SUMMARYField studies were conducted in order to determine the components of yield and response of maizeto soil applied copper and sulphur in the rainforest-savannah transition zone and Guinea savannah zone of Nigeria. Five CuO treatments, 0, 1, 3, 6 and 9 kg/ha, were applied to maize in 1988–90, and grain yield was consistently and significantly increased by the 1 kg/ha CuO treatment. Five S treatments, 0, 10, 20, 40 and 80 kg/ha, were applied to maize in 1989–90, and all treatments increased grain yield significantly in 1989, but only the 10 kg/ha S treatment increased grain yield significantly in 1990. Higher amounts of applied S increased leaf S significantly in the Guinea savannah zone.

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Field trials designed to investigate the effects of nitrogen, phosphorus and potassium on the yield, starch and dry matter content of white guinea yam were conducted over a four year period in south western Nigeria. The optimum level of nitrogen for successful production of white guinea yam was 35 kg ha"" 1 in a forest Alfisol that had been under cultivation for at least two years. Nitrogen, phosphorus and potassium had no significant effect on starch content, but large applications of phosphorus and potassium significantly decreased dry matter accumulation.

S U M M A R YA two year field study designed to investigate the effect of spacing on yield of two cowpea cultivars in the savanna and forest zones of Nigeria showed that cowpea yields were larger in the savanna than in the forest zone at all spacings employed. Inter-row spacing wider than 60 cm and intra-row spacing closer than 10 cm could be used for improved seed yield in the savanna zone, while 60 x 20 cm (83 333 plants ha~') would be optimum for cowpea production in the forest zone.Spacing is an important factor affecting the yield of cowpea. In Nigeria, cowpeas are grown sole or mixed with other crops at very irregular spacings. For successful production of this crop, knowledge of optimum inter-row and intrarow spacing is required. Few such reports are available (Remison, 1980;Nangju et al., 1975;Ezedinma, 1974). A small plant population (wide spacing) can lead to a significant reduction in yield of cowpeas (Ezedinma, 1974) and Enyi (1973) showed that yields increase with increasing plant density. Kayode (1984) reported a non-significant response to spacing by yam (Dioscorea rotundata) in the savanna zone, whereas yield increased with spacing in the forest zone. Willey and Heath (1961) stated that the yield of plants depends not only on plant density (plants per unit area) but also on the spatial arrangement of these plants (plant rectangularity), i.e. the ratio of the distance between plants within the row to the distance between the rows. This paper reports on the response of cowpea to spacing and plant rectangularity in the savanna and rainforest zones of Nigeria. MATERIALS AND METHODSA field experiment was carried out in the early growing seasons of 1980 and 1981 at Ibadan (7° 25' N, 3° 52' E) in the rainforest zone and at Mokwa (9°1 8' N, 5° 4' E) in the southern guinea savanna zone of Nigeria. Two indeterminate cowpea cultivars, Ife Brown and New Era, were sown at four inter-row spacings (30, 40, 50 and 60 cm) and five intra-row spacings (10, 15, 20, 25 and 0014-4797/85/0000-0974 $05.00© 1985 Cambridge University Press

Field studies on the effect of nitrogen levels, plant population and soil nutrient status on maize yield and yield components at eight different sites representing six different ecological maize growing zones of Nigeria conducted during 1976Nigeria conducted during , 1977Nigeria conducted during and 1978 indicated that N significantly influenced yield at six of the eight sites, NIFOR, Umudike and Uyo (rain forest zone), Mokwa and Samaru (Savanna zone) and Riyom (derived savanna zone). Optimum N required for maize varies between 50 and 100 kgN in the different zones. There was no response to spacing except at Umudike, Uyo and Riyom, and no significant interaction between N and spacing at any location.Nitrogen application significantly influenced earweight at lkenne, NIFOR, Umudike, Uyo and Mokwa.Nitrogen application had a significant effect on the number of cobs at NIFOR, Uyo and Mokwa, while spacing significantly influenced number of cobs at all locations except Uyo. Husk weight was significantly increased by nitrogen application at four locations, NIFOR, Uyo, Mokwa and Samaru, but spacing had no effect on this yield component in any of the other locations except Umudike. There was an N by spacing interaction on husk weight at Umudike and Mokwa.Nitrogen application significantly influenced lodging at tasseling at Uyo, Mokwa and Riyom, whilst there was a significant effect of spacing at Uyo and Samaru. At harvest, lodging was not influenced by N application but spacing significantly affected lodging at Ibadan and NIFOR.Both N application and spacing had no significant effect on shelling percentage at any location.The high correlation between relative yield and available Zn(r = 0.77"), Cu (r = 0.63) and Fe (r = --0.66) of the soil at all the locations has shown that:(i) other nutrients besides N, P and K affected maize yield, therefore the inclusion of other nutrients besides N, P and K in the fertilizer recommendation for maize should be encouraged; (ii) different levels of NPK fertilizer plus other nutrients are required in each ecological zone and this can be determined by soil testing.Response of maize to varying plant population at different soil fertifity status depends on climatic conditions, spacing and genetic factors. It has been shown that differences in light utilization by varieties are related to their level *Significant at 5% level. Fertilizer Research 2:1 77-191 {1981) 016 7-1 731/81/0023-01 77 ¢02.25