Growth and Distribution of Roots of Dioscorea Alata L. Do Not Respond to Mineral Fertiliser Application

Water yam (Dioscorea alata) is a staple crop for many of the poorest in the tropics. Yam tuber response to fertiliser application has been reported to be erratic. We hypothesised that this variable response could be due to the development of a suboptimal root system that would not allow the plant to capture the nutrients added with the fertiliser. To verify this hypothesis we studied during two seasons (2006 and 2007) in Central Cote d'Ivoire, the growth of yam root system in relation to plant growth as affected by fertiliser input, using sequential root coring. In addition, we assessed the horizontal and vertical distribution of yam roots using the wall profile method in the same site. Three root types (seminal, adventitious and tubercular roots) were identified. Roots grew within the mound until 100 days after planting (DAP) and afterwards, extended radially in the soil outside of the mound remaining in the first soil horizon (15 cm depth). Maximum root extension was observed at 160 DAP. Root length density and root mass density were higher at higher soil temperature whereas the average root diameter decreased at higher soil temperature. Specific root length did not change with soil temperature. Root growth parameters were not affected by fertiliser application. Roots were mostly distributed in clumps. As tuber yield formation was independent from root growth we conclude that the root system of this cultivar did not limit tuber productivity. Other factors have to be considered to explain the variable response of yam to mineral fertiliser additions.