Soil Chemistry (continued)
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matter in general. Carbon, oxygen, and hydrogen are most
abundant; the content of nitrogen is often about one-tenth
that of carbon, whereas the content of phosphorus or sulfur is
usually less than one-fifth that of nitrogen (Table 1). The num-
ber of organic compounds into which these elements are incor-
porated in soil is very large, and the elucidation of the chemis-
try of soil organic matter remains a challenging problem.
* ppm, parts per million.
Table 1 - Average percentages of total carbon, total nitrogen, and organic phosphorus in selected soils
Soil % C % N % P
Sand 2.5 0.23 0.04
Fine sandy loam 3.3 0.23 0.06
Medium loam 2.3 0.22 0.05
Clay loam, well drained 4.6 0.36 0.10
Clay loam, poorly drained 8.0 0.43 0.05
Peat 46.1 1.32 0.03
Besides oxygen, the most abundant elements found in
mineral soils are silicon, aluminum, and iron. The distribution of
chemical elements will vary considerably from soil to soil and,
in general, will be different in a specific soil from the distribu-
tion of elements in the crustal rocks of the Earth. The most
important micro or trace elements in soil are boron, copper,
manganese, molybdenum, and zinc because these elements
are essential in the nutrition of green plants. Also important
are cobalt, which is essential in animal nutrition, and selenium,
cadmium, and lead, which may accumulate to toxic levels in
soil. The average natural distribution of trace elements in soil
is not greatly different from that in crustal rocks (Table 2). This
similarity indicates that the total content of a trace element in
soil usually reflects the content of that element in the soil par-
ent material and, generally, that the trace element content of
soil often is not affected directly by pedochemical processes.
Table 2 - Average amounts of trace elements commonly found in soils and crustal rocks
Trace element Soil, ppm* Crustal rocks, ppm*
As 6 1.8
B 10 10
Cd 0.06 0.2
Co 8 25
Cr 100 100
Cu 20 55
Mo 2 1.5
Ni 40 75
Pb 10 13
Se 0.2 0.05
V 100 135
Zn 50 70
