Page 132 - Ilmu Tanah

The Chemistry and Fertility of Soils under Tropical Weeds 119

7.2 The Release of Potassium from Organic Matters

In addition to those adsorbed in the surfaces of soil solids (organic and non-
organic) and those contained in the primary minerals, the soil K is also contained in
soil organic matters (Table 7.1 and Table 7.2). This soil K is originated from the soil
solution absorbed by plant roots and wandering in the plant tissues. This means
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that this plant nutrient is K originally released into the soil solution by desorption
and soil weathering. The decomposition of organic matters containing K may also
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contribute some amount of this K . However, the amount of K from organic
matters is generally lower than that from soil minerals.
Different from that from the soil minerals, the K from soil organic matters is
released through several enzymatic reactions catalyzed by various enzymes like
phosphatases, proteases, and arylsulfatase which work on the decomposition of
organic P, organic N, and organic S, respectively. In addition to releasing the
2-
2-
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related nutrient element i.e. H 2 PO 4 and HPO 4 , NO 3 , and SO 4 , these processes of
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course release several other plant nutrients including K . The higher the soil
enzymatic activities and the higher the available organic matters may contribute
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higher K into the soil solution. The soil enzymes are not only produced by plant
roots but also by microorganisms and macroorganisms (earthworms), the
populations and activities of which are stimulated by the presence of roots.
The rate of K release from organic matters is dependent on the soil enzymatic
processes involved. As shown in Table 7.3, some important factors control the
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rates of soil enzymatic activities releasing K . First, the amount of substrates is the
most important factor since soil enzymes work very specifically on particular
substrates. Phosphatases work only on the decomposition of organic P,
arylsulfatase only on organic S, and proteases only on organic N. The amount of
substrates may control the rates of organic matter decomposition. Second, the
activities of soil enzymes may determine the rates of organic matter
decomposition. Effective roots and organisms may produce significant amount of
enzymes that may cause the increase in organic matter decomposition. Third, as
shown by the reactions in Table 7.4, all enzymatic reactions need water to
progress. This means that the rates of organic matter decomposition and the
release of K may increase in moist soils.

Abdul Kadir Salam and Nanik Sriyani – 2019