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The nutrient composition of leaves and petioles of three yam species were studied at fortnightly intervals, and in the tubers when harvested at maturity. Highest concentrations of plant nutrients were recorded in D. alata, while D. cqyenensis had the lowest. The lamina was more appropriate for determination of N and P contents, while the petiole was more satisfactory for K. Peak nutrient concentration occurred between late June and early July, suggesting that that was the period of maximum demand for nutrients. The dependence of yields on nutrients was demonstrated.

Field experiments were made in the rainforest zone of Western Nigeria over three cropping seasons on the effects of potassium fertilizer of one local cassava cultivar 53101 and two improved cultivars 60506 and 60447 which were F x hybrids of the former. The results showed that K-fertilizer increased the starch yields only slightly. Cultivar 60506 produced significantly higher starch yields (P < 0-01) than the local strain, 53101, while cultivar 60447 gave lower starch recovery than the local cultivar.A definite influence of K application on the cyanide (HCN) content of the tuber roots could not be established. The peeled tubers of the improved cultivars contained much less cyanide than those of the local 53101 strain (P < 0-01). Potassium had the highest (0-72-1-28% dry weight basis) while phosphorus had the lowest concentration (0-06-0-12%) in the peeled tuber. Owing to its higher yielding capability the unfertilized 60506 removed larger amounts of potash (average 110-9 kg K 2 O/ha) from the soil than unfertilized 53101 (average 82-2 kg K 2 O/ha).

The results of field experiments on the influence of nitrogen fertilization on the yield components, starch and prussic acid content of two of the cassava cultivars recommended for cultivation in Nigeria are presented. The two cultivars 53101 (local) and 60506 (improved) were harvested at 9, 12 and 15 months. In general, the improved cultivars appeared to respond better to N fertilization, but high rates of N (150 kg N/ha) tended to reduce the yield performance of the two cultivars. The local cultivar produced maximum yields of 31-5, 37-5 and 55-7 fresh tuber t/ha with 60 kg N/ha at 9, 12 and 15 months, respectively, while the improved cultivar gave maximum yields of 31-5 and 48-4 t/ha with 120 kg N/ha at 9 and 12 months, respectively, and 64-1 t/ha with 90 kg N/ha at 15 months. By allowing the crop to grow up to 15 months, the starch yield was more than doubled and about three times the yield obtained at 9 months. The 53101 strain seemed to produce maximum starch yields with 60 kg N/ha while the 60506 type tended to give maximum starch yields with 90 kg N/ha. Influence of N treatments was significant up to 12 months of age but not at 15 months.The HCN content of both cultivars diminished considerably with the age of the plant, dropping at 15 months to about half the level at 9 months. While the HCN concentration of the local cultivar tended to increase with higher N-fertilization, that of the improved cultivar diminished, particularly at 12 months after planting. At 15 months however, the HCN content of both cultivars rose with increasing Nfertilization.

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