Abstract: Aluminum (Al) toxicity is a major limitation to leguminous crop production in acidic soils and fertilizer treatment could ameliorate the condition. In this study, direct and residual effects of fertilizers on the growth and yield of cowpea and soybean grown with or without Al treatment were evaluated on an alfisol. The investigation involved a greenhouse (2 kg soil/pot) experiment with two factors: Fertilizer types- (Control, Organic, Inorganic and Organomineral) and Al rates (0, 50, 100 μM AlCl3) at the Agronomy Department, University of Ibadan, Nigeria. Treatment combinations were replicated three times in completely randomized design, giving 36 experimental units for each crop. Growth parameters (plant height and number of leaves) and biomass yield were determined and data Analyzed using ANOVA while treatment means were separated by Duncan’s Multiple Range Test. High Al rate (100 μM) reduced growth and yield of the crops while moderate application (50 μM) enhanced their performance. Organomineral fertilizer promoted crops’ performance better than the other fertilizer materials while the application of inorganic or organic fertilizers with 50 μM AlCl3 proved most effective. Organic fertilizer had the highest residual effects equivalent to that of organomineral fertilizer applied with 50 μM AlCl3, confirming that organic based fertilizers could be used to minimize the deleterious influence of Al toxicity on the production of these legumes in acid soils.
INTRODUCTION
Aluminium is the most abundant metal in the earth’s crust constituting approximately 7% of its mass, occurring in a number of forms in soils (R’bia et al., 2011). It is generally accepted that Al toxicity is a primary factor limiting plant growth on acid soils (Kochian, 1995). Toxic effects of Al on plant growth have been attributed to several physiological pathways, although the precise mechanism has not yet been understood (Kochian et al., 2004). Proposed mechanism of Al toxicity include Al interactions with the root cell wall, Al disruption of plasma membrane and membrane transport processes and Al inhibition of mineral uptake and metabolism, especially that of Ca and P (Rout et al., 2001; Akinrinde, 2006).
Besides salinity, Al toxicity is among the widespread problems of ion toxicity stress in plants. Deficiency of phosphorus, calcium and magnesium coupled with presence of phytotoxicity substances (soluble Al and manganese) are responsible for the fertility limitation of acid soils as aggravated by industrial pollution and nitrification. Poor growth in acid soils could be related directly to Al saturation (Akinrinde et al., 2004). In acid soils, Al toxicity limits plants’ growth due to series of chemical factors and interactions including toxicities of hydrogen, Al and manganese. Oluwatoyinbo et al. (2005) showed that acid soils cover about 17 million hectares of land (representing about 18% of total land area) in Nigeria.
Soil acidity is normally corrected by application of calcitic and dolomitic lime. Liming has a beneficial effect on plant growth under Al stress and alleviates Al toxicity of plants (Bessho and Bell, 1992). However, in many developing countries where subsistence agriculture prevails, economic and logistic reasons make it difficult for the resource poor farmers to apply high rates of fertilizer P and/or lime (Akinrinde et al., 2004; Anetor and Akinrinde, 2006). It has been discovered that over liming may also cause negative effects on plant growth and soil properties (Ahmad and Tan, 1986). Deficiencies, for example, of some nutrients such as P, Zn, B and Mn can be induced by lime application (Nikolic et al., 2000; Shaaban et al., 2007). However, a number of workers have shown that the addition of green manures and animal waste to acid soils can reduce Al toxicity and increase crop yields (Berek et al., 1995; Hue et al., 1994; Hue, 1992; Duruigbo et al., 2007). The application of organic residue to acid soils in order to minimize the need for lime and fertilizer P application would be beneficial to resource poor semi-subsistence farmers (Haynes and Mokolobate, 2001).
The present study sought to investigate the short-term and long-term (residual) effects of different types of fertilizer (organic, inorganic and organomineral), Al treatments and the interaction of the two on the growth and yield of legumes (cowpea and soybean) on an alfisol.
MATERIALS AND METHODS
The study was carried out in the greenhouse at the Department of Agronomy, University of Ibadan, Nigeria from July to December, 2007. The experimental soil was an alfisol collected from Ajibode village near the University of Ibadan and cropped thrice with cowpea (Vigna unguiculata) before the study reported here. The pre-cropping physico-chemical properties of the soil were determined (Table 1).
The experimental design used was Completely Randomized Design (CRD) with two crops, cowpea (IT93K-452-1) and soybean (TGX1485-1) replicated three times. It was a factorial experiment with two factors; Al at three levels (0, 50 and 100 μM) and four fertilizer treatments (control, organic, inorganic and organo-mineral).The total treatment combinations was 12 for each of (soybean and cowpea).
| Table 1: | Pre-cropping soil physical and chemical properties |
The fertilizers used for the study include: Organo-mineral fertilizer (Sunshine organic fertilizer and NPK in ratio 2:1), NPK 15-15-15 and Sunshine organic fertilizer. The experimental treatment also involved the application of three levels of Al (0, 50 and 100 μm) using AlCl3..6H2O. The fertilizer materials were applied at the rate of 200 kg P2O5 ha-1 before planting while Al was apply at fourth week of growth. Watering and weeding of pots were also carried out. The treatment combinations were: control, Organic Fertilizer (OF), Inorganic Fertilizer (IF), Organomineral Fertilizer (OMF), Al50, Al50 and OF, Al50 and IF, Al50 and OMF, Al100, Al100 and OF, Al100 and IF, Al100 and OMF.
The parameters measured during the growth period include plant height and number of leaves per plant. The experiment was carried out twice. Aluminium and fertilizer treatments were used only in the first experiment while the second experiment evaluated the residual effects of the treatments on cowpea. The first experiment was terminated after eight weeks while the second experiment was terminated after six weeks. The plants were oven-dried at 70°C to a constant weight. Dry weights of samples were recorded.
Statistical analysis: Data were analyzed using the Statistical Analytical System (SAS 2003 to carry out Analysis of Variance (ANOVA) and in separating the means multiple comparisons were performed across treatments using Duncan’s Multiple Range Tests (DMRT).
RESULTS AND DISCUSSION
Pre-cropping soil fertility status: The physical and chemical properties of the soil used are given in Table 1. The particle size distribution of the soil indicated that the textural class was loamy sand. It was slightly acidic (pH 6.5) and the organic matter content (25.22 g kg-1) was below the critical level of 26 g kg-1 given by Adeoye and Agboola (1985) Cation exchange capacity and exchangeable Ca and Mg (8.59 and 4.42 cmol kg-1, respectively) in the soil were high compared with 2.6 and 0.26 cmol kg-1 proposed as critical values by Agboola and Corey (1973). The aluminium content of the soil was 0.0046 mg kg-1 while Total N (0.84 g kg-1) content and available P (8.62 mg kg-1) were marginal in the soil.
First cropping: The immediate or short-term effect of fertilizer treatment revealed that the test crops (cowpea and soybean) performed better (Table 2, 3) under organomineral fertilizer treatment especially with respect to number of leaves.
| Table 2: | Effects of the interaction between fertilizer and aluminium treatments on No. of leaves of cowpea at successive weeks after planting |
| Means with the same letter (s) in the same columns are not significantly different from each other p = 0.05 (DMRT). ns: Not significant | |
| Table 3: | Effects of the interaction between fertilizer and aluminium treatments on number of leaves of soybean at successive weeks after planting |
| ns: Not significant | |
There was a general increase from 4 to 7 week after planting with the highest number of cowpea leaves (27.3) obtained from plant treated with organomineral and this could be adduced to the fact that organomineral combined the benefits of organic manure such as nutrient supply, soil structure improvement and inorganic fertilizer (high and fast nutrient release) that enhanced vegetable production (Olaniyan et al., 2006). Also there was no significant difference in terms of number of leaves (Table 3) of soybean irrespective of fertilizer treatment. However, the data summarizing the effect of Al treatment, interaction between fertilizers and Al treatments on number of leaves of cowpea and soybean (Table 2, 3, respectively) indicate no significant difference among the various treatments throughout the period of experiment between the treated and untreated plants. This suggests that Al rates applied had no significant effect on the leaf production of the test crops. Sole fertilizer effects (Table 4, 5) were significant for both crops throughout the duration of the experiment. The tallest plants (20.9 and 30.5 cm) cowpea and soybean, respectively were obtained at 8 week after planting with sole application of inorganic fertilizer.
This could be adduced to the fast release of nutrients by inorganic fertilizer. This is in consonance with the study of Mokwunye and Vlek (1986) which revealed that the judicious use of N and P fertilizer can bring out substantial yield increase in West Africa. With respect to the effects of interaction between fertilizer and Al treatments on cowpea height (Table 4) indicate that there were significant differences (p<0.05) among the treatment means for cowpea. The highest cowpea height (23.3 cm) was obtained when 50 μM was applied with Inorganic Fertilizer (IF). Plant height was improved with the application of inorganic fertilizer by 5.19 and 15.57% compared with organomineral and Organic Fertilizer (OF), respectively. This trend was maintained throughout the experiment with IF proved to be most effective fertilizer for cowpea height. On the other hand, the interaction between fertilizer and Al on soybean plant (Table 5) indicates that treatment combination of 50 μM level of Al and OF was observed to favour the attainment of the highest soybean plant (34.8 cm) while the lowest (24.4 cm) was obtained at 8 WAP from sole application of 50 μM Al. This observation shows that organic fertilizer application had a beneficial effect on Al detoxification as it had been established by many researchers (Ahmad and Tan, 1986; Hue and Amien, 1989; Ostatek-Boczynski et al., 1995).
Organomineral fertilizer increased cowpea biomass production by 16.8 and 5.8% compared with inorganic and organic fertilizers, respectively this is in consonance with the findings of Singh et al. (2011) which stated that combined application of organic manures and inorganic fertilizers increased the dry matter accumulation. It was also observed that Al application reduced the performance of the crops, when applied at the highest rate (100 μM). The results of the sole application of Al (100 μM) for both crops (Table 6) show that the least biomass production (9.3 and 12.9 g pot-1) were obtained from cowpea and soybean, respectively, Petra and Proctor (2000) observed similar trends. Dry matter (Table 7) indicates that cowpea was reduced by 14.79% with 100 μM compared with 50 μM application level. This confirms that Al limits plants growth at high concentration; it is also in line with the submissions of Kochian (1995) which reported that Al toxicity is a primary factor limiting plant growth on acid soils.
There were significant interactions between Al and fertilizer treatment on cowpea (Table 7) dry matter yield. The highest cowpea dry matter (3.1 g pot-1) was obtained with the application of 50 μM Al and inorganic fertilizer while the least dry matter weight (1.5 g pot-1) was obtained from 100 μM Al.
| Table 4: | Effects of the interaction between fertilizer and aluminium treatments on cowpea height (cm) |
| Means with the same letter (s) in the same columns are not significantly different from each other p = 0.05 (DMRT) | |
| Table 5: | Effects of the interaction between fertilizer and aluminium treatments on soybean height (cm) |
| Means with the same letter (s) in the same columns are not significantly different from each other p = 0.05 (DMRT) | |
| Table 6: | Effects of the interaction between fertilizer and aluminium treatments on cowpea and soybean biomass weight (g pot-1) |
| Means with the same letter (s) in the same columns are not significantly different from each other p = 0.05 (DMRT). ns: Not significant | |
This could be attributed to the fact that inorganic fertilizer increases the crop yield. Peter et al. (2000) reported that targeted sufficient and balanced quantity of inorganic fertilizer will be necessary to make nutrients available for high yield without polluting the environment.
| Table 7: | Effects of the interaction between fertilizer and aluminium treatments on cowpea and soybean shoot dry weight (g pot-1) |
| Means with the same letter (s) in the same columns are not significantly different from each other p = 0.05 (DMRT) | |
Second cropping: Evaluation of the residual effects of the fertilizer treatments (Table 8) showed that production of leaves in cowpea followed the same trend with control plants having the lowest number of leaves (9.7) at 6 weekafter planting. The leaves however increased in number with inorganic fertilizer application and there was no significant difference among the fertilizer types.
| Table 8: | Residual effects of the interaction between fertilizer and aluminium treatments on number of leaves of cowpea |
| Means with the same letter (s) in the same columns are not significantly different from each other p = 0.05 (DMRT) | |
| Table 9: | Residual effects of the interaction between fertilizer and aluminium treatments on cowpea shoot dry weight (g pot-1) |
| Means with the same letter(s) in the same column are not significantly different from each other p = 0.05 (DMRT) | |
Organic fertilizer addition increased dry matter production by 24.67 and 48.67% compared with organomineral and inorganic fertilizer, respectively. This could be adduced to the fact that organic improve moisture availability and nutrients supply. Organic additions also supply N to the legume crops which could be limiting factor for the grain legumes if biological activity is low. Carsky (2003) observed that organic additions substantially increased cowpea grain yield.
The 50 μM Al applications favoured the growth and yield of cowpea unlike the case with 100 μM Al application level. Combined Al at 50 μM and organomineral treated plants (Table 8, 9) produced more leaves (13.7) and dry matter (2.43 g pot-1) than the other treatment combinations.
CONCLUSION
It is evident from this study that high Al concentrations in soils would reduce the growth and yield of cowpea and soybean while moderate Al amounts (e.g., 50 μM) would enhance the performance. Organomineral fertilizer application also promoted the growth and yield of the two crops more than the other fertilizer types. However, combined applications of 50 μM Al and IF or OF proved to be the most effective in enhancing crop growth. Expectedly, organic fertilizer had the highest residual effect while OMF was the best in ameliorating acid soils with moderate Al concentration.