The Important Facts about Tropical Weeds
3.1 The Concept of Weeds 3.2 Weeds as Pioneer Vegetation
Abdul Kadir Salam and Nanik Sriyani – 2019
The Chemistry and Fertility of Soils under Tropical Weeds 35
Chapter 3
The Important Facts about Tropical
Abdul Kadir Salam and Nanik Sriyani – 2019
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
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The Chemistry and Fertility of Soils under Tropical Weeds 37
decreased including Burkholderia, Gp6, Gp1 and Gp4 in the applied treatments at genus level as compared to control treatment.
As pointed out previously, weeds also excrete several substances among which are H+ and organic acids. These substances may lower the soil pH and cause the changes in the adjacent soil chemical and fertility properties. The decrease in soil pH may cause the increase in soil metal elements released from the speeded soil mineral weathering. The presence of organic acids may also cause parts of the metal elements to dissolve in soil water. The increase in soil organic matters by the presence of weeds may increase the soil CEC, which also indirectly influence the dissolved metal elemens.
Even though the impact of weeds related to their controls and their root excretion like alellophatic substances and organic acids is significant, the negative effects of weeds on animal and human health are not much in the current literature. Among those reported are the allergic reaction caused by Ambrosia artemisiifolia and Rhuss spp. and the poisoning effect of Solanum nigrum and Rhuss spp. on animals. Data of weed effect on human and animal heath in Indonesia is limited, less than those in some developed nations. It was not less than 280.000 sheeps and 264.000 goats were predicted dead every year in the United States of America caused by poisonous weed consumption. Some weeds like Cynodon dactylon (Grintingan), Eleusine indica (Lulangan), dan Sorgum halapense (Glagah), as well as Mimosa invisa (Kucingan) and Cyperus rotundus (Teki) may cause allergy and wounds on animals and Ipomoea triloba (Mantangan) may cause diare on goats (Sembodo, 2010). Some of these weeds are depicted in Fig. 3.1 and Fig. 3.2.
A high amount of labors, time, and fund are also needed yearly to control weeds, not because of the weed effects on crop production, but of the value of natural beauty of lands. For example, the weed control in recreation lands, golf courses, dams for recreation, etc. The presence of weeds may decrease the values of the natural beauty of these lands and lower their selling prices. The presence of weeds in golf courses may enhance the cost of maintenance significantly. Other impact difficult to predict is the destruction of esthetic value caused by the presence of weeds around roadsides, buildings, offices, campuses, sport fields, rail- road sides, irrigation ditches, etc. In addition to decrease the natural beauty, the presence of weeds may also enhance the maintainance cost.
Abdul Kadir Salam and Nanik Sriyani – 2019