Page 15 - Ilmu Tanah

2 The Chemistry and Fertility of Soils under Tropical Weeds

al., 2019), soil heavy metals (Fe, Mn, Zn, Cu, and Pb) (Salam, 1999), and soil
enzymatic activities (Salam, 1996; Salam et al., 1997a; 1997b; 1999b).
Some of these soil chemical properties are directly and some are indirectly
+
affected by weeds. Soil pH is directly affected by the root excretion of H and
organic acids (Song and Huang, 1988; Walker et al., 2003; Calvaruso et al., 2010).
The degree of excretions is dependent on weed species. For example, soil planted
with Arachis pintoi is reported to show lower ambient soil pH than is that planted
with Pennisetum purpureum (Lestari, 2018; Ontia, 2018; Salam et al., 2019).
Soil exchangeable K is indirectly affected by the presence of weeds, particularly
related to the decrease in the ambient soil pH that may ease the detachment of
+
some weakly held structural K attacked by H ions (Salam et al., 2019). The
excretion of soil enzymes by weed roots (Duxbury and Tate III, 1981; Bolton et al.,
1985; Reddy et al., 1987; Rejsek, 1991; Sakai and Tadano, 1993; Joner et al., 1995;
Salam, 2014; 2017) may also ease the detachment of part of the organically bound
K into the soil solution.

1.1 The General Chemistry of Soils

Soil is a complex and dynamic chemical system comprising a matrix containing
solid (mineral and organic matters), liquid (solution), and gas (air) component with
ions (cation and anions) and molecules dissolved in soil liquid dynamically
interacting with those attached to or structurally bound in soil solid and those
contained in soil air. Parts of ions and molecules in the soil system may chemically
converts into any of the soil components in a system. The soil chemical system is
basically affected and governed by two major variables i.e. soil pH (soil reaction)
and soil E (redox potential).
The effect of pH is apparent in all soils both in dryland and in wetland. Like
those in dry land soils which are always oxidative, the role of soil E in wetland soils,
which are always reductive, is not significant. The roles of soil E is, however, very
obvious in soils subjected to flooding and draining like in paddy soils. Soil is
oxidative with high redox potential upon draining and is reductive with low redox
potential upon flooding. Draining may cause, for example, the existence of more
2- 2-
SO 4 and less S and FeS due to oxidation reaction. Conversely, flooding may
2- 2-
cause more S , FeS, and low SO 4 . The effects of draining and flooding on the
3+
chemistry of paddy soils are shown in Table 1.1. Draining may cause higher Fe at
2+
low pH and Fe(OH) 3 at high pH, and conversely flooding may cause higher Fe at
Abdul Kadir Salam and Nanik Sriyani – 2019