🧫 Calcareous Soils

This lesson explains calcareous soils, their occurrence, and their influence on nutrient availability and crop management.

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Definition and Occurrence

Calcareous soils contain free calcium carbonate (CaCO3) in quantities sufficient to effervesce visibly when treated with dilute hydrochloric acid (HCl). These soils typically have a pH between 7.5 and 8.5, and the CaCO3 content may range from a few percent to over 50% of the soil weight. The CaCO3 may be present as finely divided powder throughout the soil matrix, as concretions (kankar nodules), or as a distinct calcareous layer (calcic horizon).

Calcareous soils are widely distributed in India, particularly in:

• Arid and semi-arid zones — Rajasthan, Gujarat, parts of Maharashtra, Karnataka, and Andhra Pradesh
• Indo-Gangetic alluvial plains — where CaCO3 accumulates as kankar nodules at varying depths
• Black cotton soils (Vertisols) — of central and peninsular India

Effects of CaCO3 on Nutrient Availability

High CaCO3 content and elevated pH significantly affect the availability of several essential plant nutrients:

Phosphorus

At high pH, phosphorus reacts with calcium to form insoluble calcium phosphates (tri-calcium phosphate, hydroxyapatite), drastically reducing P availability. Freshly applied soluble P fertilizers are rapidly converted to less available forms in calcareous soils, a process called P fixation.

Iron

Iron availability decreases dramatically above pH 7.0. In calcareous soils, iron exists as insoluble Fe(OH)3 and Fe2O3, and iron deficiency (lime-induced chlorosis) is the most common nutritional disorder. Symptoms appear as interveinal chlorosis on young leaves.

Zinc

Zinc availability decreases 100-fold for each unit increase in pH. Zinc deficiency is widespread in calcareous soils, causing stunted growth, small leaves, and interveinal chlorosis in crops like rice, maize, wheat, and citrus.

Manganese

Manganese availability also decreases at high pH due to oxidation to insoluble Mn4+ forms. Manganese deficiency causes interveinal chlorosis in middle-aged leaves.

Boron and Copper

Both become less available in calcareous soils; boron due to adsorption on CaCO3 surfaces and copper due to complexation with organic matter at high pH.

Management of Micronutrient Deficiencies

Soil Application

• Iron — FeSO4 (ferrous sulfate) at 25–50 kg/ha, but effectiveness is limited as Fe2+ is rapidly oxidized to insoluble Fe3+ at high pH; application with organic manure improves efficiency
• Zinc — ZnSO4 at 25 kg/ha; most effective micronutrient fertilizer in calcareous soils; can also be applied as Zn-EDTA chelate at lower rates
• Manganese — MnSO4 at 25 kg/ha; band placement near roots is more effective than broadcast

Foliar Application

Foliar sprays bypass the soil chemistry problem entirely and deliver nutrients directly to leaves:

• Iron — 0.5% FeSO4 solution; or iron chelates (Fe-EDDHA is most effective in calcareous soils as it remains stable at high pH)
• Zinc — 0.5% ZnSO4 with 0.25% lime (to prevent leaf burn)
• Manganese — 0.5% MnSO4 solution

Other Management Strategies

• Organic matter — FYM, compost, and green manure release organic acids during decomposition that temporarily lower pH in the rhizosphere and form soluble chelates with micronutrient metals, increasing their availability
• Acidifying fertilizers — ammonium sulfate, elemental sulfur, and sulfur-coated urea lower pH in the root zone
• Band placement of phosphorus — reduces contact with CaCO3, slowing fixation
• Use of chelated fertilizers — EDTA, DTPA, and EDDHA chelates keep micronutrients in soluble form
• Selection of tolerant crops/varieties — efficient genotypes that exude organic acids or phytosiderophores from roots to solubilize Fe and Zn

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Summary Cheat Sheet

Key Recall Points

• Calcareous soils contain significant CaCO3 and often maintain high pH.
• Common constraints are Fe, Zn, and P availability issues.
• Effective management combines targeted nutrient placement, chelates, and organic inputs.

Exam Traps

• High soil test P does not always mean high plant-available P in calcareous conditions.
• Foliar micronutrients help quickly, but root-zone correction is still required.
• Carbonate effects differ with moisture and bicarbonate-rich irrigation water.

References

3 sources • [1] [2] [3]

[1]

Soil Chemistry References on Calcareous Soil and Nutrient Fixation

Book

[2]

ICAR Recommendations for Micronutrient Management in Calcareous Soils

Official

[3]

Problematic Soil Notes for BSc Agriculture

Book