Chemistry Reference
In-Depth Information
Table 4
Estimated nutrient deficits for N, P and K, and corresponding
amounts required to be mined (Ore), projected to 2050.
2011
2050
d
n
1
r
2
n
g
|
3
7 billion
9 billion
Population
million tonnes:
Element
Ore
Element
Ore
N
19.51
-
28.40
-
P (grade 20% P
2
O
5
)
7.82
89.51
10.47
119.95
K
31.72
38.22
47.42
57.15
to be mined of nutrients, unless there are major changes in fertiliser production
and supply. The scale of the problem is particularly severe for K, as production
needs to triple by 2050 to meet projected global demand. In contrast, assuming
a grade of 20% P
2
O
5
, world production of P needs to increase by about 70%,
less if higher grades are mined.
5 Mineral Dissolution Rates in the Soil System
The demand for P and K fertilisers is currently met by apatite and soluble K
salts, respectively. Although minerals such as feldspar can contain small
amounts of P
19
(up to 1% P
2
O
5
), there is practically no widely available
alternative to apatite. This means that the P deficit will be satisfied by the
search for new resources and reserves of phosphate rock, to continue current
mining practice. In contrast, there are many alternative sources of K, if we
consider potassium silicate minerals. The difficulty with these is that they
release their K slowly, by virtue of their slow dissolution rates.
Examples of K-bearing silicate minerals are given in Table 5, together with
their typical K
2
O content. Table 5 also shows dissolution rate data, for
dissolution reactions involving an acid reaction mechanism.
20
Inspection of
reaction rates shows that these vary by 9 orders of magnitude, which means
that different potassium silicate minerals will behave very differently if used as
sources of K.
Potassium feldspar occurs very widely in a range of igneous and sedimentary
rocks, as one of three polymorphs: sanidine (typically from volcanic rocks),
orthoclase (granites) and microcline (metamorphic rocks and syenites). It is
one of the commonest minerals on the continental crust.
21
Having a log
dissolution rate of 210.06 mol m
22
s
21
, it dissolves only very slowly.
Leucite and nepheline belong to the feldspathoid family; they occur typically
in very specific volcanic rocks and in syenites. These rock types are
uncommon, tending to occur in association with igneous rocks in rift valleys
and other locations within continental plates.
Comparing the dissolution rates of feldspar, leucite and nepheline shows
that there is a considerable difference, and this depends in detail on the
