Potentially Toxic Effects of Phosphogypsum on Palexerults in Western Spain

We studied effects of phosphogypsum (PG) rates ranging from 1.4 to 84.2 Mg ha −1 applied to the Ap horizon of a plinthic Palexerult on the ionic composition of the soil solution, as well as their impact on biomass production in wheat [ Triticum aestivum (L.), cv. Jabato]. Similar soil samples were treated with highly pure gypsum (G) or the industrial byproduct red gypsum (RG) at rates equivalent to PG applications of 4.2 and 33.7 Mg ha −1 for comparison with the action of PG. All treatments increased the Ca, Mg, K, Na, and SO 4 concentrations to the same extent; however, RG at high rates (30.6 Mg ha −1 ) raised the Na and Mg concentrations more markedly than did the other two amendments. Phosphogypsum provided the greatest increases in F, Al, and Si, the last two as a result of the corrosive effects of F on soil silicates; the increases were all proportional to the applied PG rate, and as a result, the higher PG rates increased the activities of all Al ionic species relative to G, RG, and the control. Application of G and RG at rates equivalent to PG applications of 4.2 and 33.7 Mg ha −1 increased biomass production compared with the control. Phosphogypsum increased biomass production at lower rates (0–16.8 Mg.ha −1 ), but biomass was virtually zero at rates above 67.3 Mg ha −1 The highest PG rates increased plant Al and F contents, the last reaching toxic levels for cattle.