Accelerated erosion as a result of human and animal activities, including tillage is a major environmental and economic problem throughout the world. The severity of soil erosion and changes on soil as a whole is a result of a combination of factors which include tillage and management practices (Moore et al, 1986). Conventional agricultural practices require extensive tillage usually carried out by incorporating residue into the soil using a moldboard or animal drawn plow. The soil surface will then be further tilled using a harrow to provide a seedbed devoid of clod. Once row crops were planted, a cultivator is used, often several times, to keep the weeds down. Thus, in the process the soil was tilled repeatedly at great cost in terms of time and energy. More importantly, this conventional tillage operation usually left the soil bare immediately after plowing until later in the period when crop growth was sufficient to provide ground cover. This means that conventional tillage left the soil unprotected during the early part of the cropping period and can severely be affected by erosion and runoff pressure especially in sloping lands. In addition, Steiner (2002) cited that soil organic matter decomposes more rapidly in the tropics compared to subtropical and moderate climates because of the higher temperature. In case where machineries are use, conventional tillage increases soil inversion and thereby increased soil aeration which accelerates organic matter breakdown. Further, the fast breakdown of soil organic matter releases more CO2 in the atmosphere that contributes to global warming.
Until recently, conventional tillage is the common practice in most agricultural land. However, in the past two decades or so, several developments in the field of agriculture have dictated drastic changes in tillage practices. First, the availability of herbicides capable of controlling most of the major weeds has become available at reasonable cost. This development reduced the need for cultivating and even plowing in some cases. Second, dramatic increases in fuel costs forced tractor-dependent farmers to seek means of reducing their tillage operation costs. Third, the increasing environmental awareness has forced a re-evaluation of soil erosion as source of off-site water pollution. These major developments have triggered scientist and farmers the opportunity to examine the effects of reduced-tillage methods, most of which allow less erosion than the conventional tillage systems.
Conservation tillage system varies with specific field operations involved. Zimincheck et al. (2001) described the practices could range from stubble mulch tillage system (disk undercut weeds and crop residues to loosen soil and kill weeds); reduced tillage which could be of several combinations (moldboard plow + minimum secondary tillage; moldboard plow with no other tillage; tandem disk with most residues left in the surface) and the no-tillage system (no primary tillage only rotary tillage or similar implement to the soil for seed planting). These conservation tillage all involve less tillage system in comparison to the conventional tillage which involve plowing with 2-3 times harrowing and one time furrowing then crop planting. Sometimes subsequent tillage system follows with series of cultivation (hill-up operation and weeding). The no-tillage system permits direct planting in the residues of the previous crop and utilizes only localized tillage necessary to plant the seeds. Such system minimizes incidence of soil erosion. The no-tillage gives better soil protection than conventional tillage as confirmed by field research (Basic et al., 2001). This happens as the conventional tillage system leave about 1-5% of the soil surface covered with crop residues. Reduced tillage system commonly leaves 15-25% soil coverage, while the no-tillage system, about 50-75% of the land is covered with residues. These differences in residues land cover have marked effects on both soil erosion and runoff (Hussain et al., 1998).
Numerous studies have showed that the conventional up and down slope ploughing is the least favorable method (Laflen and Moldenhauer, 1979; Basic et al., 1991; Edwards et al., 1993; Schultz and Malinda, 1994; Meyers and Wagger, 1996; Rejman, 1997). It leads to higher erosion whereas ploughing across the slope and the no-tillage is more effective in terms of erosion control. The no-tillage system that maintains the soil cover with crop residues results in less erosion than the conventional tillage system. Likewise, surface runoff is generally decreased (Rondal and Kon, 2000), although the differences are not pronounced as with soil erosion. Baker and Laflen (1983) have shown that even in soils with high erosion potentials, soil loss is far below the tolerable limit under the no-tillage system. The high percentage of soil cover (42-76%) of the no-till system is responsible for the low soil loss. Consequently, the no-till system also provides low nutrient losses (Warren et al., 1997). The low nutrient losses specifically nitrogen happens because the finer fraction of the soil are among the first one to be carried out through erosion, which mostly contain nitrogen and in no-till system, less fine fraction are carried out through erosion.
On crop yield, conservation tillage generally provides yields equal or even greater than those from the conventional tillage provided that the soil is not poorly drained and can be kept free of weeds through the use of chemicals (Philips et al., 1980). Conservation tillage steadily improves soil fertility and water use efficiency. This together with timely planting leads to increasing yield (Steiner, 2002). Reasons for the low yield in poorly drained soil include lower soil temperature and incidence of certain plant diseases, which may be higher somewhat in higher moisture condition. Also certain weeds tend to be more problem on wet soils, thus exerting limitation on the practice of conservation tillage.
Conservation tillage has variable effects on soil properties depending on the particular system chosen. However, evidence suggests that the no-till system have some effect. For example, the no-till system leaves the upper 10-cm of soil lower in total porosity than conventionally tilled system. Additionally, in the no-till system, moisture is higher in the upper soil layer due to the reduced evaporation brought about by the residues left on the surface. This effect is sometimes coupled with greater leaching losses of nitrate especially in poorly drained areas (Doran, 1982). As to energy and labor cost requirements, previous documentation have shown that the primary reason for a farmer to adopt the conservation tillage practices is due to their low labor and energy requirements which would vary depending on the conservation tillage chosen. Ploughing or hoeing is replaced by ripping, pot holing or is completely abandoned in the case of no-tillage. Study suggests labor requirements are cut in half by the farmers switching to conservation tillage.
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