Allelopathy in ecological sustainable agriculture

Abstract

The indiscriminate use of new agricultural technology [agrochemicals (fertilizers, pesticides etc.) and multiple cropping in irrigated areas], for the success of modern agriculture has (a) made our soils sick, (b) caused environmental pollution, (c) development of resistance in pest (weeds insects, pathogens etc) and (d) toxic residues in our food. The new technology in a short span of 35-40 years has caused these major problems, which have severely deteriorated our soil health, human and livestock health, environment and quality of life. This indicates the new technology is not sustainable over long periods. Modern agriculture is exploitive of growth resources and has caused various problems such as environmental pollution through (a) contamination of underground drinking water resources with pesticides and nitrates, (b) contamination of food and fodder with residues of pesticides, nitrates and antibiotics, (c) both 'a' and 'b' cause harm to farm workers (Pretty 1995), (d) poor soil health and soil productivity and (e) poor quality of rural life. There is growing evidence that grain yields of cereals in cereal based rotations in modern agriculture cannot be sustained at the current levels e.g. rice-wheat rotation. In the last 16 years, there is a decline of 25-30% in the grain yields of both rice and wheat in Philippines, Indonesia, India and Pakistan despite the use of recommended cultural practices (Pretty 1995). Therefore, the recent emphasis in agriculture has shifted from a primary goal of maximizing yields over the short term, to a sustainable productivity over long periods of time. The knowledge of ecological interactions occurring within an agroecosystem and the sustainable functioning of the system as a whole has become the overall approach. Sustainability can be achieved in an agriculture that is ecologically sound , resource conserving and not environmentally degrading. Therefore, several definitions of sustainable/ecologica1/ecofriendly/organic agriculture are available, but for this chapter, the ecological sustainable agriculture means that "farmer grow the crops with the resources available on the farm, reduces dependence on off-farm inputs and maintains soil productivity and clean environment over a long period of time". Hence, farmer conserves the resource base to minimize artificial inputs from outside the farm and manage pests (weeds, insects, nematodes, pathogens) through internal regulating mechanisms based on ecological principles and processes (Stinner and House, 1987). Thus, ecological sustainable agriculture strives for the integrated use of a wide range of pests, nutrients and soil management technologies (Pretty 1995). In the last few years, some good books on similar aspects have been released (Franco and Ponte, 1988; Gliessman, 1990a; Narwal, 1998; National Research Council, 1989 and Pretty 1995). All plant spp. and their residues produce secondary metabolites called allelochemicals. Allelopathy means any process involving secondary metabolites produced by plants, microorganisms, viruses, fungi that influence the growth and development of agricultural and biological systems (International Allelopathy Society, 1996). Although allelochemicals are produced by all plant parts, but the leaves and roots are mainly responsible for their production and release. The ecological agriculture maintains diversity of plant spp. on the farm through various types of multiple cropping systems viz., mixed cropping, crop rotations etc., hence, allelopathy assumes great significance. Allelopathy may be used to increase crop production through avoidance of negative impacts, exploitation of stimulatory effects, management and development of allelopathic crops and varieties to suppress pests (weeds, insects, nematodes, pathogens) and use of allelochemicals as pesticides and growth regulators (Einhellig, 1985). The productivity of monoculture declines after few years mainly due to build up of pests and soil sickness etc. These problems could be overcome through the adoption of crop rotations and intercropping systems, which exert detrimental effects on the pest through various chemical interactions (Allelopathy) and cause physical hindrances to restrict movement of pests. Likewise, the bio-diversity provided by crop mixtures has a smothering effect on pest and diseases. These indirect synergistic effects also contribute to higher productivity in crop rotations and intercropping systems. If these effects are utilized properly it is possible to reduce the use of chemical pesticides and herbicides. This chapter reviews allelopathic effects in crop production and possible use of multiple cropping systems, plant residues, plants and varieties rich in allelochemicals to control pests and supplement nitrogen requirement through Biological Nitrogen Fixation by legumes in ecological sustainable agriculture.