Presentation on theme: "Potential acid sulfate soil Soils which become acid when drained due to oxidation of pyrite (FeS 2 ) WRB Potential acid sulfate soil contains sulfidic."— Presentation transcript:
Potential acid sulfate soil Soils which become acid when drained due to oxidation of pyrite (FeS 2 ) WRB Potential acid sulfate soil contains sulfidic soil material that contains pyrite but has not oxidized to an extent that the soil-pH dropped to a value below 3.5
Formation of pyrite Fe 2 O 3 + 4SO 4 2- + 8CH 2 O + 1/2O 2 = 2FeS 2 + 8HCO 3 - + 4H 2 O Iron must be present Sulfur must be present Anaerobic condition must prevail to reduce SO 4 2- & Fe 3+ Organic matter as energy source for the microbes The process increases pH
Location of pyrite in the landscape In delta regions and lagunes where sea water is meeting fresh water. Inland wetland areas which are enriched with ferro iron and sulfate from higher parts of the landscape Soil material with high content of pyrite is called sulfidic soil materials
Oxidation of pyrite If the soil is drained pyrite will be oxidized: 4FeS 2 + 15O 2 + H 2 O -> 2 Fe 2 (SO 4 ) 3 + 2H 2 SO 4 pH drops significantly and not only ferro iron but also ferri iron will be mobile. Soils which become very acid due to oxidation of pyrite are classified as actual acid sulfate soils
Oxidation of pyrite might form a sulfuric horizon Definition of sulfuric horizon A sulfuric horizon must: have a soil-pH < 3.5 (in 1:1 water suspension); and have –yellow/orange jarosite [KFe 3 (SO 4 ) 2 (OH) 6 ] or yellowish-brown schwertmannite [Fe 16 O 16 (SO 4 ) 3 (OH) 10.10H 2 O] mottles; or –concretions and/or mottles with a Munsell hue of 2.5Y or more and a chroma of 6 or more; or –underlying sulfidic soil materials; or –0.05 percent (by weight) or more of water-soluble sulphate; and have a thickness of 15 cm or more.
Agriculture problems actual acid sufate soils Low soil pH Aluminium toxidity Salinity (from sea water) Phosphorous deficiency (precipitation of aluminiumphosphates) H 2 S toxidity if flooded N-deficiency due to slow microbial activity Ingeneering problems as soil acidity attacks steel and concrete structures
Environmental problems Ochre polution of Danish watercourses Severe ochre polution of Danish streams has frequently occured due to drainage of farmland. The ochre polution was believed to be due to oxidation of pyrite. In order to prevent ochre polution of the streams a mapping of potential acid soils was conducted The mapping should be done within a 3 years period Based on the mapping a legislation should be made to stop the ochre polutions of the streams.