Promiscuous (naturally nodulating) IITA (International Institute of Tropical Agriculture) soybean (Glycine max (L) Merrill) varieties and elite varieties of maize (Zea mays L.) were evaluated in four experiments between 2007 and 2009, for their growth and yield responses to an area without history of soybean cultivation, some soil fertility management options and for soybean fertilizer replacement value (FRV) to companion and subsequent non-legume maize crop. These experiments were conducted at Abakaliki in the derived Savanna of Southeastern agro-ecological zone of Nigeria, located at latitude 060 19´ 407´´ N, longitude 080 07´ 831´´ E and an altitude of about 447m above sea level, with a mean annual rainfall of about 1700mm to 2060mm spread between April and October. The maximum mean daily temperature is between 270 C - 310 C with abundant sunshine and a high humidity all through the year. The soil is shallow with unconsolidated parent materials (shale residuum) within 1m of the soil surface, described as Eutric leptosol.

The first experiment assessed twelve IITA promiscuous soybean varieties (TGx 1740-2F, TGx 1904-2F, TGx 1904-4F, TGx 1903-5F, TGx 1909-3F, TGx 1844-4E and the selected six varieties used in Experiment II), for their growth and yield performances in the derived savanna belt of Southeastern Nigeria. These varieties showed high adaptable potentials by exhibiting significant good growth and high yield components. Varieties like TGx1740-2F, TGx1485-1D, TGx1904-6F, TGx1908-8F, TGx1903-5F, TGx1844-18E, TGx1904-2F and TGx1903-7F produced seed grain of up to 6.0-7.5 tons/ha, seed weight between 29.1-33.5 g/plant, nodule number between 20.9-37.1 and a vigorous growth to a height of up to 30.8-69.3 cm with girth size of 1.2-1.5cm. These qualities observed were good evidence that soybean can be successfully cultivated in Abakalki climatic conditions and that with application of the management options implicated in this study, soybean can be a veritable resource among the resource-constrained smallholder farmers for food and for their soil fertility improvement without the costly external fertilizer inputs.

In Experiment II, eight soil fertility management options (lime at 10 tons/ha, wood ash (WA) at 10 tons/ha, urea at 20 Kg/ha, poultry manure (PM) at 20 tons/ha, muriate of potash (MOP) at 30 Kg/ha, single super phosphate (SSP) at 40 Kg/ha, NPK (15:15:15) at 40 Kg/ha and a control) were evaluated for their effects on the growth and yield of six selected soybean varieties (TGx1876-4E, TGx1903-7F, TGx1485-1D, TGx1844-4E, TGx1904-6F and TGx1908-8F) from Experiment I. A soil test was carried out before planting (BP) and after harvesting (AH), which indicated that the area was acidic with pH values between 5.50 (BP in 2008) and 5.85 (AH in 2009), but with high available phosphorus (24.57 mg/kg AH in 2009) while other elements were low. Poultry manure was found highly significant (P<0.05) in improving the growth and yield of the six soybean varieties, seedling emergence (71.1%), plant height (39.59 cm), the girth size (1.27 cm), number of branches (3.13), number of nodule/plant (25.25), number of pods (106.2), weight of pods/plant (39.78 g), number of seeds/plant (209.80) and weight of seeds/plant (23.68 g). Wood ash was next in improving the growth and yield parameters but not with the same degree with PM. Lime was next to WA, followed by urea, NPK (15:15:15), MOP, SSP, and the control in their effects. TGx1485-1D responded better in terms of number of nodules per plant (12.70) and number of pods per plant (119.2) than others, but TGx 1903-7F had the highest number of seeds per plant (160.5) and weight of pods (31.28 g/plant). TGx 1904-6F (medium maturing) had the least number of nodules (9.39) but was second to the highest in terms of number of seeds (145.4). No one variety responded better than others across all the parameters.

The fertilizer replacement value (FRV) of soybean residual manure (SRM) was evaluated in Experiment III on the growth and yield of subsequent three maize varieties. SRM + NPK (15:15:15) at 200 Kg/ha significantly (P<0.05) influenced the growth and yield parameters of maize varieties than soybean residual manure alone, NPK (15:15:15) alone and the control. Where SRM + NPK (15:15:15) were applied, it gave the highest shelling weight (18.75 g) per plant and 1000 seed weight (196.73) but was third in influencing harvest index (HI) with 0.56 as against 0.59 arising from NPK (15:15:15) and 0.57 from control. However, SRM alone contributed almost one half (9.08 g) of the shelling weight arising from SRM + NPK (15:15:15), but had the least HI (0.53). Also, SRM + NPK (15:15:15) had the highest undehusked cob weight (29.42 g) and dehusked cob weight (23.38 g) per plant. Composite maize breed, Suwan produced the highest shelling weight (14.59 g) and was second to Oba super II in HI (0.60) with 0.58. But the local (Ikom white) yielded the highest 1000 seed weight 209.38 g, followed by Suwan (193.02 g) and Oba super II (171.10 g).

The fertilizer replacement value of twelve soybean varieties on the growth and yield of a companion crop was evaluated in Experiment IV by intercropping soybean (30 x 15 cm) with maize (75 x 25 cm). The yield performance of the maize varieties showed that the HI of Oba super II was lower (0.46) in 2008 than in 2009 (0.59), and was like that for Suwan, 0.46 (2008) and 0.55 (2009) and 0.43 (2008) and 0.57 (2009) for Ikom white, indicating that by intercropping soybean with maize, maize growth and yield could be sustained successfully without inorganic fertilizer application and without the maize developing any deficiency symptoms.

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Literature Review

Materials and methods



There is a growing concern all over the continent of Africa over the decline in the productive capacity of the continent’s soil resources due mostly to declining soil fertility with cultivation. Agricultural productivity is reported to have actually declined over the past 45 years in many African countries which has been blamed on soil degradation as its major cause (Bluffstone and K√∂hlin, 2011). Sanchez (1987) had earlier observed that soil fertility depletion is the fundamental cause of low per capita food production among smallholder farmers in Africa who remove huge amounts of nutrients from the soil without returning any at the rate of 22 kg N, 2.5 kg P and 15 kg K per hectare over the past 30 years in 37 African countries (Anon, 2003). However, reports show that where farmers applied fertilizers at all, very little are used as low as less than 20 kg/ha which is strikingly low compared with the 200 kg/ha common in European agriculture (Tittonell et al., 2008). “African Green Revolution” in which fertilizer use is expected to rise from 8 kg/ha to at least 50 kg/ha annually by 2015, was launched in Abuja, Nigeria to indicate the need for increased fertilizer use in Africa, known as “Abuja declaration 2006”. Almost all agricultural intensification to guarantee food security for all, hinges on heavy use of fertilizers (ENDA, 1977), but the tropical soils do not respond well to some of the temperate farming practices involving the use of fertilizers, herbicides and pesticides (Houngnandan et al.,2000).

There is a strong nexus between soil fertility management and demographic growth rate especially in Africa where food production is lagging behind demand for food. Rapid population growth and urbanization consequently led to increased demand for land especially for cultivation of food crops to avert hunger. The consequent severe pressure on soil productivity made most soils lose their fertility quickly (Kang et al., 1984, Kang and Reynolds, 1986, Spore, 2009). The more the population the more access to good agricultural land is restricted in regions where land area per capita is continually decreasing, yet it is these regions where the demand for agricultural products is continually rising (Spore, 1994) and consequently requiring land use intensification. Soil study and fertility interpretations of the Southeastern Nigeria indicated the following categorizations of soil fertility guide for fertilizer application needs......

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