How does soil fertility affect yam growth?

Yams (Dioscorea spp.) are known to be very demanding in terms of soil fertility. The influence of deleterious soil fertility on the growth of yams, however, has not yet been studied. A field study was carried out in Côte d'Ivoire, in a fertile forest site and in a low fertility savanna site, submitted to identical climatic conditions, to determine how soil fertility affects Leaf Area Index, Leaf Area Ratio, Crop Growth Rate, Tuber Bulking Rate, Radiation Interception, Radiation Use Efficiency and final tuber yields in D. alata and D. rotundata, grown in the presence of adequate fertilizers. While germination and establishment of both species were not affected by the site, growth parameters of D. alata were superior to those of D. rotundata. This resulted in higher yields of D. alata, when compared to D. rotundata. Only the radiation use efficiency was higher in D. rotundata than in D. alata. Both species reached higher Crop growth rate and Tuber bulking rate and produced higher fresh tuber yields at the forest site than at the savanna site, reflecting the known demand of yams for high fertility soil. Correlation analyses showed significant relationships between the mean Leaf Area Index measured over the entire growth period, Tuber Growth Rate calculated between tuber initiation and harvest, and fresh tuber yields, suggesting that the development of leaf area is determining the growth rate of the tuber which itself determines the fresh tuber yield. The decrease in D. alata tuber yields at the savanna site was explained by a decrease in Leaf Area Index, while the decrease in D. rotundata tuber yields at this site was explained by a decrease in Radiation Use Efficiency. Both effects might have been the consequence of a suboptimal nitrogen and potassium nutrition at the savanna site.