Growth and Nutrient Use Efficiencies of Yams (Dioscorea spp.) Grown in Two Contrasting Soils of West Africa

Fertilization is an important management strategy of yams (Dioscorea spp.) especially when grown in degraded soils. A field study evaluated the leaf numbers, leaf area indices, crop growth, yields, and nitrogen (N) and potassium (K) use efficiencies of D. alata and D. rotundata in C？te d'Ivoire when grown in two contrasting soils with and without fertilizer. D. alata had a lower number of leaves per vine, although leaf area indices were higher, and the leaves were retained for a longer period than in D. rotundata. In all situations, the yields of D. alata were significantly higher, and fertilizers promoted growth of shoots, roots, tubers, and, thus, final yields especially in the low fertile savanna soil. The beneficial impact of fertilizer on yields was significantly lower in the fertile forest soils. The nutrient use agronomic efficiencies indicated the impact of both N and K in promoting yields especially under nonfertilized conditions. 1. Introduction Yams (Dioscorea) are a vital component of the agricultural sector of West Africa, both in terms of food, social, and cultural values [1]. In these regions, D. alata and D. rotundata are cultivated in a range of soils, from degraded smallholdings through low fertile savannas to newly cleared forest sites, which have a high degree of fertility. Most farmers of the region cultivate yams in smallholdings under low-fertility conditions, except in some instances where short fallows are incorporated into the cropping systems to rejuvenate the soil fertility [2]. Thus, despite increases in land areas being cultivated in West Africa, yam production has remained static [3], indicating a gradual decline in yields per unit land area. Crop management plays an important role in procuring high yields in tropical tuber crops [4]. A major constraint for enhancing yam productivity is low soil fertility, both in terms of macro- and micronutrient deficiency [5]. This is because yams are high-nutrient-demanding species [6], and when planted in low-fertility soils under subsistence conditions as done in smallholder systems of West Africa, yields are low, varying between 9 and 10 t？ha？1 compared with a potential yield of 51 t？ha？1 for D. alata and 27 for t？ha？1 D. rotundata [7]. Hence, research on possible methods to enhance yam productivity is needed to produce this culturally important and economically viable species especially under conditions of West Africa. Research on the impact of fertilizers on Dioscorea yams is scarce. Some studies show the lack of yield response of Dioscorea to applied phosphorus [8], and to both