Geology Reference
In-Depth Information
5.5.6 Soil Processes
A soil receives inputs of ions from the weathering of parent material,
mineralization of organic matter, pollution, and specific additions, such
as fertilizers. The processes described above control the mobility, or
bioavailability, of these ions (Figure 15). Certain ions, such as sodium
and nitrate, are poorly held in soil, and so readily leached out. On the
other hand, phosphate and copper are strongly held. From the point of
view of soil as a source of plant nutrients, it is beneficial for a soil to hold
su
cient nutrients to meet plant requirements, and to hold the nutrients
in a form that can be released to supply the plant over the growing
season. Increasingly, we are using soils as a sink for pollutants, when the
desirable result is that the pollutant is tightly held in order to prevent its
dispersal into the wider environment.
QUESTIONS
(i) Explain the origin and characteristics of the charges on soil
components that make up the CEC of a soil.
(ii) Using the information in Figure 8, how much N, P, and S are
stored per hectare in the soil humus? How much N, P, and S are
released annually per hectare by mineralization of the humus?
It is commonly found that about twice as much P and S is
released from the soil than predicted, but that the amount of N
agrees well with the prediction. What process could account for
this?
A soil solution contains 3
10
6
mol PO
4
-P L
1
,5
10
4
mol SO
4
-S L
1
, and 2
10
3
mol NO
3
-N L
1
. If the annual
drainage is 250 mm of water, how much P, S, and N are lost
annually per hectare by leaching? (P
¼
31; S
¼
32; and N
¼
14).
(iii) A typical rate of addition of Pb in sewage sludge to agricultural
soils in the UK is 1200 g Pb ha
1
year
1
. If this amount is added
to a soil of bulk density 1.2 g cm
3
containing 5% humus with a
binding capacity of 80 cmol
c
kg
1
, what percentage of the
binding capacity to a depth of 20 cm is used annually to hold
the lead? (Assume that all the added Pb is bound to the humus.
Pb
¼
207.)
(iv) Using the values for solubility product below, and assuming
that the solubility of the hydroxides controls the metal ion
concentration in solution, calculate the theoretical concentra-
tions of Fe
21
,Fe
31
, and Cr
31
at pH 2, 5, and 7.
