Page 26 - Ilmu Tanah

The Chemistry and Fertility of Soils under Tropical Weeds 13

The decomposition of organic matters may release not only the inorganic N, P,
and S, but also all other nutrient elements structurally bonded in organic matters.
Therefore, the decomposition or organic matter may also increase the soluble and
adsorbed species of all nutrient elements in soils. The nutrient elements released
by the decomposition of organic matters therefore may complement the nutrient
elements released by the weathering processes. These elements may affect and
participate in some or all soil chemical properties and processes (Table 1.4)

Table 1.4. The main chemical properties and reaction of soils.

No. Properties Reactions
1 Soil Reaction (pH) Precipitation/Dissolution
2 Soil Redox Potential (E) Chemical Weathering
3 Cation Exchange Capacity (CEC) Adsorption/Desorption
4 Organic C or Organic Matter Ion Exchange
Contents
5 Total N Chelation
6 Exchangeable Cations: Oxidation/Reduction (Redox)
+
2+
2+
Base Cations (Ca , Mg , K ,
+
Na )
3+ +
Acid Cations (Al , H )
7 P and S Decomposition
8 Heavy Metal Cations:
2+ 2+ 2+
Nutrients ((Fe , Mn , Cu ,
2+
Zn )
2+
2+
2+
Non-Nutrients (Pb , Cd , Hg )
9 Enzymatic Activities
(Phosphatase, Urease, Protease,
Arylsulfatases)

1.2 Weed Effects on the Chemistry and Fertility of Soils

+
Like plants and vegetation in general, weeds produce H ions and organic acids
released into the soil solution. As mentioned previously, these substances may
acidify the ambient soil solution. Plants and vegetation also produce CO 2 that also
acidify the soil solution. However, weeds are significantly different because they
Abdul Kadir Salam and Nanik Sriyani – 2019