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The Chemistry and Fertility of Soils under Tropical Weeds 23

3+
2+
precipitates of Fe may dissolve in soils when the concentration of Fe or Fe is
low. The dissolution may occur more readily at low pH.
The third mechanisms may in general occur relatively slowly, comprising two
major mechanisms i.e. mineral matter weathering and organic matter
decomposition (Fig. 2.1). Mineral matters and organic matters are two sources of
nutrient elements in the soil environment; mineral matters come from volcanic
materials containing mostly Si, Al, and O while organic matters come from the
living things containing mostly C, H, and O. These materials may release soluble
nutrient elements upon weathering or decomposition processes. The weathering
of mineral matters is depicted previously in Eq. 1.2 and the decomposition of
organic matters in Eq. 1.4.
Like all other vegetation, weeds also excrete some substances into the soil
environment which directly or indirectly related to the soluble nutrient elements in
+
soil water and the adsorbed elements in soil solids. The first is H , which may of
course acidify the soil ambience. The second is the organic acids, which may also
acidify the soil ambience (Robert and Berthelin, 1986). Among the organic acids
produced in the rhizosphere are of amino acids (aspartic and glutamic), of phenolic
acids, of aliphatic acids (oxalic, malic, citric, and tartaric) (Robert and Berthelin,
1986). Soil microorganisms living in the rhizosphere also produce some organic
acids including oxalic, citric, formic, 2-ketoglutamic, lactic, malic, and tartaric acids,
and lichenic acids (Robert and Berthelin, 1986). The third is the evolved CO 2 by
weed roots and microorganisms which may react with water molecule producing
+
H ions which may also acidify the soil ambience. The fourth is the soil enzymes
which may intensify the soil organic matter decomposition. All these substances
may drive the changes in the soil chemical properties.

2.2 The Release of Nutrient Elements from Soil Minerals

and Organic Matters

Nutrient elements in the soil environment are in general of two categories;
readily available and slowly available. Readily available nutrient elements are
dissolved in soil water (free ions), relatively fast balanced by dissolved complexes
or chelates and by those adsorbed onto the soil mineral and organic adsorption
sites (exchangeable elements); while slowly available nutrient elements are those

Abdul Kadir Salam and Nanik Sriyani – 2019