Soil properties in a sugarcane plantation after the application of treated sewage effluent and phosphogypsum in Brazil

In a field experiment performed in Lins County (São Paulo State, Brazil), treated sewage effluent (TSE) irrigation increased sugarcane yield but caused an excessive increase in the exchangeable sodium percentage (ESP) and clay dispersion after 16months due to an intense irrigation regime (2500mm/16months) with sodium rich effluents. After two additional complete cycles with lower TSE irrigation rates (1200mmyear−1), 1700kgha−1 of phosphogypsum was added to a section of the irrigated plots to evaluate its residence time and its implications on Na+ dynamics and other soil properties. Undisturbed soil cores were taken 2years after phosphogypsum application to verify soil physical properties up to 0.2m depth, and disturbed soil samples were taken every year up to 1m depth for chemical analyses. After 5years of consecutive TSE irrigation (2005–2010), soil acidity (pH≈5) and basic cations (Ca≈12, Mg≈6 and K≈2mmolckg−1) were maintained in adequate conditions for plant development without the necessity of liming, while acidity was increased (pH≈4.5) and Ca (≈9mmolckg−1), and the Mg (≈4.5mmolckg−1) concentration decreased in the rainfed without phosphogypsum treatment. An increase in water retention capacity at −30 (from 0.14 to 0.17m3m−3) and −1500kPa (from 0.08 to 0.12m3m−3) potentials was also observed in all TSE irrigated treatments. The plots with a phosphogypsum treatment showed average increases of 2mmolckg−1 of Ca2+ and 7mgkg−1 of S-SO42− in all soil profiles and an average reduction of 2mmolckg−1 of Na+ up to 0.4m from 2008 to 2009. However, the extent of the chemical effects was greater after the first year compared to the second year. The high concentration of Na+ found in previous studies performed in the same area returned to low concentrations after continued TSE irrigation at lower rates, even without the phosphogypsum application. An unusual phosphorus migration was observed to the 0.4–0.8m soil layer as a result of TSE irrigation, most likely due to a high pH and a Na carbonate-dominated TSE.