Plant Nutrients - Soils and Food Security - page 195

has been dismissed. However, almost all of these studies were carried out when
conventional K was a fifth of the price it reached in 2008.
The recent high price of K has led, in some countries, to a reduction in use of
conventional fertilisers, simply because of cost, and renewed interest in
alternative sources. 25 While the possible use of feldspars is of interest because
of their high K content and widespread occurrence, the use of micas has also
been investigated.
Experiments using zinnwaldite (8% K), a mica associated with tin mineralisa-
tion, and the feldspar orthoclase (10% K) have been carried out in the Czech
Republic to investigate the response of barley. 25 As with many previous studies,
the response to feldspar was not significant, but a statistically significant increase
in yield was observed for treatments with zinnwaldite. In other studies to
investigate olive tree growth, 26 a significant response was observed following
application of glauconite (9% K 2 O). In both of these studies, it is likely that the
cation exchange reaction enabled these micas to act as sources of K to the soil
solution.
Overall, the pathways by which minerals yield their nutrients to the soil
solution for uptake by plants are summarised in Figure 5. The potassium
silicate minerals, feldspars and feldspathoids, only release their K following
weathering, and this typically produces K-bearing clays or micas. Micas yield
K as a consequence of cation exchange reactions; they also undergo
weathering, which tends to 'open up' the layered structure, facilitating release
of K. 27 The potassium salts are directly soluble in the soil, with very rapid
dissolution rates, resulting in K becoming immediately available to plants. The
phosphate mineral apatite dissolves in soil systems, but 10 000 times more
slowly than a simple salt, such as KCl.

d n 1 r 2 n g | 3

Figure 5

Summary of processes controlling release of nutrients from potassium
silicate minerals, potash salts and the calcium phosphate mineral apatite to
the soil solution, and so to a form available for plant uptake.