Ferrous Sulphate as a Soil Amendment: Correcting High pH and Alkaline Soils

In agricultural districts throughout the Great Rift Valley of Kenya—encompassing parts of Naivasha, Nakuru, Gilgil, Baringo, and Kajiado—farmers frequently contend with alkaline soils having pH values ranging from 7.8 to 8.6. These soils are often classified as calcareous or sodic, characterized by high concentrations of free calcium carbonate (CaCO₃) and exchangeable sodium. At soil pH levels above 7.5, phosphorus precipitates as insoluble calcium phosphates, and essential cationic micronutrients (iron, manganese, zinc, copper) become tightly immobilized, leading to severe multi-nutrient deficiencies.

Chemical Hydrolysis and Acid Generation

While elemental sulphur is traditionally used for soil acidification, it requires soil temperatures above 20°C and several months of active microbial oxidation by Thiobacillus bacteria before sulphuric acid is produced. Ferrous Sulphate Heptahydrate (FeSO₄·7H₂O), by contrast, acts rapidly via direct chemical hydrolysis:

Step 1 (Hydrolysis): Fe²⁺ + 2H₂O → Fe(OH)₂ + 2H⁺
Step 2 (Oxidation): 4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃↓

As demonstrated by the net stoichiometric equation:

4FeSO₄ + O₂ + 10H₂O → 4Fe(OH)₃↓ + 4H₂SO₄

Every mole of ferrous sulphate applied generates one equivalent of sulphuric acid (two moles of H⁺), immediately neutralizing soil carbonate and bicarbonate ions:

2H⁺ + CaCO₃ → Ca²⁺ + H₂O + CO₂↑

This reaction converts insoluble calcium carbonate into soluble calcium ions, improving soil structure while driving rhizosphere pH downward into the bioavailable zone (pH 6.2–6.8).

Application Rates and Soil Buffering Capacity

The exact quantity of Ferrous Sulphate Heptahydrate required depends directly on soil texture and carbonate buffering capacity:

• Sandy to Light Loam Soils (Low Buffering): Apply 150 to 250 kg per hectare (15 to 25 g/m²) to lower soil pH by approximately 0.5 units.
• Clay Loam to Volcanic Soils (High Buffering): Apply 350 to 600 kg per hectare (35 to 60 g/m²) to overcome strong carbonate reserves.
• Orchard Tree In-Furrow Application: Mix 200 g to 500 g with organic compost per tree planting hole during establishment.

Best Management Practices in the Field

For optimal results, broadcast dry ferrous sulphate crystals uniformly across tilled soil, lightly disc or rototill into the top 10–15 cm of the root zone, and apply irrigation immediately. Applying the material alongside organic compost or well-rotted animal manure enhances performance: organic humic and fulvic acids chelate the newly generated Fe²⁺ ions, preventing premature re-precipitation and sustaining prolonged micronutrient availability.

KIKI'S Industrial Chemicals supplies bulk Ferrous Sulphate Heptahydrate 98% Min in 50 kg heavy-duty woven polypropylene bags with heat-sealed polyethylene inner moisture liners (Net Wt 50.000 kg, Kaiser Exports India, Lot VP/KE/26/27/05) from our central warehouse on Enterprise Road, Nairobi.