Geology Reference
In-Depth Information
CHAPTER 5
The Chemistry of the Solid Earth
IAN D. PULFORD
Chemistry Department, University of Glasgow, Joseph Black Building,
Glasgow, G12 8QQ, UK
5.1 INTRODUCTION
Soil is the product formed when the rocks of the earth's crust are
exposed at the surface and are subjected to various physical, chemical,
and, eventually, biological weathering processes. The minerals in these
rocks are predominantly silicates, which dominate the characteristics of
most soils. Table 1 shows those elements that are found in the crust
above an average concentration of 1% and their corresponding soil
content. The importance of aluminosilicates in soil is clear from the
enrichment factors of approximately 1 for O, Si, and Al. Some loss
occurs of K, Fe, Ca, Na, and Mg as a result of soil processes. But two
elements, C and N, show considerable enrichment in soil because of the
crucial role played by organic matter.
Table 1 Average elemental concentrations in the earth's crust and in soil, and
the enrichment factor in soil
Average concentration in crust
(%)
Average concentration in soil
(%)
Soil:crust
ratio
Element
Oxygen
47
49
1.0
Silicon
28
31
1.1
Aluminium 8.2
7.2
0.88
Iron
4.1
2.6
0.63
Calcium
4.1
2.4
0.59
Magnesium 2.3
1.2
0.52
Sodium
2.3
1.2
0.52
Potassium
2.1
1.5
0.71
Carbon
0.05
2.5
50
Nitrogen
0.0025
0.20
80
Source: adapted from Sposito, The Chemistry of Soils, Oxford University Press, 1989.
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