Characteristics of arid soils: solonetz, solonchaks

Soils in Hot Arid and Hot Semi‑Arid Environments (Cambridge A‑Level Geography – Topic 10)

1. Climate & Distribution of Hot Arid / Semi‑Arid Zones

• Latitude: mainly 20° S – 30° N (subtropical high‑pressure belt) and on leeward sides of major mountain ranges.
• Mean annual temperature: 20 – 30 °C; summer maxima often > 40 °C.
• Mean annual precipitation:
  • Arid: < 250 mm yr⁻¹
  • Semi‑arid: 250 – 500 mm yr⁻¹
• Potential evapotranspiration (PET) ≈ 2–3 × precipitation → strong moisture deficit.
• Rainfall pattern: erratic, short, intense events that generate flash floods.

2. Dominant Physical Processes Shaping Arid Landscapes

• Physical weathering – thermal expansion, salt‑crystal growth; chemical alteration is limited.
• Aeolian processes – wind erosion and deposition produce dunes, deflation hollows and dust‑covers.
• Fluvial flash flooding – rapid transport and sorting of sediments, formation of alluvial fans and ephemeral channels.
• Salt‑crust formation – capillary rise of soluble salts and surface evaporation create crusts that protect the surface but greatly reduce infiltration.

3. Arid‑Soil Families (WRB) Relevant to the Syllabus

Only the families below are required for the Cambridge exam. The first two are examined in detail; the remaining three are given concise diagnostic criteria.

• Solonetz – sodium‑rich, hard sub‑soil horizon (alkaline soil).
• Solonchaks – thick saline surface horizon (saline soil).
• Calcisols – accumulation of secondary calcium carbonate (caliche) in the sub‑soil; often a hard, cemented layer (B‑calcareous horizon).
• Gypsisols – accumulation of gypsum (CaSO₄·2H₂O) in the profile; a B‑gypsic horizon may be hard or soft depending on moisture.
• Cambisols – weakly developed soils with a thin, weakly structured A‑horizon; often the initial stage before more advanced arid soils develop.

4. Developmental Sequence of Arid Soils

Under prolonged aridity and limited leaching, soils tend to follow a predictable pedogenic pathway:

1. Cambisol stage – shallow, weakly structured A‑horizon on recent alluvium or aeolian deposits.
2. With time, soluble salts (Na⁺, Cl⁻, SO₄²⁻) accumulate at the surface → Solonchak formation.
3. If the parent material is silicate‑rich and sodium dominates exchange sites, dispersed clay develops → Solonetz (alkaline horizon).
4. In carbonate‑rich settings, precipitation of CaCO₃ forms a caliche layer → Calcisol.
5. Where gypsum is abundant in the parent material or groundwater, gypsum precipitates → Gypsisol.

Thus, the type of arid soil that ultimately develops depends on (a) the chemistry of the parent material, (b) the balance between salt input and leaching, and (c) the duration of aridity.

5. Saline vs Alkaline Soils – A Quick Box

6. Solonetz – Sodium‑Rich Alkaline Soil

• Parent material: loamy to clayey silicate soils with high exchangeable Na⁺.
• Diagnostic horizon: solonetzic horizon – dense, hard, sodium‑rich sub‑soil.
• Depth of solonetzic horizon: 0.5 – 2 m.
• Key chemical indicators:
  • Exchangeable Sodium Percentage (ESP) > 15 %.
  • Sodium Adsorption Ratio (SAR) > 15.
• Physical properties:
  • Clay particles are dispersed → slick, low‑permeability horizon.
  • Cracks on drying; surface horizon may be slightly saline but usually less than in solonchaks.
• Typical vegetation:
  • Halophytic grasses – e.g. Stipa capensis.
  • Salt‑tolerant shrubs – e.g. Atriplex spp.
• Management / reclamation:
  • Gypsum (CaSO₄) amendment – replaces Na⁺ with Ca²⁺ (see reaction below).
  • Deep ripping or sub‑soiling to break the hard horizon.
  • Followed by leaching where water is available.

7. Solonchaks – Saline Surface Soil

• Parent material: loamy to sandy sediments where evaporite salts accumulate.
• Diagnostic horizon: solonchakic horizon – thick saline surface layer.
• Thickness of saline horizon: 0.1 – 0.5 m (up to 1 m in extreme cases).
• Key chemical indicator: Electrical conductivity (EC) > 4 dS m⁻¹.
• Dominant salts: NaCl, Na₂SO₄, and other soluble evaporites.
• Physical properties:
  • Crusty surface; often a slick, hard crust.
  • Permeability improves below the crust.
• Typical vegetation:
  • Extreme halophytes – e.g. Salicornia europaea, Atriplex nummularia, Salsola soda.
• Management / reclamation:
  • Leaching with large water volumes (5–10 × annual precipitation) where water is available.
  • Where leaching is impractical, land is used for extensive grazing of salt‑tolerant livestock or for salt extraction.

8. Gypsum Amendment Reaction (Solonetz Reclamation)

Resulting sodium sulfate is soluble and can be removed by subsequent leaching, improving soil structure and permeability.

9. Calcisols – Carbonate‑Rich Soils

• Diagnostic feature: Accumulation of secondary calcium carbonate (caliche) forming a hard B‑calcareous horizon.
• Typical texture: Loamy to clayey; often a thin A‑horizon over a cemented caliche.
• Physical effect: Caliche acts as a barrier to root penetration and water movement.
• Vegetation: Sparse xerophytic shrubs; occasional drought‑tolerant trees where caliche is thin.
• Management: Mechanical breaking of caliche combined with gypsum or organic amendments; water‑efficient irrigation where possible.

10. Gypsisols – Gypsum‑Accumulating Soils

• Diagnostic feature: B‑gypsic horizon with significant gypsum (CaSO₄·2H₂O) accumulation.
• Texture: Often loamy to sandy; gypsum may cement particles when dry.
• Physical effect: Gypsum is relatively soluble; when moisture fluctuates, it can cause swelling‑shrinkage cycles.
• Vegetation: Halophytes and some salt‑tolerant grasses; gypsum can be a valuable mineral resource.
• Management: Gypsum itself is not a problem; the issue is waterlogging or salt accumulation – leaching and drainage are key.

11. Cambisols – Weakly Developed Soils

• Diagnostic feature: Thin, weakly structured A‑horizon; no significant accumulation of salts, carbonates or gypsum.
• Typical setting: Recent alluvial or aeolian deposits in arid/semi‑arid zones.
• Significance: Represents the early stage of soil development; can evolve into any of the more advanced arid families depending on climate, parent material and time.

12. Soil‑Vegetation–Ecosystem Relationships

• Plant functional types: halophytes (salt‑tolerant), xerophytes (drought‑adapted), occasional C₄ grasses in less saline patches.
• Primary productivity: typically < 200 g C m⁻² yr⁻¹; limited by water scarcity and soil salinity/alkalinity.
• Biodiversity: low species richness; communities dominated by a few well‑adapted species that can complete life cycles quickly after rare rain events.

13. Human Use, Management & Sustainability

• Agricultural potential: limited; only salt‑tolerant crops (e.g., barley, quinoa, sorghum) can be grown with intensive reclamation.
• Case study – Gypsum reclamation in the Kyzylkum Desert (Uzbekistan):
  • Application: 10 t ha⁻¹ gypsum over three years.
  • Result: ESP reduced from 30 % to 12 %.
  • Outcome: Establishment of Atriplex pastures supporting sheep grazing.
• Water constraints: Leaching requires 5–10 × the amount of annual precipitation; in most arid zones this is not feasible without large‑scale irrigation.
• Sustainable alternatives:
  • Extensive grazing of native halophytes.
  • Salt extraction for industrial use.
  • Conservation of natural desert habitats.

14. Comparison of Solonetz and Solonchaks

15. Key Points for Examination

1. Define solonetz and solonchaks and describe their diagnostic horizons.
2. Explain how high ESP and SAR (alkaline) versus high EC (saline) affect soil structure, permeability and plant growth.
3. Compare depth, texture, salinity level, vegetation and reclamation methods using the table above.
4. Identify the main vegetation types associated with each soil and give two example species.
5. Discuss the principal reclamation methods (gypsum amendment vs. leaching) and why water availability determines which is practicable.
6. Briefly describe the diagnostic features of Calcisols, Gypsisols and Cambisols and indicate the typical parent material that leads to each.