Page 49 - Ilmu Tanah

36 The Chemistry and Fertility of Soils under Tropical Weeds

The worst impact of weeds in agriculture is directly related to the economy of
small and big farmers. Reported by Bridges (1999), about 80% of the worst impact
of weeds occurs in agricultural crop production and the rest 20% occurs in forestry,
pasture, recreation land, and real estates. The negative effect of weeds may be
higher than that had been predicted. The real effect is not easy to calculate due to
the difficulty in estimating the negative effects on sectors other than agriculture.
The negative effect is in general originated from two sources, i.e. the decrease
in farmer’s benefit and the increase in production cost. The decrease in the
quantity and quality of crop production may directly decrease the farmer’s income
due to the decrease in selling price. The increase in production cost is caused by
the more intensive weed controls that include cost for labors and expenses for
herbicide application and crop harversting.
The effect of weeds on the environment may be of two kinds. The first is the
negative effect caused by the methods of weed control applied, particularly the
manual control that may cause more intensive soil erosion as well as the chemical
control that may polute the environment, particularly soil and water, and may
depress non-target species, particularly the predators and parasites of pests and
pathogenic plants. The second is the disturbance on the composition of flora that
may destabilize the ecosystem. This is because most weeds are not indigenous
vegetation, which may displace indigenous vegetation due to their much faster
growth. This process may change the composition and balance of flora and fauna
of a particular region, which may eventually cause a chain effect that may be
noticed after years. For example, the indigenous species of Kalimantan forest
Melastoma beccarianum may be displaced by Acacia mangium purposively
introduced. Seeds of Acacia mangium may grow earlier and faster compared to
those of Melastoma beccarianum particularly in the presence of high light intensity
(Osunkoya et al., 2005).
The ability of weeds to displace indigenous vegetation is a consequence of
their fast and competitive growth characteristics, prolific seed production and
propagation, as well as by their allelopathic characteristics. Alang-alang (Imperata
cylindrica) (Fig. 3.1) may quickly dominate particular lands due to its allelopathic
characteristic. An in vitro observation showed that the allelopathy of alang-alang
may have depressed the height and the length of seedlings, and lowered the dry-
weight of corn (Sriyani et al., 1996). Ge et al. (2017) shows the allelopathic effect
of root extract of Alternanthera philoxeroides on the inhibition of the growth of
native plant species through toxic effects on soil enzyme activities and the
microbial community. He shows that the relative abundance of microorganisms

Abdul Kadir Salam and Nanik Sriyani – 2019