Diagnosis of Iron Compounds in Some Basrah Soils Fertilized with Iron and Agricultural Sulfur

Abstract

This study aimed to diagnose native iron compounds in selected calcareous soils of Basrah Governorate, Iraq, and to evaluate the effect of iron and agricultural sulfur fertilization on iron chemical behavior under wetting–drying cycles. Ten soil samples were collected from different locations and subjected to laboratory incubation experiments using three levels of iron (0, 25, and 50 kg Fe ha⁻¹ as FeSO₄·7H₂O) and three levels of agricultural sulfur (0, 1000, and 2000 kg S ha⁻¹), in the presence of organic matter. Soil samples were incubated under two wetting–drying cycles, and key chemical properties including pH, electrical conductivity (EC), soluble iron, and sulfate were measured. The Ion Activity Product (IAP) and Saturation Index (SI) were calculated to identify dominant iron minerals.

Results indicated that hematite (½α-Fe₂O₃) was the dominant native iron mineral in all studied soils during the first wetting–drying cycle (SI > 1), while siderite (FeCO₃) and Fe(OH)₃ were not detected (SI < 1). The application of iron and sulfur fertilizers reduced SI and IAP values, reflecting increased iron availability in soil solution. During the second wetting–drying cycle, mineral composition shifted, with the formation of Fe(OH)₃, hematite, and siderite (SI > 1), although hematite remained dominant. The increase in SI and IAP values was associated with enhanced iron activity due to fertilization and sulfur-induced acidification.

The study highlights the significant role of agricultural sulfur in improving iron availability in calcareous soils through pH reduction and mineral transformation, particularly under repeated wetting–drying conditions.