Chapter 2Inorganic Solids in Soil
Soil Chemistry includes:
• Components of soil– Inorganic (soil minerals, salts, metals)– Organic (aggregates, humus, plant residues)– Solution – Gases
• Processes important to plant growth and environmental applications– Ion exchange capacity– Sorption/Complexation
Composition
Soils are:• porous
• open systems (to atmosphere, hydrosphere, biosphere)
• multi-component
• products of weathering
• dynamic, constantly changing, not static
Soil Solids
• >90% solids are inorganic in most soils.
• USDA major size fractions used by soil scientists in U.S.:
clay (<2 m)
silt (2 m – 50 m)
sand (50 - 2000 m = 0.05 - 2 mm)
Engineering uses ASTM or other systems
Common elements in soil Element
Approximate % by weight
Oxygen 46.6Silicon 27.7
Aluminum 8.1
Iron 5.0
Calcium 3.6
Sodium 2.8
Potassium 2.6
Magnesium 2.1
All others 1.5
Common elementsO and Si are two most common elements by
weight and volume
O > Si >>> Al > Fe >> C, Ca, Mg, K, Na
‘aluminosilicates’ and ‘silicates’ – minerals that are made up of Si-O-Al and Si-O molecular framework
www.indiana.edu/~geol116/week2/sillmin.jpg
Essential and toxic ions
• Macronutrients: H, C, N, O, Mg, P, S, K, Ca– Animals also need Na, Cl
• Micronutrients: B, Cl, V, Mn, Fe, Cu, Zn, Mo– Animals also need F, Si, Cr, Ni, Co, As, Sn, Se, I
The list changes with progress in experimental techniques
http://www.soils.wisc.edu/~barak/soilscience326/listofel.htm
Nutrients in plants and animals
• Light atomic weight, concentrated at earth’s surface (heavier metals form the core of earth).
• Evolution took advantage of elements abundant at the surface.
• Plants can tolerate a much wider range of mineral concentrations than animals.
• Chemical spatial variability of soils causes variations in mineral concentrations of plantsanimals should eat a variety of plants and plants
grown on different soils.
Minerals in Soil as Sources of Elements and Ions
Mineral: "natural, inorganic homogenous compound with definite chemical composition and ordered atomic arrangement”
http://www.chem.wisc.edu/~newtrad/CurrRef/BDGTopic/lab/Crystlab.html
Examples of ionic crystalline solids
Unit Cell
Smallest repeating 3-D array of atoms in a crystal.
Minerals are often reported in half-cell formulas for simplification – be aware!
Ex: Kaolinite
Unit cell = Si4Al4O10(OH)8
Half-cell = Si2Al2O5(OH)4
http://www.gly.fsu.edu/~salters/GLY1000/6_Minerals/6_Minerals_index.html
http://gpc.edu/~pgore/myart/silicate.gif
science.kennesaw.edu/.../silicon/sil1cone.htm
Soil clay minerals Silica Tetrahedrons – one building
block of soil minerals
Crystal pictures are from Bob Harter at Univ. of New Hampshire http://pubpages.unh.edu/~harter/crystal.htm#2:1%20MINERALS
Figure 1: Single silica tetrahedron (shaded) and the sheet structure of silica tetrahedrons arranged in a hexagonal network.
http://www.britannica.com/ebc/article-80127
Aluminum Octahedrons – another building block or layer in minerals
Single octahedron (shaded) and the sheet structure of octahedral units.
http://www.britannica.com/ebc/article-80127
Various linkages of the tetrahedra create classes of silicates:
www.indiana.edu/~geol116/week2/sillmin.jpg
www.winona.edu/geology/MRW/minrx.htm
