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The Chemistry and Fertility of Soils under Tropical Weeds 113

researchers report that the release of K and other nutrient elements from soil
mineral increases with the decrease in soil pH (Salam, 1989). This suggests that any
chemical reactions that may decrease the soil pH may in turn drive the detachment
of K from soil minerals. The chemical reactions may be naturally driven like root
excretion and antropogenically origin like soil contamination by acid industrial
waste.
Root excretion may also enhance the organic matter decomposition through
the soil enzymatic reactions. Various soil enzymes produced by roots and by
microorganisms and macroorganisms associated with root existence, like
phosphatases, arylsulfatases, and proteases, may speed up the decays of organic
matters containing the functional K. The amounts of soil enzymes produced by
these living things may determine the amount of K released into the soil
environment. Any factors that may support these living things may enhance the
increase in soil enzymatic activities and the release of K into the soil environment.
+
The enhancement of K released by the root-related production of H and
enzymes is expressed in the increase in dissolved K and exchangeable K. Salam et
al. (2019) report that the enhancement of exchangeable K by weed presence is
related to the characteristics of weeds, among which is the Root-To-Shoot Ratio
(RST). Weeds with high RST like Arachis pintoi may drive more intensive release of
K from K-sources in the soil environment (Ontia, 2018; Salam et al., 2019).

7.1 The Release of Potassium from Soil Minerals

Potassium in soils is contained in mineral and organic matters, which are not
easily available to plant root absorption. Some of K reserves in soil are listed in
Table 7.1, which mainly exist as silicate minerals, like feldspars, and organic
+
matters. Potassium is absorbed by plant roots as K ions and therefore the
+
structural K in the silicate minerals must be released as K to be available to plants.
+
The mechanism by which K ions are released into the soil solution from mineral K
is called dissolution or chemical weathering. Similarly, organic K must be released
+
as K into the soil solution through a chemical mechanism called decomposition.
Therefore, the soil K in the soil environment is not only supplied by mineral sources
but also by organic sources. The root excretion may enhance the soil available K
from soil minerals particularly in young soils with enough K-minerals and from soil
organic matters containing K. Inputs of organic matters into soils from root
Abdul Kadir Salam and Nanik Sriyani – 2019