Transcript Potential acidity analytical results for lime free samples

Potential acid sulfate soil
Soils which become acid when drained due
to oxidation of pyrite (FeS2)
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
Fe2O3 + 4SO42- + 8CH2O + 1/2O2 = 2FeS2 + 8HCO3- + 4H2O
Iron must be present
Sulfur must be present
Anaerobic condition must prevail to reduce SO42- & Fe3+
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
Fluvisols and gleysols
Histosols
Oxidation of pyrite
If the soil is drained pyrite will be oxidized:
4FeS2 + 15O2 + H2O -> 2 Fe2(SO4)3 + 2H2SO4
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 [KFe3(SO4)2(OH)6] or yellowish-brown
schwertmannite [Fe16O16(SO4)3(OH)10.10H2O] 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
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Low soil pH
Aluminium toxidity
Salinity (from sea water)
Phosphorous deficiency (precipitation of
aluminiumphosphates)
• H2S 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.
Normal stream
Ochre from drains
Ochre poluted streams
Sampling area
Camp site and equipment for mapping
potential acid sulfate soils
Sampling area
Travelling to sampling site
Augering in wetland
Samples
Soil description scheme
Determination of colour and pH
Potential acidity
analytical results for lime free samples
A sample is potential acid sulfate if:
pH drop below 3.0 within 16 weeks of oxidation and
pH drops more than one unit within that period
Potential acidity
lime containing samples
Potential acid sulfate if: %pyrite x 34 meq/100g > (Ca + Mg) meq/100g
Potential acid sulfate soil classes
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Class 1: > 50% acid sulfate soil profiles
Class 2: 20-50% acid sulfate soil profiles
Class 3: 2-20% acid sulfate soil profiles
Class 4: <2% acid sulfate soil profiles
• An acid sulfate soil profile is a profile containing
at least one acid sulfate soil sample
Map showing potential acid sulfate soils
Red 50%-100%
Yellow 20%-50%
Green 20%-2%
Blue: <2%
Potential acid
sulfate soil
Area statistics
Ochre investigation areas
if the farmer wants to drain