Soil chemical changes under irrigated mango production in the Central São Francisco River Valley, Brazil

Irrigated areas in Brazil's Central São Francisco River Valley have experienced declines in productivity, which may be a reflection of changes in soil chemical properties due to management. This study was conducted to compare the chemical composition of soil solutions and cation exchange complexes in a five-year-old grove of irrigated mango (Mangifera indica L. cv. Tommy Atkins) with that of an adjacent clearing in the native caatinga vegetation. A detailed physiographic characterization of the area revealed a subsurface rock layer, which was more undulating than the current land surface, and identified the presence of a very saline and sodic (1045 microS cm(-1), sodium adsorption ratio [SAR] = 5.19) ground water table. While changes in concentrations of Ca, Mg, and K could be attributed to direct management inputs (fertilization and liming with dolomite), increases in Na suggested average annual capillary rise from the ground water table of 28 L m(-2). Accordingly, soil salinity levels appeared to be more dependent on surface elevation than the elevation of the rock layer or sediment thickness. The apparent influence of land surface curvature on water redistribution and the solution chemistry was more pronounced under irrigated mango production. In general, salinity levels had doubled in the mango grove and nearly tripled under the canopies, after only five years of irrigation. Though critical saline or sodic conditions were not encountered, the changes observed indicate a need for more adequate monitoring and management of water and salt inputs despite the excellent water quality of the São Francisco River.