Physico-chemical properties of pre-experiment soils
Table 1 presents the physico-chemical properties of the pre-experimental soil. The soil in the experimental area had a silt loam texture with bulk density of 1g/cm3 and porosity of 62.26%. It had moderately acidic pH value of 5.64 in a 1:2.5 soil : water solution, 2.73% SOC, 4.74% OM, 0.24% TN, 11:1 C:N and 49.6 ppm P as well as 24.6, 2.29, 0.21, 46.15 cmol(+)/kg of CEC, K, Na and Ca, respectively (Table 1). The pH value of the soil is optimum for maize production according to FAO (2006). Landon and Thevenot (1991) rated a SOC value of 2.73% medium. The TN content of the soil was 0.24%, which is in the range of medium according to Tekalign (1991) with a classification of soil TN as < 0.1, 0.1–0.15, 0.15–0.25 and > 0.25% as very low, low, medium and high, respectively. The C: N ratio falls under medium (Landon and Thevenot, 1991)). The available P and CEC are high (Tekalign, 1991). The exchangeable Ca and K are in the range of very high while exchangeable Na is in the range of low (Tekalign, 1991, Dunn et al., 2002).
Table 1
Physico-chemical properties of pre-experimental soil
Para | pH | OC | OM | TN | C:N | P | CEC | Ca | K | Na | Sa | Si | Cl | TC | BD | Po |
Value | 5.64 | 2.73 | 4.74 | 0.24 | 11:01 | 49.6 | 24.6 | 46.15 | 2.29 | 0.21 | 35.28 | 42 | 22.72 | Silt loam | 1 | 62.26 |
Note: Para – parameter, OM- Organic matter, Sa – Sand, Si- Silk, Cl – Clay, TC – textural class, BD – bulk Density, and Po – Porosity, pH (H2O), OC (%), OM (%), TN (%), P (ppm), CEC (cmol/kg), Ca (cmol/kg), K (cmol/kg), Na (cmol/kg), Sa (%), Si (%), Cl (%), BD (g/m3) and Po (%) |
Heavy metal concentration in pre-experiment soil
Table 2 presents the heavy metal concentration of the pre-experimental soil. The mean concentration of heavy metals in pre-experimental soil generally decreased in the order of Fe > Mn > Pb > As > Zn > Cr > Cd > Ni > Cu > Co (Table 2). The mean concentrations of the metals were below the maximum tolerable limit of FAO/WHO (2012) of metal concentration in soil. However, Cd was higher than the maximum tolerable limit of 3 mg/kg dry weight. This result unveils that the soil is slightly contaminated with Cd. Similar finding was reported by Tefera et.al. (2018) on the research farm in Maddawalabu University in Ethiopia, with Cd value of 0.88 mg/kg. Similarly, Bekele et al., (2020) around same general area reported a Cd value with 0.16 mg/kg. The research farm used for conducting this study had been used for undertaking various agricultural researches with relatively huge application of various forms of agrochemicals such as pesticides and fertilizers. The agro-chemicals might have attributed to the heavy metals in soil besides the inherent natural sources.
Table 2
Mean concentration (mean ± SD) of heavy metal in pre-experiment soil
| Heavy metal concentration (mg/kg dry weight) |
| As | Pb | Zn | Cd | Cu | Ni | Co | Fe | Mn | Cr |
| 16.60 ± 2.76 | 28.17 ± 1.20 | 11.23 ± 0.93 | 4.34 ± 0.90 | 0.54 ± 0.002 | 3.30 ± 0.32 | 0.33 ± 0.01 | 213.13 ± 6.56 | 129.86 ± 4.47 | 4.54 ± 0.12 |
MPL | 20.0 | 50.0 | 1000.0 | 3.0 | 300.0 | 50.0 | 50 | 50000 | 2000 | 75.0 |
MPL, maximum permissible limit for agricultural soils according to FAO/WHO (2001), SD: standard deviation |
Chemical composition and heavy metal concentration of bio-slurry
Chemical composition of bio-slurry
Table 3 shows that the bio-slurry had 7.52 pH, 6.24% OC, 0.54% TN, 11.56 C:N, 262.2 ppm, available P, 10.33 ppm available K and 39 cmol(+) kg− 1 CEC as well as 10.3, 0.39 and 52.34 exchangeable K, Na and Ca, respectively (Table 3). The bio-slurry is slightly alkaline in reaction, which is in line with the optimum range (Azim et al., 2018). The CEC value of the bio-slurry was 39 cmol(+) kg− 1, which is below the recommend value for mature compost (Azim et al., 2018). This study used fresh and non-composted bio-slurry, which might be the reason for the low CEC and C: N.
Table 3
Chemical composition of bio-slurry
Parameter | pH | OC % | OM % | TN | C:N | P (ppm) | Cmol (+)/kg |
CEC | K | Na | Ca |
Value | 7.52 | 6.24 | 10.6 | 0.54 | 11.56 | 262.2 | 39 | 10.3 | 0.39 | 52.34 |
ppm: parts per million, cmol(+): cent mol of cation |
Concentration of heavy metal in bio-slurry
The mean concentration of heavy metals in bio-slurry was found to generally decrease in the order of Fe > Mn > Zn > Pb > Co > As > Cd > Cu > Ni > Cr (Table 4). According to Canada Council of Ministers of the Environment (CCME) and USA compost quality standard, the concentration of all heavy metals, except Cd, was below the maximum recommended for compost to use as fertilizer for soil amendments (CCME, 2005, Brinton, 2000). Our findings go well with this finding because the bio-slurry used in this study is fresh and non-composted, and is sourced from cows grazing on natural pastures. Natural pastures in remote areas are often less likely to be polluted by heavy metals.
Table 4
Mean concentration (mean ± SD) of heavy metal in bio-slurry
| Mean concentration of heavy metals (mg/kg dry weight ± SD) |
As | Pb | Zn | Cd | Cu | Ni | Co | Fe | Mn | Cr |
| 10.48 ± 0.63 | 17.69 ± 0.83 | 66.13 ± 3.1 | 8.80 ± 0.22 | 7.57 ± 0.79 | 3.59 ± 0.54 | 13.39 ± 0.33 | 550.87 ± 15.20 | 322.90 ± 4.94 | 0.26 ± 0.03 |
MPL | 13* | 150* | 700* | 3* | 400* | 62* | 34* | 6154** | 471** | 210* |
MPL: maximum permissible limit, SD: standard deviation |
Source * Canada Council of Ministers of the Environment (CCME), 2005
**William et al. 2012
Effects of bio-slurry and chemical fertilizer on soil properties
Table 5 presents the results of physicochemical analysis of post-experiment soil samples collected from the 100% bio-slurry and 100% chemical fertilizer treated soils and control soils. The application of bio-slurry and chemical fertilizer influenced the physicochemical properties of the soils. The soil pH, OC, OM, TN, C: N and K increased while P and CEC declined over the control. The increase in soil pH, OC, OM, TN, C: N and K over the control soils due to the application of bio-slurry was relatively higher than that of chemical fertilizer application. The increment in these soil parameters, due to bio-slurry application, is in line with the findings from a previous study that application of bio-slurry increases those parameters over the control (Terefe et al., 2018, Haile and Ayalew, 2018, Hossain et al., 2018, Hussain et al., 2019).
The application of bio-slurry and chemical fertilizer also improved soil physical properties as compared to pre-experiment soils. In accordance, particle size distribution (sand, silt and clay) improved due to the application of bio-slurry and chemical fertilizers. In addition, the application of both bio-slurry and chemical fertilizers improved (reduced) soil bulk density as compared to the control (Table 5). Similar finding was reported in a previous study that the application of both bio-slurry and chemical fertilizers also improved (increased) soil porosity slightly over the control, which is in agreement with findings from previous studies (Kumar et al., 2015, Du et al., 2018, Yamika et al., 2019, Xu et al.,2019, Zheng et al., 2020).
Table 5
Physicochemical properties of pre-and post-experiment soil
Soil property | Post-experiment soil | Pre-experimental soil (control soil) | |
Fertilizer rate | |
100% RD BS | 100% RD CF | |
pH (H2O) | 5.9 | 5.8 | 5.64 | |
OC % | 3.12 | 2.93 | 2.73 | |
Organic matter % | 5.39 | 5.05 | 4.74 | |
TN % | 0.27 | 0.25 | 0.24 | |
C:N | 11.56 | 11.72 | 11.38 | |
P (ppm) | 24.78 | 23.01 | 49.6 | |
CEC (cmol/kg) | 22 | 18 | 24.6 | |
Ca (cmol/kg) | 52.69 | 65.87 | 46.15 | |
K (cmol/kg) | 2.53 | 2.42 | 2.29 | |
Na (cmol/kg) | 0.22 | 0.20 | 0.21 | |
Sandy % | 45.25 | 49.28 | 35.28 | |
Silt % | 20 | 26 | 42 | |
Clay % | 30.72 | 28.72 | 22.72 | |
Textural class | Loam | Loam | Silt loam | |
Bulk density | 0.98g/cm3 | 0.99g/cm3 | 1g/cm3 | |
Porosity % | 63.02 | 62.64 | 62.26 | |
| RD BS: recommended dose bio-slurry, RD CF: recommended dose of chemical fertilizer |
Effects of bio-slurry and chemical fertilizer application on soil heavy metals concentration
The mean concentrations of heavy metals in soils treated with bio-slurry declined in the order of Mn > Fe > Pb > As > Zn > Cd > Cr > Ni > Cu > Co. Likewise, the mean concentrations of heavy metals in soils, treated with chemical fertilizers decreased following the pattern of Fe > Mn > Pb > As > Zn > Cd > Cr > Ni > Cu > Co (Table 6). The concentration of most of the heavy metals declined in post-experiment soils except Cu, Fe and Zn. Applying bio-slurry increased the the mean concentrations of essential metals (Cu, Fe and Zn) in post-experiment soils. In addition, applying chemical fertilizers increased the mean concentrations of Cu and Fe in post-experiment soils. The increase in the mean concentrations of these metals may attribute to the fact that original content of the bio-slurry and probable contamination of the chemical fertilizers with the stipulated heavy metals. The increase in the mean concentrations of Zn in post experimental soil (after amendment with bio-slurry and chemical fertilizers) is in line with findings from previous studies (Akther et al., 2019, Bakar Ijaz et al., 2021) that higher Zn concentration in soils amended with compost and chemical fertilizer. Santos et al. (2010) also reported increment in Cu and Zn after different organic amendments of experimental soils. The mean concentration of Cd in soils treated with both bio-slurry and chemical fertilizers exceeded the permissible limit (FAO, 2006). This indicates that both bio-slurry and chemical fertilizer treated soil could be unsafe for agriculture with respect to Cd toxicity at least in the study area. Though the mean concentration of Cd in the pre-experiment was already above the recommendation, the application of bio-slurry further raised its concentration in the post-experiment soils.
Table 6
Effect of bio-slurry and chemical fertilizer on the mean concentration ± SD of heavy metals in post-experimental soil
Treatment | Heavy metal concentration (mg/kg of dry weight ± SD) | |
As | Pb | Zn | Cd | Cu | Ni | Co | Fe | Mn | Cr | |
Control | 16.60 ± 2.76 | 28.17 ± 1.20 | 11.23 ± 0.93 | 4.34 ± 0.90 | 0.54 ± 0.00 | 3.30 ± 0.32 | 0.33 ± 0.01 | 213.13 ± 6.56 | 129.86 ± 4.47 | 4.54 ± 0.12 |
100% BS | 12.56 ± 0.76 | 20.04 ± 0.33 | 11.26 ± 1.00 | 3.92 ± 0.70 | 0.71 ± 0.05 | 2.65 ± 0.22 | 0.31 ± 0.02 | 126.69 ± 8.32 | 126.95 ± 6.24 | 3.77 ± 0.17 |
100% CF | 14.58 ± 1.13 | 21.27 ± 2.04 | 10.69 ± 0.24 | 4.04 ± 0.08 | 0.62 ± 0.26 | 2.93 ± 0.35 | 0.30 ± 0.05 | 129.03 ± 1.69 | 125.59 ± 0.81 | 3.39 ± 0.24 | |
MPL | 20.0 | 50.0 | 1000.0 | 3.0 | 300.0 | 50.0 | 50 | 50000 | 2000 | 75.0 | |
BS: bio-slurry, CF: chemical fertilizer, MPL, maximum permissible limit for agricultural soils (FAO/WHO, 2001), SD: standard deviation | |
Effects of bio-slurry and chemical fertilizer application on heavy metals concentration in maize grain
Table 7 presents the effects of bio-slurry and chemical fertilizer application on the mean concentration of heavy metals in maize grain. In accordance, the mean concentration of heavy metals in maize grains generally followed this order: Fe > Mn > Ni > Zn > Cr, in soils treated with bio-slurry, Fe > Mn > Ni > Zn > Co > Cr in chemical fertilizer and Fe > Ni > Mn > Co > Zn > Pb > Cr in control soils. The concentrations of Cu, Co, Cd, As and Pb were under the detection limit in maize grain grown in bio-slurry treated soils. Besides, the mean concentrations of As, Cd, Cu and Pb in chemical fertilizer treated maize and As, Cd and Cu in control maize were below the detection limit (Table 7). Generally, the mean concentrations of As, Pb, Cd and Cu; As, Cd and Cu; and Co were under the detection limit in maize grain grown in bio-slurry and chemical fertilizer; bio-slurry, chemical fertilizer and control; and control, respectively.
The mean concentration of Co, Ni and Pb in maize grain was above the maximum permissible limit as per the standard set for diet of humans (FAO/WHO, 2001). The mean concentrations of Ni exceeded the permissible limit (1.63 mg/kg) in all the treatments while that of Co (0.1 mg/kg) exceeded the limit in maize grain grown in soils treated with chemical fertilizers as well as in controls. These indicate that maize grain grown in both bio-slurry and chemical fertilizer treated soils may not be safe for human consumption with regard to Ni and Co toxicities. Similarly, maize grain grown in control may not be safe attributing to its higher concentration of Pb. The concentrations of heavy metals (Zn, Fe, Mn and Cr) in maize grain grown in bio-slurry treated soils was higher than maize grains grown in chemical fertilizer treated soils. Only the mean concentrations of Ni and Co were higher in maize grain grown in chemical fertilizer treated soils. This shows that bio-slurry has higher potential of increasing heavy metal concentrations in maize grains. This might attribute to the difference in terms of heavy metal concentrations of chemical fertilizers and bio-slurry. In addition, the application of bio-slurry and chemical fertilizers differently influenced the physico-chemical properties of the soils. The accumulation of heavy metal in plants is strongly influenced by different soil parameters such as soil pH, OM, redox potential, total metal contents, and CEC (Wilson et al., 2019). At a higher pH, metals often tend to form mineral phosphates and carbonates which are insoluble whereas at low pH they tend to occur as free ionic species or as soluble organo-metals and are more bioavailable to plants (Egbeda et al.,2015). Metals are more soluble and more bioavailable in the soil solution with low pH, therefore, the heavy metal in maize gain might partially attribute to the low pH of the study soils that favors plant uptake of heavy metal. In addition, the application of fertilizers increased OM and reduced heavy metal concentration in maize grain. This might be that high organic matter content causes immobilization of heavy metals. Low organic matter content has low adsorption strength and could increase metal mobility and bioavailability (Oladejo et al., 2017).
Table 7
The mean concentration of heavy metal in maize grain
Treatment | Concentration of metals in mg/kg dry weight |
As | Pb | Zn | Cd | Cu | Ni | Co | Fe | Mn | Cr |
100% BS | ND | ND | 9.90 ± 0.24 | ND | ND | 11.29 ± 0.47 | ND | 50.66 ± 0.47 | 16.45 ± 0.98 | 0.12 ± 0.02 |
100% CF | ND | ND | 8.33 ± 0.42 | ND | ND | 12.71 ± 0.82 | 5.87 ± 1.0659 | 22.79 ± 1.03 | 15.16 ± 0.86 | 0.09 ± 0.02 |
Control | ND | 2.36 ± 0.33 | 10.77 ± 1.07 | ND | ND | 28.14 ± 3.39 | 11.28 ± 0.85 | 30.44 ± 2.91 | 16.62 ± 1.12 | 0.15 ± 0.02 |
MPL | 0.1 | 0.1 | 50 | 0.03 | 2.0 | 1.63 | 0.01 | 425 | 500 | 2.3 |
MPL, maximum permissible limit, FAO/WHO 2001, ND, not detectable |