Page 129 - Ilmu Tanah Book

116 The Chemistry and Fertility of Soils under Tropical Weeds

The structural K in the soil minerals is more difficult to release. This element is
released through mineral weathering which is controlled by several factors among
which are soil pH and soil moisture. As those for all primary minerals, the
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weathering of K containing minerals is enhanced by the presence of water and H
ion. Therefore, the release of K from soil minerals can be enhanced by lowering
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soil pH in the presence of water. The elimination of the released K from soil
solution through some mechanisms like plant root absorption and leaching may
also speed up the mineral weathering.
The weathering of orthoclase (KAlSi 3 O 8 ) may exemplify the above mechanisms.
The weathering of this K-mineral as shown in Fig 1 may progress in the presence of
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water molecule and H ion. Following the mass action law, the reaction may speed
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up in the presence of high amounts of water molecule (moist soils) and H ion (acid
+ 3+
soils) and low concentration of K (and also those of Al and Si(OH) 4 ). This means
that the weathering process can be enhanced by moistening the soil sample and/or
+ 3+
lowering the soil pH and/or eliminating K (and also Al and Si(OH) 4 ). However,
intensive weathering of orthoclase may end up with the existence of more stable
secondary minerals not containing K, like montmorillonite, kaolinite, and gibbsite.

3+
+
+
KAlSi 3 O 8 + 4 H 2 O + 4 H K + Al + 3 Si(OH) 4
Primary Mineral:
Orthoclase/K-Feldspar

Low [Si(OH) 4] High [Si(OH) 4]:
3+
3+
Al +3 H 2 O  2 Al + 2 Si(OH) 4 + H 2O 
3H +Al(OH) 3 6 H + Al 2Si 2O 5(OH) 4
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Secondary Mineral: Gibbsite Secondary Mineral: Kaolinite

Gibbsite
Al 2 Si 2 O 5 (OH) 4 + 5 H 2 O 
3 Si(OH) 4 + 2 Al(OH) 3
Secondary Mineral: More Intensive Weathering:
Kaolinite  Gibbsite Si(OH) 4 leached

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Fig. 7.1. The release of K in the weathering of K-containing primary minerals.
Abdul Kadir Salam and Nanik Sriyani – 2019