution of salts at the root zone. Keeping the soil moisture levels higher between irrigation events effectively dilutes salt concentrations in the root zone, thereby reducing the salinity hazards.

v. Land Shaping Technologies:

The management of the location of salts in relation to root zone or seed placement is one of the tools to grow crops on the saline bed. Modification in the irrigation practices and shaping the land to obtain a more favourable salt distribution in the soil matrix is well known alternative. As the salts tend to accumulate in the ridges when using furrow type irrigation, the direction of movement of applied water and dissolved salts can be managed in order to get a favourable salt distribution. With irrigation, salts leach out of the soil under the furrows and build up on the ridges. Where soil and farming practices permit, furrow planting may help in obtaining better stands and crop yields under saline conditions.

Alkali soils mostly found in semi-arid and sub-humid regions need adequate surface drainage of excess water to reclaim its sodicity. Water management has a crucial importance for managing alkali soils which poses peculiar problems due to clogging action of dispersed soil particles and low stability of soil aggregates which limits the air and water permeability.

So, alkali soil with its inherent problems viz. poor drainage/low infiltration rate, create water logged, anaerobic condition and to get free from this situation we need to adopt surface irrigation techniques such as furrow or basin type flood method or sprinkler irrigation method.

Strategy # 3. Chemical Amelioration:

Different chemical amendments are used to remove part or most of the exchangeable sodium through the replacement by calcium ions in the root zone. Addition of chemical amendments followed by leaching is the pre-requisite to remove the excess soluble salts produced in amelioration processes out of the desired depth.

The amendments can be classified as follows:

Suitability of different amendments to reclaim alkalinity largely depends on the nature of the particular soil and cost of the amendments. In general below pH 7.0 the application of ground limestone is effective for calcium poor soil but its efficiency decreases above pH 7.0 due to decrease in its solubility. Degraded alkali soil can be managed by the application of ground limestone.

However, application of lime is not effective for reclamation of sodic soils having pH more than 8.5 due to predominance of Na₂CO₃. So, in this situation it will be advantageous to supply calcium ions through soluble calcium salts and use of acid or acid forming substances to solubilise the insoluble calcium products.

The reactions of the chemical amendments in sodic soil are given below:

a. Gypsum (CaSO₄.2H₂O):

It is a white natural deposit which is sparingly soluble in water. It is a cheap amendment and used extensively to reclaim the sodicity.

When gypsum is applied to reclaim the sodic soils, the excess sodium ions of the soil exchange phase get replaced by calcium ions through following reactions:

Gypsum Requirement:

The amount of gypsum required to lower the initial ESP of a sodic soil to a desired level is known as gypsum requirement which is expressed as cmol kg^-1 soil or meq 100 g^-1 soil. The requirement of gypsum will be governed by solubility and purity of gypsum, nature and type of clay minerals and nature of extent to which the exchange process takes place in soil.

GR (cmol kg ^-1 soil) = (E_Nai– E_Naf) * CEC

The laboratory method proposed by Schoonover is widely used for assessing the gypsum requirement of sodic soils. Gypsum should be applied 1.2 to 1.5 times the theoretical value of GR for increasing its use efficiency. Application of gypsum helps to increase the hydraulic conductivity of soil. (Table 3.9)

Strategy # 4. Biological Amelioration/ Phytoremediation of Salt Affected Soils:

Reclamation of salt affected soils with different degree of severity can be done through a plant assisted approach generically termed ‘phytoremediation’ which is typically achieved by the ability of plant roots to increase the dissolution rate of inherent or precipitated calcite (CaCO₃) or dolomite (CaCO₃.MgCO₃) as the source of Ca²⁺ and/or Mg²⁺, thereby resulting in enhanced levels of Ca²⁺ and/or Mg²⁺ in soil solution to effectively replace Na⁺ from the cation exchange complex.

Qadir et al. (2007) has mentioned the multidimensional advantages of phytoremediation which are as follows:

1. No financial outlay to purchase chemical amendments,

2. Accrued financial or other benefits from crops grown during amelioration,

3. Promotion of soil-aggregate stability and creation of macrospores that improve soil hydraulic properties and root proliferation,

4. Greater plant-nutrient availability in soil after phytoremediation,

5. More uniform and greater zone of amelioration in terms of soil depth, and

6. Environmental considerations in terms of carbon sequestration in the post amelioration soil.

Phytoremediation is a viable solution for resource-poor farmers and is particularly effective when used on moderately saline-sodic and sodic soil.

Selection of Salt Tolerant Crops:

The crops are selected based on their tolerance to salinity. Generally plants develop the salt tolerance based on the mechanisms of adjustment to osmotic pressure of cell by internal mechanism or by salt exclusion by the plant roots during absorption of water and nutrients.

Proper Seed Placement:

Plants are generally most sensitive to salinity at the early stages of growth but become tolerant to some extent at the later periods of growth. Salts accumulate on the ridges away from wet zone when irrigation is applied. Therefore, seeds can be sown on the salt free zone.

Proper Raising of Seedlings:

Germinating seeds are severely affected by osmotic effects of salinity, resulting in higher mortality and thus poor crop stand. Higher seed rate and/or closer plant spacing may be used to ensure good crop stand. In case of transplanted crops, the seedlings per hill should be increased and it will be better to use older seedlings.

Use of Mulches:

Mulching will be helpful in reducing the rate of evaporation checking the upward movement of salts. Organic matter addition improves the soil physical condition and increase the water holding capacity keeping the salt in diluted condition.

Residue Management:

Crop residues act as mulching materials and will be useful in reducing the evaporative loss of water. It has also beneficial effect on maintenance of soil properties.

Afforestation:

Raising forest and horticultural crop plants on barren salt affected lands will be a feasible option for management of such soils. Agro-forestry, pastoral, silvi-pastoral practices can be followed for improving the productivity of saline soils.

Sodicity tolerant trees could also be grown by specific techniques. Field experiments have shown that some tree species such as Eucalyptus tereticornis, Prosopis juliflora and Acacia nilotica could be grown in highly sodic soils, if the soil of a pit, 90 cm × 90 cm, is improved by the application of gypsum and farmyard manure and seedlings of the tree species planted there in. This measure creates a favourable soil condition for root growth and establishment of the trees. It has also been reported that Casuarinas can reclaim salt affected soil.