Field study was conducted during the rainy seasons (and 2010 at the Research Farm of the International Institute of Tropical Agriculture (IITA) Minjibir, Kano State, Nigeria; to compare responses of six cowpea varieties to tillage. The treatments consisted of tillage systems (zero tillage, flat tillage and ridge tillage) as the main plot and cowpea varieties (IT89KD-391, IT90K-277-2, IT97K-461-4, IT97K-499-35, IT98K-131-2, and IT98K-506-1) as the sub-plot. The treatments were arranged as split plot laid out in a randomized complete block design with four replications. Zero tillage was significantly superior in influencing days to maturity, canopy height, intercepted Ewansiha et al.; ARRB, 5(3): 275-284, 2015; Article no.ARRB.2015.030 276 photosynthetic active radiation (IPAR) and leaf area index but not total dry matter and grain yield of cowpea. A positive and significant association was recorded for days to physiological maturity and canopy height across the tillage systems with a correlation coefficient of ≤ 40% for days to maturity and ≥50% for canopy height. Though flat tillage showed a positive correlation with intercepted photosynthetic active radiation (IPAR) and leaf area (LAI), but this association was not significant. However, ridge tillage showed a positive and significant correlation with IPAR and LAI (r =0.378*** and 0.384***). All the tillage systems showed a high and significant positive correlation with cowpea dry matter and fodder yield (r= 0.54*** to 0.77***). Across varieties, grain yield was better with flat tillage than zero and ridge tillage systems, with 10% yield advantage over the two. Zero and ridge tillage were similar in their effects on grain yield. Except for IPAR, LAI and total dry matter (TDM), the interaction effect of cowpea varieties and tillage systems was significant. Our result point to the fact that extensive soil tillage (especially, conventional tillage) may not be necessary for cowpea production in this agro-ecology with a high percentage of sand and a sandy loam as soil textural class. Following our result, we may recommended varieties for the different tillage systems as follows: IT98K-131-2, IT97K-461-4, IT90K-277-2, IT98K-506-1 (grain) and IT89KD-391/IT97K-461-4 (best for fodder) for zero tillage system; IT90K-277-2, IT97K-499-35, IT98K-131-2 and IT98K-506-1(grain) and IT89KD-391/IT97K-499-35 (best for fodder) for flat-tillage; IT98K-131-2, IT90K-277-2, IT98K-506-1 and IT97K-464-4(grain) and IT90K-277-2/ IT89KD-391 (best for fodder) for ridge tillage).

The agronomic responses of three contrasting soybean [Glycine max (L.) Merr.] varieties to plant populations were examined in two distinct agro-ecological locations (at Samaru Zaria and Samaru-Kataf), both in the Guinea savanna of northern Nigeria in 2009, 2010, and in 2011 growing seasons. Three soybean varieties: TGx1835-10E, TGx1904-6F, and TGx1448-2E differing in maturity duration (early, medium, and late maturing, respectively), were evaluated at four plant populations (266,700, 333,300, 533,300, and 666,700 plants ha –1) using a split plot arrangement in randomized complete block design with three replications. The plant populations were the main plots, whereas varieties were subplots. The proportion of intercepted photosynthetically active radiation (IPAR) and leaf area index (LAI) increased with increasing plant population at both locations, indicating that high leaf area indices and high degree of canopy closure at higher plant population intercepted more light than the canopy at lower population and subsequently resulted in relatively high grain and fodder yields. At both locations, optimum plant populations ranged from 533,300 to 666,700 plants ha-1 across the years. The northern Guinea savanna location (Samaru Zaria) produced more pods m-2 , grain yield, and fodder at higher plant populations than that at the southern Guinea savanna (Samaru-Kataf). Varieties TGx1448-2E and TGx1904-6F intercepted higher proportion of IPAR had higher LAI and produced a greater number of pods m-2 , seeds m-2 , grain yield, and fodder than TGx1835-10E at both locations in years of good rainfall. These data suggest that soybean yields in the Guinea savanna of northern Nigeria can be increased using higher plant populations than those currently recommended.

The Nigerian savanna zone is currently witnessing increasing intensities of crop and livestock production activities that are resulting in increased soil erosion, de-vegetation of the land area, and desert encroachment especially during the dry season. These may have led to the presence of several over-grazed and bare ground areas that is highly susceptible to soil erosion. At harvest, both crop and residues are removed from the field, thus limiting potential nutrient recycling between crop and soil, and further impoverishing the nutrient status of the soils. The Nigerian savanna zone soils therefore have low total nitrogen, organic carbon, available phosphorus and cation exchange capacity. They also have very poor moisture holding capacity, and are therefore said to have poor fertility status and very low buffering capacity. The present study aimed to determine effects of grain legumes on soil potential in the arid systems was therefore imposed on the soils in the Zaria area. Results show that sole legumes and legume/maize treatments generally resulted in higher organic carbon contribution than sole maize treatment, suggesting that sole maize grown continuously on one farm for years could degrade the organic carbon content of the soil. Also, the sole legume and legume/maize treatments resulted in improvements in soil nitrogen in the range between 65.6 and 84.8 %, while nitrogen under sole maize resulted in only 5.9 % increase. In 2001, groundnut/maize intercrop resulted in significantly higher maize grain yield (1.49 t ha-1) than the other treatments. Comparing between maize grain yields in 2001 and 2002, maize grain improved by 20.1% under sole maize, 95.0% under maize in sole groundnut, 92.8% under maize in soybean, and 98.4% under maize in cowpea. This would confirm that the sole legume planting for two years restored fertility status of the soils and enhanced the soil organic carbon and total nitrogen, to have resulted in greater maize grain yield than under the sole maize despite the 120 kg N fertilizer applied to the sole maize.