Sodic Soils: Properties

Introduction

Sodic soils are those with a high proportion of sodium adsorbed to the soil particles compared to other cations such as Ca, Mg, and K. Exchangeable sodium percentage (ESP) is a criterion used to identify sodic soils. When a soil with high ESP is wetted, the bonds between soil particles are weakened. As a result, the clay particles swell and often become detached and disperse at high water content. This leads to poor physical properties of soils. In classification system developed in U.S.A., a soil with an ESP >15 is considered as “sodic,” and in Australia, the criterion is >6. These differences are owing to the influence of several soil factors including salt levels, pH, organic matter, and clay mineralogy on the adverse effects of ESP on soil properties. Sodic soils are widespread in arid and semiarid regions of the world extending up to 30% of the total land area (Table 1). Use of saline water, including waste and effluent waters containing sodium salts, for irrigation induces sodicity in soils. Sodicity is a latent problem in many salt- affected soils where deleterious effects on soil properties are evident only when salts are leached below a threshold level.121 While soil salinity reduces plant growth, directly affecting physiological functions through osmotic and toxicity effects on plants, sodicity causes deterioration of soil physical properties indirectly affecting plant growth and survival. Sodic soils are subjected to severe structural degradation and exhibit poor soil-water and soil-air relationships; these properties (Table 2) adversely affect root growth, thereby restricting plant production and making it difficult to work in soils when they are wet or dry.

Yield Decline in Sodic Soils

Sodic soils make the paddocks prone to waterlogging, poor crop emergence and establishment, gully erosion, and in some instances tunnel erosion. Because of the heterogeneity in the accumulation of sodium by soil particles, these symptoms may be observed only in certain parts of the paddock. Generally, patchy growth and barren patches are visible in a number of spots in a paddock, while the rest of the field may look normal. However, the effects of sodicity are fully realized in the harvested yield. The actual yield obtained in sodic soils is often less than half of the potential yield expected on the basis of climate,

TABLE 1 World Distribution of Sodic Soils

Continent

Country

Area of Sodic Soils (000 ha)

North America

Canada

6,974

U.S.A.

2,590

South America

Argentina

53,139

Bolivia

716

Brazil

362

Chile

3,642

Africa

Algeria

129

Angola

86

Botswana

670

Cameroon

671

Chad

5,950

Ethiopia

425

Ghana

118

Kenya

448

Liberia

44

Madagascar

1,287

Namibia

1,751

Niger

1,389

Nigeria

5,837

Somalia

4,033

Sudan

2,736

Tanzania

583

Zimbabwe

26

South Asia

Bangladesh

538

India

574

Iran

686

North and Central Asia

China

437

U.S.S.R.

119,628

Australasia

Australia

339,971

Source: Bui et al.111

particularly rainfall and evapotranspiration.13,41 Relative yield of cereals grown in dryland sodic soils in Australia in relation to average root zone ESP is given in Figure 1.

Swelling and dispersion of sodic aggregates destroy soil structure, reduce the porosity and permeability of soils, and increase the soil strength even at low suction (i.e., high water content). These adverse conditions restrict water storage and transport. Soils are, therefore, either too wet immediately after rain or too dry within a few days for optimal plant growth. Thus, the range of soil water content that does not limit plant growth and function (“nonlimiting water range”) is very small.151 Dense, slowly permeable sodic subsoils reduce the supplies of water, oxygen, and nutrients needed for obtaining maximum potential yield. During the rainy season, even with prolonged ponding of water on the surface, only a small increase in water content occurs in subsoil. The low porosity leads to slow internal drainage and water redistribution within the profile.161 This reduction in water storage causes crop water stress during prolonged dry periods. Subsoil as a source of water and nutrients becomes more important in dryland cropping regions than in irrigated soils.

TABLE 2 Physical and Chemical Properties of a Typical Sodic Soil Profile in South Australia

Properties

0-20 cm

20-40 cm

40-100 cm

Chemical properties

pH1;5 (water)

7.9

8.9

9.2

ECc (dS/m)

0.4

3.8

4.9

Organic carbon (%)

1.2

0.6

0.3

Exchangeable sodium (%)

6.2

14.6

24.5

CaCOj (%)

0.1

2.8

4.5

Boron (mg/kg)

1.2

22.0

38.5

Water soluble Al(OH).f (mg/kg)

0.0

1.2

2.6

Physical properties

Spontaneously dispersed clay (%)

1.2

8.6

9.4

Swelling (mm/mm)

0.04

0.18

0.20

Hydraulic conductivity at saturation (mm/day)

22.8

4.5

2.3

Penetrometer resistance at lOOkPa suction (MPa)

1.8

4.2

4.8

Aeration porosity (%)

9.7

4.8

3.9

Bulk density (Mg/m3)

2.0

2.2

2.3

Final infiltration rate in the field (mm/hr)

0.2

Relative yield of cereals grown in Australian sodic soils in relation to average root zone ESP

FIGURE 1 Relative yield of cereals grown in Australian sodic soils in relation to average root zone ESP.

Salt Accumulation in Root Zones of Sodic Soils

Soils with sodic subsoils are characterized by moderate to high exchangeable sodium and, in many cases, with high pH (>8.5) where carbonate and bicarbonate minerals are present. Subsoil sodicity restricts drainage beyond the root zone and as a result salts accumulate in this zone. The concentration of accumulated salts fluctuates with rainfall pattern, input of salt from agronomic practices, and soil weathering, as schematically explained in Figure 2. Dryland salinity or “seepage salinity” in many countries is associated with rising saline groundwater tables. However, the extent of subsoil salinity, also called “transient salinity,” not associated with saline groundwater is large in many landscapes dominated by subsoil sodicity. A relationship between rainfall, subsoil ESP, and ECe for northeastern Australian soils has been reported.1"1 In dryland regions with annual rainfall between 250 and 600 mm, sodic subsoils have an ECc between 2 and 20 that can dramatically affect crop production through osmotic effects during dry periods. Laboratory measured ECc will increase several folds under field conditions as the soil layers dry in between rainy days. The combination of poor water storage and osmotic stress enhance water stress to crops under dryland cropping.

Root Zone Constraints in Dryland Sodic Soils

Multiple problems occur in soils with subsoil sodicity. Soil compaction, crusting, and induration of subsoil layers re quire “physical” reclamation. Sodicity, salt accumulation, and alkaline pH require “chemical” reclamation. All of these conditions cause, in addition to water stress, macro- and micronutrient deficiency, and toxicity owing to Na% Cl", HCO,", C032", B, Al(OH)4", and others. Low organic matter and biological activity compound these problems encountered in sodic subsoils.

Management of Dryland Sodic Soils

Major criteria in increasing productivity in dryland sodic soil are improved water storage and transport in the root zone and crop water use efficiency. More information is available on agricultural management in sodic soils that is more relevant to irrigated lands.161 Reclamation procedures involving high costs are prohibitive in dryland regions because of low benefit/cost ratio.

Diagnosis of multiple problems with large variations, vertically and horizontally across the paddock, is primarily important. Gypsum is the most commonly used compound to reclaim sodic soils. Subsoil reclamation may involve higher rates of gypsum application or deep placement of gypsum by deep ripping or deep plowing. Salt-tolerant plant species may alleviate subsoil salinity. Plants that can tolerate ion toxicity such as boron, carbonate, sodium, and chloride have also been identified. Strategies to improve subsoil fertility may include 1) mechanical means of placing nutrients deeper in the profile; 2) using nutrient sources of lower or higher mobility; 3) using deep-rooted legumes to fix nitrogen at depths; and 4) selection of plant species and genotypes better suited to acquiring nutrients from subsoils. Future research is needed on developing plants that modify the rhizosphere and adapt to edaphic conditions. Farming systems should be developed to prevent accumulation of salts and toxic elements in the root zone of sodic soils.

Salt accumulation in sodic subsoils

FIGURE 2 Salt accumulation in sodic subsoils.

Conclusions

Soils with a high proportion of exchangeable sodium are considered as sodic soils. On wetting these soils, clay particles swell and disperse degrading soil structure, and on further drying soils become dense. Poor water storage and restricted movement of air and water in soil profile lead to yield decline of many crops. Sodic subsoils cause accumulation of salts in the rootzone. In dryland sodic soils multiple problems such as salinity and toxic elements in addition to sodicity occur making management decisions difficult. Soil amelioration with gypsum and choice of tolerant crops are useful in farming these soils.

References

  • 1. Bui, E.N.; Krogh, L.; Lavado, R.S.; Nachtergaele, F.O.; Toth, T.; Fitzpatrick, R.W. Distribution of sodic soils: the world scene. In Sodic Soils: Distribution, Properties, Management and Environmental Consequences-, Sumner, M.E., Naidu, R., Eds.; Oxford University Press: New York, 1998; 19-33.
  • 2. Rengasamy, R; Olsson, K.A. Sodicity and soil structure. Aust. J. Soil Res. 1991,31, 821-837.
  • 3. French, R.J.; Schultz, J.E. Water use efficiency of wheat in a mediterranean-type environment. 1. The relation between yield, water use and climate. Aust. J. Agric. Res. 1984,35,765-775.
  • 4. Rengasamy, P. Sodic soils. In Methods for Assessment of Soil Degradation-, Lai, R., Blum, W.H., Valentine, C., Stewart, B. A., Eds.; CRC Press: New York, 1997; 265-277.
  • 5. Letey, J. The study of soil structure: science or art. Aust. J. Soil Res. 1991,29, 699-707.
  • 6. Oster, J.D.; Jayawardane, N.S. Agricultural management of sodic soils. In Sodic Soils: Distribution, Properties, Management and Environmental Consequences; Sumner, M.E., Naidu, R., Eds.; Oxford University Press: New York, 1998; 125-147.
  • 7. Shaw, R.J.; Coughlan, K.J.; Bell, L.C. Root zone sodicity. In Sodic Soils: Distribution, Properties, Management and Environmental Consequences; Sumner, M.E., Naidu, R., Eds.; Oxford University Press: New York, 1998; 95-106.
 
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