Page 42 - Ilmu Tanah

The Chemistry and Fertility of Soils under Tropical Weeds 29

element concentrations but also by the changes in other soil properties like soil pH
and soil E. The decrease in soil pH is shown to stimulate not only the nutrient
release through cation exchange but also through dissolution of soil secondary
minerals.

-
2+
Fe(OH) 2  Fe + 2 OH ……. Eq. 2.10

-]2
2+
K = [Fe ] [OH ……. Eq. 2.11

The decrease in soil E may reduce the related nutrient element in soil
secondary minerals like Fe 2 (HPO 4 ) 3 and release the nutrient element cations into
the soil solution. The trivalent Fe cation ion the structure of Fe 2 (HPO 4 ) 3 is released
2+
2+
as Fe ion in soil water. Therefore, Fe is more readily available in flooded soils
3+
like paddy soils, while Fe in dry soils is not available because its association among
which with P.

2+
2-
Fe 2 (HPO 4 ) 3 + 2 e -  2 Fe + 3 HPO 4 ……. Eq. 2.12

On the other hand, S is not available in flooded soil because it presents as FeS
precipitates. The S may be oxidized upon the increase in soil E by draining the
2-
3+
flooded soils following Eq. 2.13. FeS may dissolve producing Fe and SO 4 .

+
2-
3+
-
FeS + 4 H 2 O  Fe + SO 4 + 8 H + 9 e ……. Eq. 2.13

In addition to soil secondary minerals (precipitates), there are also primary
minerals containing various kinds and amounts of nutrient elements. Some
primary minerals present in the soil environment and their chemical weathering
are listed in Table 2.4. The primary minerals may release their structural nutrient
elements through the process of weathering (Table 2.2 and Table 2.3). One
example of the weathering process is the release of structural K from K-feldspar
(Orthoclase, KAlSi 3 O 8 ) as shown in Eq. 2.14.

+
3+
+
KAlSi 3 O 8 + 4 H 2 O + 4 H  K + Al + 3 Si(OH) 4 ....... Eq. 2.14

+
Free K is released into the soil water increasing the soil water K concentrations.
This reaction is a non-equilibrium reaction and so it will progress and speed up in
+
the presence of water and H ions.
Abdul Kadir Salam and Nanik Sriyani – 2019