Drain flow and related salt losses as affected by phosphogypsum amendment in reclaimed marsh soils from SW Spain

Phosphogypsum (PG), a by-product of the P industries, has been widely applied as an amendment to reduce Na saturation in soils. However, some concern arises due to its content of some metals and radionuclides. Thus, it is necessary to optimize the PG rates to avoid excessive accumulation of such pollutants in soil. To evaluate the effects of PG on drain flow and on the displacement of Na by Ca in soils, a three-year field experiment was conducted in a commercial farm located in a reclaimed marsh area from SW Spain. It involved two treatments (control — without PG — and 25Mgha−1 of PG, applied in April 2003 and again in September 2004), done in triplicate in 250×20m plots, following a randomized block design. Drainage flows were monitored, and drain-water samples were analyzed for Na and Ca concentrations. The recorded hydrographs revealed unexpected high lateral flows of water within these soils. No statistically significant differences were found in drained volumes, nor in Na and Ca losses through drainage, between both treatments in each of the three growing seasons (Gossypium hirsutum L. in 2003 and 2005; Beta vulgaris L. in 2003–2004). Losses of Na and Ca in each irrigation event were linearly related to the drainage to irrigation volume ratio (p<0.001); Ca, however, showed significantly different slopes for both treatments. Cation concentration, Na adsorption ratio (SAR), and electrical conductivity (EC) in the saturation extract of soil at different depths were significantly affected by PG and by the oscillation of saline water table during the experiment. The efficiency of PG in displacing Na by Ca was estimated to be about 30%, with the PG-attributable changes in Ca and SAR also appearing in deeper soil layers and in adjacent control plots.