A growth analysis study involving monthly excavation of Dioscorea esculenta plants revealed that the root system developed fully in the period before tuber initiation, and extended radially for a distance between 2.3 and 4.3 m. Primary roots initially remained in the top 10 cm of the soil profile, but descended to approximately 30 cm near their full extension. Tuber number increased from initiation around 21 weeks after planting (WAP) until maximum vine growth was reached around 33 WAP, but tuber dry weight continued to increase throughout the senescence period of the vine, to 45 WAP. Tubers lost dry matter but not fresh weight during dormancy in the soil, to 55 WAP. The results indicate that a distance of at least 4.5 m is required to separate experimental fertilizer treatments, and that post-establishment burial of fertilizers around the mound or ridge risks damage to roots near their base, while inter-row application is accessible to roots and may be preferable. I N T RO D U C T I O NThe yield of yams (Dioscorea spp.) is influenced by a range of agronomic, physiological and environmental factors, whose interactions are not well understood. High variability within yam plantings has hampered progress to characterize yield determinants in the crop and is largely a product of the mode of vegetative propagation (Okoli, 1980;Onwueme and Haverkort, 1991). Despite a long growing season (7-12 months depending on species and climate) the size of the initial planting piece (tuber sett) is highly influential on final yield (Enyi, 1972a;Ferguson, 1977;Nwoke et al., 1977). In addition to the sett size, large variability among plants results from differences in sprouting time and vigour, both influenced by the sett's dormancy status and which part of the mother tuber is used (whole tubers or top, middle or tail sections) (Okoli, 1980). Virus and nematode loads undoubtedly add to variability in most plantings. In addition, staking, mounding and mulching practices are known to influence yield, but the extent of influence has varied among reported studies and depends on genotype and environmental conditions (Enyi, 1972b;Maduakor et al.,
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