Page 44 - Ilmu Tanah

The Chemistry and Fertility of Soils under Tropical Weeds 31

Some factors clearly govern the process of the primary mineral weathering in
the soil environment (Table 2.2) the most important of which are soil moisture
content and soil pH. All of the chemical weathering listed in Table 2.4 shows the
importance of water molecule. None of these reactions occur in the absence of
water. As stated previously, Eq. 2.14 shows that the weathering of orthoclase is
speeded by the presence of water molecule. Shown in Eq. 2.14 and Table 2.4, the
+
presence of H also speeds up the chemical weathering. Lowering the soil pH may
surely increase the rate of soil mineral weathering. The importance of soil pH in
mineral weathering is also previously shown in Table 1.3.
It is always true that the nutrient elements in soil water are not supplied only
by inorganic sources like soil primary and secondary minerals but also by organic
sources. Soil organic matter may reach the amount of 5% in mineral soils and
therefore it may serve significant amounts of nutrient elements in the soil
environment particularly for such nutrient elements as N, S, and P. The soil organic
+
matters may decompose and release these nutrient elements in the forms of NH 4 ,
- 2- - 2-
NO 3 , SO 4 , and orthophosphates (H 2 PO 4 and HPO 4 ), respectively (Table 2.3 and
Table 2.4 and Fig. 1.7, Fig. 1.8, and Fig. 1.9). These nutrient elements may
compensate their depletion caused by plant root absorption or other mechanisms.
The decomposition of organic matters in the soil environment is controlled by
several important factors. As shown by Eq. 1.4, the soil enzymatic activity and soil
moisture are the two most important factors. The rate of organic matter
decomposition is positively related to the activity of soil enzymes such as
phosphatases, proteases, and arylsulfatases and soil moisture content. This means
that the release of the organic matter structural elements can be speeded by
increasing the soil enzymatic activities and soil water content. The absence of
water may stop and the absence of soil enzymes may retard the soil organic matter
decomposition. Gianfreda (2015) states that all processes and functions that occur
in the rhizosphere are dominated by the activities of plant roots, rhizosphere
microorganisms and root-microorganism interactions, and enzymes are recognized
as main actors of all activities occurring in rhizosphere environments. In general,
rhizosphere enzymes show higher activities than those in bulk soil because the
rhizosphere soil is richer in organic C substrates. Enzymes produced and released
by roots alter the availability of nutrients in the rhizosphere through the hydrolysis
of C-substrates and organic forms of nutrients such as N, P and S.
In addition to soil enzymes and water, there are other factors controlling the
rate of organic matter decomposition i.e. soil pH, soil temperature, and soil C/N
ratios. Soil enzymes cause a direct effect while soil water, soil pH, soil

Abdul Kadir Salam and Nanik Sriyani – 2019