@ Belghemmaz, because gypsum is a neutral salt (pH is ~6.7) and has a salt index, or osmotic potential, of 8. It typically has 20 to 22% calcium, and 16 to 18% sulfur. You may find generally gypsum presence in soil when the soil formed from sedimentary rocks in the marine environment with high S and Ca deposits.
CaSO4 is a salt of a weak base (Ca(OH2)) and strong acid (H2SO4) meaning that this salt gives out a hydrolyzation reaction below:
Ca + 2 H2O == Ca(OH)2 + 2 H+
Therefore its solution is slightly acidic. In this case, if the pH is above 8.2 there should not be any dominance of CaSO4 in the soil. When the carbonates become dominant the pH arises towards 8.2 (typical pH of CaCO3 is around 8.3, since anion hydrolyzation, CO3, is higher in this salt). As the Na saturation is above 15% then the pH become over 8.5. The other point is it can be used for correcting high pH in alkaline soils for this feature with some others.
The enclosed solubility diagram of Ca minerals can help you to find an answer.
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