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The Chemistry and Fertility of Soils under Tropical Weeds 25

The concentrations of free ions are relatively low compared to that absorbed
by plants. The concentrations of free ions are even lower than the dissolved
2+
element in soil water. For example, the concentration of free Zn is 40% of the
2+
total dissolved Zn and that of Cu is even lower about 2% of the total dissolved Cu
+
+
2+
2+
(Elliz and Knezek, 1982). The free K , Na , Ca , and Mg are higher, about 70 –
90% of the respective total dissolved elements. However, the amounts supplied by
all dissolved nutrient elements are also relatively low and do not suffice the plant
requirement.

Organics Plants

Minerals

Adsorbed

Complexes Free Ions Chelates

Leaching
Precipitates
(Sec. Minerals)

Fig. 2.2. The relationships between the mechanisms controlling the concentrations of
nutrient elements in soil water (Adapted from Salam, 2017).

Therefore, the rest of the nutrient elements must be supplied by other sources
of readily available nutrients and slowly available nutrient elements that include
soil organic matters, soil secondary minerals (precipitates), and soil primary
minerals (Fig. 2.2). Most of the nutrient elements in the soil environment are
contained in the soil solids. These nutrient elements are not available or slowly
available to plants. These nutrient elements must be converted into readily
available forms before being available for plant absorption. Releasing these slowly
available nutrient elements enables the use of these elements by plants and
vegetation and allows them to get into nutrient element cycles. The mechanisms
Abdul Kadir Salam and Nanik Sriyani – 2019