chapter 2 inorganic solids in soil. soil chemistry includes: components of soil –inorganic (soil...

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

socrates.berkeley.edu/~eps2/wisc/Lect4.html