ABSTRACT

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

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(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

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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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INTRODUCTION

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 Köhlin, 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.

Category

Total N (%)

Available

P

Exchangeable K

%

Organic

(Bray-2, mg/kg)

(Cmol/kg)

matter

Low

<0.15

<15

<0.20

<2.0

Medium

0.15-0.20

15.0-25

0.20-0.40

2.0-3.0

Adequate

>0.20

>25.0

>0.40

>3.0

Source: NRCRI, Umudike soil Laboratory

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Traditionally, the African smallholder farmers who produce most of the food for the developing world have enough understanding of how to manage the soil fertility of their farm lands sustainably. The changes in the natural environment were accommodated within the culture and agriculture of their specific geographical areas, until the rapid changes in population growth during the 20th century (LEISA, 2006). The traditional shifting cultivation acclaimed to be ecologically stable and biologically efficient and suitable for the fragile tropical soils with inherent resilience, was no longer feasible, as the fallow periods continued to decrease due to increased pressure on land resulting in reduced crop yields (Glen and Tipper, 2001), demanding a more technical farming system than ever, to catch up with population increase and changes in farming environment in terms of food production (Anon, 2004). The principal factors of soil quality are soil salination, pH, microorganism balance and prevention of soil contamination. Agro-forestry closely approximates the traditional shifting cultivation but suffered low acceptance by great many smallholder farmers (Giller, 2003) because the smallholder farmers will better accept any technology that can both provide for soil fertility improvement and immediate food and fibre security (Catacutan et al., 2001). The use of such research technologies and concepts can improve soil fertility, but their application or acceptance is generally bolstered when they fulfill indirect benefits. Misiko (2007), indicated that as labour force dwindles and farm sizes shrink, the resource-deprived smallholder farmers would expect high economic returns such as food, fibre, fodder and fertilizer to pay for labour and time expended on them, beyond simply improving soil fertility.

Legume crops

Patrick et al. (1957), indicated that cover crops improved soil quality by increasing soil organic matter levels over time which enhanced soil structure as well as the water and nutrient holding capacity and buffering capacity of soils, increased soil carbon sequestration to offset the risk of increased atmospheric carbon dioxide levels (Kuo et al., 1997, Sanju et al., 2002, Lal, 2003). Soil erosion is prevented due to network of roots formed. There is increased soil porosity and suitable habitat networks for soil macro fauna (Tomlin et al., 1995).

Intercropping

Intercropping is an age old traditional practice of growing two or more crops in proximity in the same field during a growing season. It promotes ecological principles such as diversity, crop interaction and other natural regulatory mechanisms (Amanor, 1994). It is a plan for

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simultaneous crop production and building up of soil fertility, prevention of nitrogen leaching risks, sometimes observed in sole crops such as grain legumes, due to changes in incorporated residue and chemical quality involving nutrient turnover. Available growth resources, such as light, water and nutrients are more completely absorbed and converted to crop biomass by the intercrop as a result of differences in competitive ability for growth factors between intercrop components. Efficient utilization of growth resources leads to yield advantages and increased yield stability compared to sole cropping. The multifunctional profile of intercropping allows it to play many other roles in the agro ecosystem, such as resilience to perturbations to weather, protection of plants of individual crop species from their host-specific predators and disease organisms, greater competition towards weeds, improved product quality and reduced negative impact of arable crops on the environment, nitrogen fixing legumes can be included to a greater extent in arable cropping systems via intercrop.

Inorganic fertilizers have the advantage of delivering nutrients more readily and directly to plants but are prone to leaching, burning of seedlings by desiccation and build up of toxic concentrations of salts that can create chemical imbalances. Fertilizers applied should provide easily available plant nutrient forms on soils which are highly leachable and low in organic matter, therefore, K20 should be applied preferably in the Sulphate form. It was found that to produce a ton of grains, the following elements will be required: 65 kg N, 11kg P205, 20 kg K20, 4 kg Mg0, 4 kg Ca0, 2 kg S, 110 kg Fe, 33 kg Mn, 43 kg Zn, 16 kg Cu; 16 kg B, 6 kg Mo (Bataglia and Mascarenhas, 1978). Balanced fertilization for sustainable agricultural production and nutrient consumption in developing countries is nowhere near to the ideal ratio of 4:2:1, nitrogen, phosphorous and potassium, which poses a serious threat to soil K and soil health (Samra, 2007).

Nitrogen (N): Soybean as a legume can fix large quantities of atmospheric N to produce yields of 3000-4000 kg/ha, if nodules formed well. Johnson et al. (1975) found that adding N to well nodulated soybeans did not increase yield. Fertilizer N added at planting delays nodulation. Gasscho et al. (1989) suggested that N application during the vegetative stages result in decrease in nodulation in proportion to the rates applied. Adding N is recommended only when adequate nodulation is not expected. One ton of soybean seed removes 60 kg of N by the plant, or about 270 kg N for a 3-ton seed crop. Nitrogen need not be applied if the crop is well inoculated with

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bacteria. Where inoculation is poor, N fertilizers should be applied at the same rate as maize (Smith, 2006). N deficiency results in reduced chlorophyll development (pale-green leaf), growth and yields.

Phosphorus (P): is absorbed by plants throughout the growing season and its availability is at maximum level at a pH of between 6.0 and 7.0. Adequate P is essential for optimal crop yields, enables a plant to store and transfer energy, promotes root, flower and fruit development and allows early maturity in plants (Elliott et al., 2009). The period of greatest demand for phosphorus starts just before the pods begin to form and continues until about 10 days before the seeds are fully developed. Much P used in seed development is taken up early, stored temporally in leaves, stems and petioles, and then trans-located later into the seed. Stunted growth is usually the only symptom of P deficiency, though some leaf cupping and discolouration are possible. One ton of soybean seed removes 5 kg of P, compared with 3 kg of P for maize. Being a lower yielder, soybeans would remove 70 % of the P contained in a maize grain crop. Soil with medium or low levels of P would benefit with the application of 20-40 kg/ha. Optimum P for loam soil is 22 mg/litre and 12 mg/litre for clay. P tends to move downhill across the field and is less likely to leach vertically into the ground water. On alkaline soils, it is best to use composted or vermicomposted manure to minimize environmental impacts (Elliott et al., 2009).

Potassium (K): Relatively large amount of K are required for growth. It has been reported that rate of uptake is highest during rapid vegetative growth and slows down as seed formation begins. Uptake continues until two to three weeks before the seed is mature. K uptake can be depressed by poor soil condition, including compaction, excess moisture and poor aeration. Most K taken up, moves to the roots by diffusion through moisture films around soil particles. As the water content of a soil decreases, moisture films around the soil particles become thinner and the path length of K ion movement increases and the movement of K to roots decreases. Potassium uptake is reported to decrease if the oxygen content of the soil is low, therefore poor aeration would require higher available K, while cold soils reduce the rate and extent of root growth which further limits K uptake. When farmers plant earlier or adopt tillage practices such as no-till that result in reduced soil temperature early in the growing season, higher levels of available K in the soil are likely to be needed for optimum growth (Yin and Vyn, 2001; Isherwood, 2006; Magen, 2007). One ton of soybeans contains 18 kg of K compared with 3.5 kg K for maize grain

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and will remove twice the amount of K as maize grown under similar conditions. Five tons of maize compared with two tons of soybeans. Soil test result of 80 mg/litre for soybean is the critical level at which point K application is required. Soils with medium or low levels of K should receive 30-60 kg/ha of K. K deficiency is easily recognized by chlorosis which starts along the outside edges of leaves and moves inward, especially in the older leaves.

Liming: Soil acidity has been recognized as one of the major limiting factors to the production of legumes and many other crops (acidity fixes most soil nutrient elements), hence liming is needed on acidic soil surface for optimum or maximum yields (Prince, 1956, Duong, 1986, Lickacz, 2002). Lime is applied to acid soils to neutralize excess acidity (very low pH) that causes reduced crop yields. Lime application raises soil pH, making the soil more productive in several ways. 1) Liming removes aluminum and iron toxicity to growing plants by making them insoluble. 2) Liming keeps phosphorus of the soil and in applied phosphates in available forms over a longer period. 3) Calcium salts in lime promote flocculation or granulation hence limestone improves soil structure, better aeration and water relations and making the soil environment more suitable for plants to grow. 4) Liming increases the activity of nitrogen fixing organisms, hence an important practice in legume production. 5) Liming promotes more rapid decomposition of manure and crop residue in the soil, thus making elements contained in them more readily available to the plant.

Wood ash: The beneficial effect of wood ash on crop growth as an alternative to lime and/ or to the use of acidity tolerant crops has been documented (Duong, 1986, Spore, 1995, Lickacz, 2002). It is the inorganic and organic powder residue left after the combustion of wood or unbleached wood fibre which contains the oxides and hydroxides of calcium, magnesium, potassium and to a lesser extent sodium. It is similar to burned or hydrated lime in its mode of action (Lickacz, 2002). It has been used to supply calcium in groundnut plots showing calcium deficiency (Spore, 1995). Many factors cont