Reclamation of a calcareous saline-sodic soil using phosphoric acid and by-product gypsum M. A. Gharaibeh , N. I. E ltaif & S. H. S h r a ’a h Department of Natural Resources and the Environment, Faculty of Agriculture, Jordan University of Science and Technology, P.O.Box 3030, Irbid 22110, Jordan Abstract Phosphoric acid is increasingly used as a source of water-soluble P, but it has not been widely tested for its effectiveness in reclaiming calcareous sodic and saline-sodic soils relative to chemically equivalent rates of gypsum. In lysimeters experiments, we showed that a calcareous saline-sodic soil can be ‘reclaimed’ using phosphoric acid and leaching with moderately saline irrigation water (sodium adsorption ratio = 4.1 and electrical conductivity = 2.2 dS ⁄ m). Phosphoric acid (50% pure) was dissolved directly in the leaching water at application rates of 450, 600 and 900 kg ⁄ ha, and phosphogypsum (80% pure) was mixed with soil prior to leaching at application rates of 15, 20, 30 and 40 t ⁄ ha. Phosphoric acid was more efficient than the chemical equivalent of phosphogypsum in improving soil hydraulic conductivity, and in reducing the exchangeable sodium percentage (ESP). The ESPs after leaching were: 10 (water only), 5.5–5.3 (phosphoric acid) and 8.2–5.9 (phosphogypsum). Keywords: Sodic soil, SAR, reclamation, phosphogypsum, phosphoric acid, hydraulic conductivity Introduction Globally, salt-affected soils cover an area of about 955 · 10 6 ha (Szabolcs, 1994), 560 · 10 6 ha of which are saline-sodic (Tanji, 1990). Such soils are common in arid and semi-arid regions and are characterized by the occurrence of sodium (Na) at levels which causes destabilization of soil structure, reduction in soil infiltration rate, increased susceptibility to crusting and fertility problems that adversely affect the growth and yield of crops (Sumner, 1993; Qadir & Schubert, 2002; Qadir et al., 2005). The management of these soils requires an understanding, not only of the simultaneous transport of water and solute within the soil, but also of how these soils affect crop production (McWilliams, 2003). Amelioration of saline-sodic soils with chemical amendments is an established technology. Some amendments supply calcium (Ca) which replaces excess exchangeable Na (e.g. gypsum), while others (e.g. sulphuric acid) help to increase the dissolution of calcite in calcareous saline-sodic soils (Gupta & Abrol, 1990; Mace et al., 1999; Qadir et al., 2001). The displaced Na + is either leached from the root zone by excess irrigation, and ⁄ or is taken up by crops; some of this uptake may be permanently removed from the field (Ayers & Westcot, 1985). However, chemical strategies used to ameliorate these soils have become too costly for subsistence farmers in developing countries during the last two decades (Qadir & Oster, 2002). Salt-affected soils represent about one-third of the total land that is suitable for agriculture in the Jordan valley (0.5 Mha), around 12% of which have a saturated paste electrical conductivity (EC e ) of 4–8 dS ⁄ m, 13% with an EC e of 8–16 dS ⁄ m and the rest have an EC e of greater than 16 dS ⁄ m (Mashali, 1989). The problems of saline and sodic soils in Jordan are likely to increase in future as a result of overuse of poor quality irrigation water. The use of moderately saline canal water with a moderate sodium adsorption ratio (SAR), containing adequate dissolved calcium and magnesium for reclamation, can potentially prevent dispersion of clay during leaching (Sumner, 1993; Mace & Amrhein, 2001), whereas the application of freshwater (low EC e ) to sodic or saline-sodic land can cause structural disruption and irreversible problems (Ayers & Westcot, 1985; Quirk, 2001). Such problems are encountered in saline-sodic soils Correspondence: M. A. Gharaibeh. E-mail: mamoun@just.edu.jo Received May 2009; accepted after revision December 2009 Soil Use and Management, June 2010, 26, 141–148 doi: 10.1111/j.1475-2743.2010.00260.x ª 2010 The Authors. Journal compilation ª 2010 British Society of Soil Science 141 Soil Use and Management