Legume Research, 38 (2) 2015 : 241-245
AGRICULTURAL RESEARCH COMMUNICATION CENTRE
Print ISSN:0250-5371 / Online ISSN:0976-0571
www.arccjournals.com/www.legumeresearch.in
Effect of superphosphate and mucuna (Mucuna pruriens) management options on soil organic matter, soil pH and physical properties of a kaolinitic sandy loam soil in Zimbabwe M.D. Shoko, P.J. Pieterse*1 and G.A. Agenbag1 Department of Agronomy, Faculty of Agricultural Sciences, Great Zimbabwe University, Box 1235, Masvingo, Zimbabwe. Received: 28-07-2014 Accepted: 26-01-2015
DOI: 10.5958/0976-0571.2015.00071.5
ABSTRACT The continuous use of fertilizer alone will eventually result in deterioration of soil pH, soil organic matter (SOM) and soil physical properties. Inclusion of a leguminous crop such as mucuna (Mucuna pruriens) in a rotational system may alleviate these problems. The major objective of this research was to investigate the effect of two phosphorus application levels and four mucuna management options on SOM, soil pH, bulk density, particle density and porosity. Incorporation of mucuna at flowering stage in combination with addition of 40 kg ha-1 P increased the SOM and porosity and decreased bulk and particle density significantly compared to other treatments. All treatments where mucuna biomass were incorporated into the soil in the absence of any P application resulted in increased soil pH (CaCl2) levels with values of > 5.2 which will be beneficial for the production of most crops. The incorporation of above-ground biomass of mucuna had positive effects on all soil properties investigated. Key words: Mucuna, Soil organic matter, Soil pH, Soil physical properties, Superphosphate. INTRODUCTION It has been shown that on the poorly buffered kaolinitic soils found in many areas in the tropics, including sub-Saharan Africa, continuous use of fertilizer alone cannot sustain crop yield and maintain soil fertility, because soil pH, soil organic matter (SOM) and physical properties such as bulk density, particle density and porosity tend to deteriorate in the long term (Juo et al., 1995). Kang (1993), working on an Alfisol soil in Nigeria, reported a soil pH decline from 6.2 to 5.1 during 10 years of continuous cropping with maize under inorganic N fertilizer regimes. Several other examples of acidification and the decline of soil organic matter and exchangeable nutrients in sub-Saharan Africa are given in a review by Franzluebbers et al. (1998).That means that there is need to find alternative organic sources which will increase soil pH, SOM and ameliorate soil physical properties. The incorporation of a legume crop at flowering will help to increase N, K,Ca and SOM levels in the soil (Shoko et al., 2007). According to Sanginga et al. (1996) and Carsky et al. (1999) the influence of mucuna (Mucuna pruriens) on soil physical properties and SOM depends on soil type,
management and environmental conditions. In many cropping systems, soil management to increase SOM and soil pH and decrease bulk density has been attempted via the use of crops as green manure and the return of crop residues of rotational legume crops such as mucuna (Becker et al., 1995; Snapp et al., 1998; Whitbread et al., 1999, 2000). Mucuna tolerates low soil fertility, acidic soils and drought conditions (Hairiah et al., 1991; Burle et al., 1992; Weber, 1996), properties which indicate its potential for surviving and producing biomass during the drier part of the year. In reviewing the challenges for research and development of legume-based technologies for the African savannas, Weber (1996) concluded that mucuna is among the species adapted to cropping systems in sub-Saharan Africa (Maasdorp & Titterton, 1997). The major aim of this study was to determine the effects of superphosphate application and mucuna management options on soil pH, SOM and some physical properties of a kaolinitic sandy loam soil in the dry savanna area of Zimbabwe. Mucuna was used in this study because it has been identified as a potential rotational cropping legume in maize production systems in these areas (Maasdorp and Titterton, 1997).
*Corresponding author’s e-mail:
[email protected]. 1 Department of Agronomy, Stellenbosch University, Private Bag X1, Matieland, 7602, Republic of South Africa.
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MATERIALS AND METHODS The experiment was carried out at the Grasslands Research Station (18o 111 S; 31o 301E ; alt. 1200 m.a.s.l.) belonging to the Ministry of Agriculture, Zimbabwe, in Marondera in Zimbabwe during 2007 and 2008. Marondera receives on average > 86% of the mean annual rainfall of 850 mm during the hot summer months (November to March). During the experimental period, a total of 1067 mm and 867 mm were received during 2007 and 2008 respectively. Mean monthly daily minimum temperature ranges from a lowest of 5.3 0C in July to 15.3 0C in January and mean monthly daily maximum temperature ranges from a lowest of 18.3 0C in June to a highest of 26 0C in October. The soils are classified as humic Ferralsols based on the FAO/UNESCO system (FAO UNESCO, 2006) and are equivalent to a Kandiudalfic Eutaudox in the USDA soil taxonomy system (Soil Survey Staff, 1994). The soils are predominantly of the kaolinitic order (Paraferallitic 5G in the FAO classification) with loamy sands of low fertility (Nyamapfene, 1991). In general these soils are slightly acidic (pH (CaCl2) = 5.2) with organic matter content of 0.33% (Nyamapfene, 1991). These soils have inherent low weatherable minerals and are deficient in nutrients such as P (Thompson and Purves, 1978; Grant, 1981). The P levels of these soils range from as low as 5 to 15 mg kg-1(Mehlic 3 method) (Grant, 1981). Soil analyses performed on soil samples (0-30 cm deep) taken before the trial started showed a mineral N content of 15 mg kg-1 at the time of sampling as well as a P content of 15.8 mg kg-1 (Mehlic 3 method), K content of 0.15 cmol kg-1, Ca content of 0.2 cmol kg-1 and Mg content of 0.03 cmol kg-1. AP level of 30 mg kg-1 is regarded as sufficient for crop production and therefore this soil clearly showed deficient P levels. The whole area was ploughed, disced and planted to Mucuna pruriens var. utilis in August 2007 (first season crop) and July 2008 (second season crop). The experimental plots were planted to a maize crop and then were lying fallow for four years before the current experiment was planted. Mucuna was planted using an inter row spacing of 45 cm and intra row spacing of 10 cm. Weed control was done twice at one month and two months respectively after planting of mucuna using mechanical methods. Irrigation was supplied during drought periods to supplement the rainfall of 242.7 mm and 148.4 mm that occurred during the growth period of the mucuna crop (July to November) during 2007 and 2008, respectively. Although this is not a normal practice with the smallholder farmers, this was done during the experimental phase so that the mucuna crop would not fail.
The experimental design was a split plot with two P treatments as main plot factors [P0 = 0 kg P ha-1 and P40 = 40 kg P ha-1] applied prior to planting a mucuna crop and four mucuna treatments as sub-plot factors [MF=mucuna incorporated at flowering, MAR= mucuna above ground biomass removed at maturity and only roots incorporated, MPR = above ground biomass except pods incorporated at maturity and F = Fallow (control)]. Single superphosphate (19.25 % P2O5, 12 % S and 14 % Ca) was broadcasted over the plots as pre-planting fertilizer for the P treatments. The P40 treatment was chosen because it is the rate of P generally recommended by extension officers in Zimbabwe for a mucuna crop, but which was not previously tested in this part of Zimbabwe. No other fertilizers were applied to the crop. The treatments were replicated four times. The plot size was 100 m2. The nett plot area was 25 m2. The remainder of the plot area was used for other destructive sampling measurements. Soils were sampled before planting mucuna in 2007 and after the incorporation of mucuna in 2007 and 2008. Soil samples were collected at 0-30 cm depth by taking five cores per plot using a 50 mm diameter auger. The five sub samples were thoroughly mixed to obtain one composite sample per plot. Subsequently 500g of soil were weighed from each composite sample and taken to the laboratory for analyses. The collected soil was analysed for pH, OM, bulk density, particle density and porosity. Soil organic matter was determined by the Losson-Ignition method where soil was heated at 360 ºC for two hours after temperatures reached 360 + 5 ºC (AOAC, 1990). Soil pH was measured using the 0.01M CaCl2 solution method (AOAC, 1990). The following models were used for the determination of the physical properties of the soil (Tagwira, 1992): Bulk density = Mass of oven dry soil (g)/ Total volume of the soil (cm3) Eqn1 Particle Density = Mass of soil solids(g)/ Volume of the soil solids(cm3) Eqn 2 % Porosity = [1-{ bulk density / particle density}] x 100 Eqn 3 Statistical analysis of the data was performed using the STATISTICA software, version 8.02 program (StatSoft, 2004). Analysis of variance (ANOVA) was conducted to determine significance of treatment effects. Means were separated using Bonferroni studentised range for testing least significant differences at the 5% level when ANOVA revealed significant (p < 0.05) differences among the treatments.
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RESULTS AND DISCUSSION In terms of all the parameters measured the data showed the same trends in both seasons and only the means over two seasons will be discussed in the following sections although the data for both seasons are presented in Tables 1 and 2. The significant (p < 0.05) interaction between P treatments and mucuna management options in terms of SOM accumulation over two seasons is illustrated in Table 1. In the P0 treatment the MF treatment accumulated significantly (p < 0.05) more SOM than the MPR, F and MAR treatments. The MPR treatment in turn had significantly higher SOM levels than the MAR and F treatments. The P40 treatment showed similar trends as the P0 treatment with the exception of the MAR treatment that accumulated significantly (p < 0.05) more SOM than the F treatment, in contrast with the P0 treatment. The MF management option at the P0 and P40 treatments and the MPR management option at the P40 treatment will increase the SOM content of the soil. Higher SOM levels will improve soil properties such as the CEC, fertility, water holding capacity and microbial biomass of sandy soils (Kang, 1993). There were significant interactions (p < 0.05) between the P treatments and mucuna management options in terms of soil pH. The P0 treatment showed significant soil pH increases in all treatments where mucuna was planted compared to the natural fallow treatment whereas in the P40
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treatment all the mucuna treatments significantly reduced the pH compared to the natural fallow treatment (Table 1). The reduction in soil pH from 5.1 to 4.6 by the MPR treatment in the P40 treatment may reduce yields of crops that are sensitive to acidity. The pH levels of the MF, MAR and MPR mucuna management options at P0 treatment are close to optimal (pH 6 - 7.5) for the availability of N, P, K, Ca, Mg and S (Tisdale et al., 1999). However some liming of about 1 300 kg CaCO3ha1 may be required to raise the pH for the F management options under P0 to increase availability of the P that is present in the soil. The above lime quantity is enough to raise the soil pH by 0.5 units in sandy soils in Zimbabwe (Tagwira, 1992). At the P40 treatment the MF, MAR and MPR management options increased soil acidity and farmers will need to lime the soils to increase pH levels. Increased mucuna crop growth in the P40 treatment (Shoko et al., 2010) could have resulted in a higher Ca uptake and the microbial decomposition of the increased amount of SOM added to the soil could also have an acidifying effect. There was a significant interaction (p < 0.05) between P treatment and mucuna treatment in terms of bulk density in each of the two seasons but the interaction was absent when the mean values over two seasons were analyzed. At both P levels the mucuna treatments significantly (p < 0.05) reduced bulk density compared to the natural fallow treatment. At
TABLE 1: Percent soil organic matter (SOM) and soil pH (CaCl2) of a sandy loam soil under different management options of mucuna and two P treatments SOM (%) Treatment
P0 1
Before planting mucuna 2007 F2 MF MAR MPR 2008 F MF MAR MPR Mean of two seasons F MF MAR MPR
0.26
Soil pH P40
P0
P40
5.12
0.26a* 0.63c 0.25a 0.43b
0.26a 0.86e 0.42b 0.77d
5.12c 5.31e 5.22d 5.22d
5.12c 5.22d 4.86b 4.60a
0.26a 0.64d 0.27a 0.45b
0.27a 0.90f 0.48c 0.82e
5.13e 5.41g 5.23f 5.21e
5.12d 4.92c 4.85b 4.64a
0.26a 0.64c 0.26a 0.44b
0.26a 0.88e 0.45b 0.80d
5.13d 5.36f 5.23e 5.22e
5.12d 5.07c 4.86b 4.62a
*Parameter means followed by the same unbold letter in a season and those followed by the same bold letter for the mean of two season are not significantly different at p = 0.05. 1 P0 = No P applied (control) and P40 = 40 kg P ha-1 applied. 2 MF = mucuna incorporated at flowering, MAR = mucuna above ground biomass removed and only roots incorporated, MPR = only mucuna pods removed and all the other above ground biomass was incorporated and F = Fallow (control).
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TABLE 2: Bulk density, particle density and porosity of a sandy loam soil under different management options of mucuna and two P treatments -3
Bulk density (Mgm ) Treatment
P0 1
Before planting mucuna 2007 F2 MF MAR MPR 2008 F MF MAR MPR Mean of two seasons F MF MAR MPR
1.5
P40
Particle density (Mgm-3) P0
P40
2.67
Porosity(%) P0
P40
42
1.5d* 1.3b 1.5d 1.3b
1.5d 1.2a 1.4c 1.2a
2.67b 2.53a 2.66b 2.53a
2.66b 2.52a 2.67b 2.53a
42a 50.2d 43.8b 48.6c
43a 52.4e 43.8b 53e
1.5d 1.2a 1.4c 1.3b
1.5d 1.3b 1.4c 1.3b
2.66b 2.52a 2.64b 2.51a
2.64b 2.51a 2.66b 2.51a
43a 52.4d 47b 48.2b
43.8a 50.2c 43.8a 48.2b
1.5d 1.25a 1.45c 1.3b
1.5d 1.25a 1.4c 1.25b
2.67b 2.52a 2.65b 2.52a
2.65b 2.51a 2.66b 2.52a
42.5a 51.4d 45.4b 48.3c
43.4a 52.2d 43.8a 50.6d
*Parameter means followed by the same unbold letter in a season and those followed by the same bold letter for the mean of two season are not significantly different at p = 0.05. 1 P0 = No P applied (control) and P40 = 40 kg P ha-1 applied. 2 MF = mucuna incorporated at flowering, MAR = mucuna above ground biomass removed and only roots incorporated, MPR = only mucuna pods removed and all the other above ground biomass was incorporated and F = Fallow (control).
both P levels the MAR treatment had the least effect on the bulk density, although it was still significantly lower than the F treatment (Table 2). A bulk density value of soil which is close to 1.2 Mg m may indicate a crumby structured soil and this can sustain plant growth and development (Tisdale et al., 1999). This bulk density ensures good water holding capacity, aeration and porosity and can be attributed to the significantly (p < 0.05) higher amounts of organic matter which the MF and MPR management options added to the sandy loam soils compared to the F and MAR management options (Table 1). -3
There was no significant (p > 0.05) interaction between P treatments and mucuna management options in terms of particle density. Both the MF and MPR managements options at both P levels significantly decreased particle density (Table 2). The significant (P < 0.05) interaction between P treatments and mucuna management options in terms of soil porosity over two seasons is illustrated in Table 2. In the P0 treatment only the MF management option resulted in porosity values of 50% and higher. In the P40 treatment both the MF and MPR management options resulted in porosity values higher than 50%. Soil with a porosity of about 50% is well aerated, well drained and supports microbial activity (Fageria et al., 1991). The MF management option ensured that such porosity levels were achieved in both P treatments.
The MF management options resulted in the highest SOM, bulk density, particle density and porosity at the P40 treatment (Table 2). This shows the potential of mucuna as an ameliorant to soil physical properties when incorporated at flowering stage. However, in this management combination it may be necessary to lime the soil to increase its pH to required levels for maize production. Similar positive effects of leguminous rotational crops on soil physical properties were observed by Wang et al. (2009) and the inclusion of mucuna as a rotational crop in a maize system on these sandy loam soils of Zimbabwe may therefore ameliorate soil physical properties. CONCLUSION The use of mucuna, particularly when incorporated into the soil in the flowering stage, appears to have a very beneficial effect on soil physical conditions. Resource-poor farmers in rural areas who do not have access to sufficient fertilizer to apply the recommended rates, will greatly benefit from using mucuna as a rotational crop because it will alleviate the deterioration of soil under continuous cropping conditions. ACKNOWLEDGEMENTS This paper is based upon work supported by the National Research Foundation under Grant Number 63607. The Grassland Research Station in Marondera, Zimbabwe is also acknowledged for providing the experimental site.
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