Long-term effects of gypsum on the chemistry of sodic soils under sugarcane

Agricultural productivity is limited in soils with high exchangeable sodium due to poor structural stability and water movement. Exchangeable sodium is often measured as exchangeable sodium percentage (ESP) which represents the proportion of the cation exchange capacity occupied by sodium. Sugarcane yield losses of 1.5–2.1 t/ha for every 1% increase in ESP in the subsoil have been observed (Spalding 1983; Nelson and Ham 2000). Sodic soils cover up to 340 million ha in Australia and they are common in drier regions of the sugar industry, comprising approximately 40% of the Burdekin and Mareeba-Dimbulah Irrigation Areas (Shaw et al. 1994; Nelson et al. 2001; Intergovernmental Technical Panel on Soils 2015). Sodic soils are often treated with gypsum (CaSO4⋅2H2O), but despite its widespread use over the past 70 years, few field studies have investigated the long-term (> 10 years) effects of gypsum on soil chemistry, especially in the dry tropics, to soil depths greater than 0.4 m, and with repeated low (<10 t/ha) gypsum application rates. In addition, the amount of gypsum required is normally calculated according to the amount of exchangeable sodium to be displaced to a given soil depth (Oster and Jayawardane 1998). However, this gypsum requirement model (GRM) has not been validated in the field. The aim of our study was to address these two knowledge gaps.

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Proceedings of the 45th Conference of the Australian Society of Sugar Cane Technologists, ASSCT 2024

45

9798331307790

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