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The Chemistry and Fertility of Soils under Tropical Weeds 21

previously, these changes may affect the soil pH, which is very closely related to
the soil mineral matter weathering through dissolution processes. The dissolution
processes are more vigorous at low pH (Salam, 1989). The changes in soil pH not
only affect the soil mineral weathering, but as pointed out previously, also affect
several other soil chemical properties such as soil CEC and BS (Salam, 2017; Salam,
2019).
Like soil mineral matters, soil organic matters are most likely affected by the
presence of weeds since weeds may also produce soil enzymes like phosphatases,
aryl-sulfatases, proteases, etc. In the presence of soil enzymes, the decomposition
of soil organic matter is speeded. The mineral matter weathering and the organic
matter decomposition may produce nutrient elements and substances that may
change the chemical properties of the soil system. These two processes that
release nutrient elements and other substances into the soil water may complete
the relationship between soil – nutrient element – water – weeds as a system.

2.1 The Soil – Water – Nutrient – Weed Relationships

The relationship between soil, water (soil water), nutrient elements, and
weeds is depicted in Fig. 2.1. Weed roots are directly connected to the soil water
where they absorb most of the nutrient elements needed for their growth and
development. As they are competitive and progressive, weeds may significantly
absorb the soluble nutrient elements from soil water. Of course, absorption by
weeds is not the only mechanism for nutrient element losses in the soil
environment. The soluble nutrient elements in soil water are also absorbed by
plants and parts of it may be adsorbed by the soil adsorption sites, precipitated, or
leached. However, the presence of weeds may lower the nutrient element losses
through all these mechanisms and increase that lost by weeds. Therefore, this
stimulates farmers to eradicate weeds employing several mechanisms.
The above explanation shows that weeds may cause a significant effect on the
nutrient element balance in the soil environment. The imbalance of nutrient
element in soil water is chemically buffered by some mechanisms mentioned
previously. The first mechanism is the detachment of adsorbed nutrient elements
as described in Eq. 2.1 called desorption.

X – NE  X – + NE ……. Eq. 2.1

Abdul Kadir Salam and Nanik Sriyani – 2019