Application of Phosphogypsum and Manure for Reclaiming Saline-Sodic Soils in Tunisia: Geochemical Effects on Soil Properties and Leachate Composition

Soil salinity and sodicity are major constraints on soil fertility, agricultural productivity, and crop yield, particularly in arid and semi-arid regions such as a large area of Tunisia, where 1.5 million hectares of farmland are salt-affected. This study aims to evaluate the potential of phosphogypsum (PG), a calcium-rich (up to 95%) by-product of phosphoric acid production, as a soil amendment for mitigating salinity and sodicity in the central-western region of Tunisia. Rather than being discarded as waste in the Mediterranean Sea, PG was tested for its ability to displace sodium (Na+) ions through calcium (Ca2+) exchange, thereby improving soil structure and chemical quality. Field trials and column leaching experiments were conducted to assess the geochemical effects of various doses of PG (10, 20 and 40t/ha) alone and in combination with organic manure on soil properties and leachate composition. Results showed that soil electrical conductivity (EC) significantly decreased from 14.49 (Control Soil, CS) to 2.26 mS/cm with 20t/ha PG with manure, while exchangeable sodium percentage (ESP) and sodium concentrations also declined. Soil pH decreased from 8.89 to 7.02 with 20t/ha PG, and organic carbon (OC) content increased from 3.27% in CS to 4.79% in soil treated with the PG and manure combination. Leachate composition analysis was used to assess the removal of soluble salts. Indeed, leached analysis revealed a marked enrichment in major cations and anions (Na+, Ca2+, Mg2+ and Cl−), indicating effective mobilization and removal of soluble salts from the soil profile. Geochemical modelling using saturation index (SI) calculations showed that leachates were saturated or oversaturated with respect to minerals such as fluorite, gypsum, anhydrite, and hydroxyapatite, suggesting possible precipitation or equilibrium conditions influenced by the pH and the ionic strength of the solution. This study highlights the promising potential of PG for reducing soil salinity and sodicity, supporting its possible application as a soil amendment within sustainable land management strategies in Tunisia. At the same time, it emphasizes the importance of assessing potential environmental risks related to PG use, particularly due to the presence of heavy metals and naturally occurring radioactive elements. A comprehensive evaluation of these risks is essential to ensure the safe and responsible use of PG in agricultural practices.