1. GENERAL INTRODUCTION 1
1.3. Cowpea and soil nitrogen fixation 9
Dadson and Acquaah (1984) reported that in N deficient soils, smaller starter doses of applied N may stimulate nodule formation and enhance the grain yield of legumes. The low soil N status of the soils is expected to encourage a positive response to Rhizobium inoculation particularly in the presence of applied phosphorus. Nodulation of faba bean was markedly restrained by N fertilization at the later growth stage of faba bean but facilitated remarkably by inoculation, and the facilitation of intercropping on nodulation was erratic (Omar and Abd-Alla, 1994).
Sangakkara and Marambe (1989) reported that inoculation increased nodulation of bush beans and to a lesser extent of mungbean. This effect was more evident with time. Nodulation was reduced in the presence of nitrogen fertilizer, and the effect was more pronounced in the
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extensively nodulating species, mungbean. Nitrogen and nodulation increased yield of both species. The study indicated the inability of bush beans to meet all nitrogen requirements by nodulation and nitrogen fixation alone. This suggests the need for some fertilizer nitrogen for tropical legumes, in addition to inoculation, to obtain yields (Sangakkara and Marambe, 1989). Otieno et al. (2007) reported that when sufficient levels of nitrogen are present in the soil, nodulation is inhibited. Nitrogen fertilizer application significantly reduced the number of nodules and nodule dry weight per plant in most species during long rains.
They further indicated that, the addition of 20 kg N ha-1 as ammonium nitrate depressed nodulation and nitrogen fixation in soybean. Nitrogen is known to impact negatively on nodulation but phosphorus has been reported to improve nodulation. Rhizobia inoculation increased number of nodules and nodule dry weight per plant for most species but the increase in the nodulation was neither translated to dry matter accumulation in the shoot and root nor to the yield and yield components (Otieno et al., 2007).
Cartwright and Snow (1962) reported that the urea treatment resulted in a delay in nodulation so that the number of nodules at the first sampling (four weeks) were reduced, while numbers at later samplings were higher since nodulation had been delayed until the root system was larger and provided a greater number of potential nodule sites. The authors further indicated that, urea treated plants showed reduced nodulation throughout the six week experimental period. It was highlighted that the advance effects on nodulation cannot be due to high concentration of combined nitrogen in the rooting medium, but it is suggested that they derive from a high level of nitrogen within the plant (Cartwright and Snow, 1962).
Davis et al. (1991) reported that cowpea, like all legumes forms a symbiotic relationship with a specific soil bacterium (Rhizobium spp). Rhizobium makes atmospheric nitrogen available to the plant by a process called nitrogen fixation. Excess nitrogen promotes lush vegetative growth, delays maturity, reduce seeds yield and may suppress nitrogen fixation. Cowpeas
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perform well under low N condition due to a high capacity of N fixation. A starter N rate of around 12.25 kg ha-1 is sometimes required for early cowpea plant development on low N soils (Davis et al., 1991).
Geetha and Varughese (2001) also reported that even though cowpea has the ability to fix atmospheric nitrogen, it requires a starter dose of nitrogen for early growth and establishment. Higher level of nitrogen tended to reduce the pod yield in their study. The authors highlighted that the reduction in yield at higher dose of nitrogen might be due to the excessive vegetative growth at the expense of pod production (Geetha and Varughese, 2001).
Abayomi et al. (2008) reported that a parameter such as plant height; number of branches per plant, number of pods per plant, pod weight and shelling percentage were significantly improved by the application of nitrogen fertilizer and hence significant increase in grain yield. It was concluded that the application of inorganic fertilizer to cowpea is beneficial, although in a small quantity of 30 kg N ha-1.
1.3.1. Soil nitrogen and maize production
Gungula et al. (2005) reported the significant differences observed in total leaf number among N rates, which is the indication that the number of leaves produced by maize plant is affected by N rates. Increasing the N rates resulted in more leaves produced per plant with the highest mean values in most cases at 120 kg N ha-1. They further indicated that higher N rates enhanced the vegetative growth of the maize and increased the source capacity of the plants by the number of leaves produced per plant. By increasing the level of N in the soil, there will be more green leaves maintained on the plants. It was also indicated that, since there are more leaves produced at higher N rates, those higher N rates will have higher photosynthetic capacity than the lower N rates.
Ding et al. (2005) reported that N deficiency decreased grain yield and plant weight. The response of grain yield to N deficiency was associated with much larger effects on biomass
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production than the harvest index. They further indicated that, different responses of grain yield to N deficiency between hybrids were mostly due to their different rate of accumulation of dry matter after anthesis. Leaf area may be decreased by N deficiency depending on the severity. Dry matter production after flowering of the N deficient plant was significantly lower in the study by Ding et al., (2005).
Thomison et al. (2004) reported that split applications of N increased grain protein concentration but had little or no effect on yield. Grain oil concentration was not influenced by the timing of N application and responded to N rate only. Their study demonstrated that N management will be an important factor in maximizing the grain protein of nutritionally enhanced maize. Feinerman et al. (1990) reported that late nitrogen application at tassel emergence did not increase corn yield. If applying nitrogen as late as tassel emergence, it is important that the fertilizer be activated either by rainfall or irrigation or soon as possible for maximizing plant availability of the nitrogen fertilizer and minimizing yield loss.
They further indicated that nitrogen fertilizer application as late as tassel emergence may increase corn yield if the plant is nitrogen deficient. Sharifi and Taghizadeh (2009) reported that maximum maize plant height of 204.6 cm was obtained with the highest nitrogen level of 240 kg N ha-1, while the least value of 181 cm was recorded at the lowest nitrogen level of 0 kg N ha-1. It was highlighted that across nitrogen levels, maximum number of kernel per ear (668) was recorded at 240 kg N ha-1 and minimum of 300.3 at 0 kg N ha-1. The number of kernel per ear increased with increasing nitrogen level.
It was revealed that nitrogen levels influenced significantly the cob length of maize hybrid.
Ear length generally decreased with decrease in nitrogen level. Nitrogen levels and maize hybrids did not show any significant variation in respect of number of ears per plant (Sharifi and Taghizadeh, 2009).
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