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INTRODUCTION
The majority of Alfisols available for crop production in the tropics are strongly weathered and of inherently low organic matter and nutrient status (Lal, 1987). In addition, Alfisols have a weak structure and are highly susceptible to crusting, compaction and accelerated erosion (Lal, 1987) leading to low crop yields. Cocoyam like any other root and tuber crops is a heavy feeder, exploiting a large volume of soil for nutrient and water (Osundare, 2004). Tillage is an important cultural practice that can be used to increase the yield of cocoyam. In the humid tropics where most farmers are poor and fertilizer is expensive, soil working and tillage methods can temporarily serve as an alternative to fertilizer application (Adekiya and Ojeniyi, 2002).
Traditionally, cocoyam is grown on heaps, ridges and occasionally on flat manually cleared soils. Until now, a wide range of mechanized tillage methods, e.g. ploughing, harrowing, ridging and discing have been used for crop production in the humid tropics, without the benefit of experimental data on soil properties and crop response. Hence, there is a need to examine the potential of growing cocoyam using the traditional tillage method, mechanized ploughing plus harrowing and ploughing plus harrowing twice and their effects on nutrients uptake and cocoyam performance. There is a lack of information on tillage requirements of cocoyam on tropical Alfisols because previous tillage studies in the tropics on cocoyam have concentrated on acidic Ultisols (Anikwe et al., 2007; Hulugalle et al., 1985).
Tillage methods for crops depend on soil type and depth, micro-climate and topography (Agbede, 2006; Howeler et al., 1993). Tillage studies have mainly compared the effect of conventional tillage practices on cocoyam yields. The few studies undertaken largely neglected minimum or traditional and conventional tillage practices and their effects on soil properties and cocoyam yield (Agbede, 2008). The few studies carried out in Nigeria and other tropical countries produced inconclusive and controversial results under the different tillage practices compared (Hulugalle et al., 1985). For instance, Villanueva (1986) investigated the effect of tillage intensity on production of upland taro (cocoyam) and found no significant differences between ploughing and harrowing once or twice. In Hawaii, taro (cocoyam) is usually grown on puddled flooded soils...