When drained, soils containing Reduced Inorganic Sulfides such as pyrite may become extremely acidic (pH SO):
The product Fe(OH), iron(III) hydroxide (orange), precipitates as a solid, insoluble mineral by which the alkalinity component is immobilized, while the acidity remains active in the sulfuric acid. The process of acidification is accompanied by the formation of high amounts of aluminium (Al, released from clay minerals under influence of the acidity), which are harmful to vegetation. Other products of the chemical reaction are:
# Hydrogen sulfide (HS), a foul-smelling gas
# Sulfur (S), a yellow solid
# Iron(II) sulfide (FeS), a black/gray/blue solid
# Hematite (FeO), a red solid 
# Goethite (), a brown mineral
# Iron sulfate compounds (e.g., jarosite, schwertmannite, gypsum, and epsomite)
# H-Clay (hydrogen clay, with a large fraction of adsorbed H ions, a stable mineral, but poor in nutrients)
The iron can be present in bivalent and trivalent forms (Fe, the ferrous ion, and Fe, the ferric ion respectively). The ferrous form is soluble in a relatively wide range of pH conditions whereas the ferric form is not soluble except in an extremely acidic environment such as muriatic acid rust remover. The more oxidized the soil becomes, the more the ferric forms dominate.  Acid sulfate soils exhibit an array of colors ranging from black, brown, blue-gray, red, orange and yellow. The hydrogen clay can be improved by admitting sea water: the magnesium (Mg) and sodium (Na) in the sea water replaces the adsorbed hydrogen and other exchangeable acidic cations such as aluminium (Al). However this can create additional risks when the hydrogen ions and exchangeable metals are mobilised.