Heavy metals status of agricultural soils should be monitored in order to prevent soil-plant pollution. This study evaluates the effect of season, agronomic practice and soil mineral composition on the levels of some heavy metals (Pb, Mn, Ni, Fe, and Zn) in the arable and oil palm soils of three Farm Settlements in Ogun-State Southwest, Nigeria. Soil samples were collected in two consecutive seasons between 2010 and 2012 and digestion was carried out using Standard Wet Acid Digestion method. Total heavy metals in the digest were determined using Flame Atomic Absorption Spectrophotometer (FAAS). The total levels of heavy metals (in mg/kg) found in the sampled soils were as follows: in the rainy season Mn (28.4 - 34.2), Fe (1599.7 - 2013.2), Pb (11.0 - 16.9), Zn (100.5 - 112.9) and Ni (11.3 - 13.8) and in the dry season Mn (32.1 - 40.1), Fe (1701.4 - 2455.5), Pb (13.0 - 18.7), Zn (105.7 - 110.4) and Ni (15.5 - 16.3). Levels of the heavy metals found in the sampled soils were significantly lower (p < 0.05) than their permissible levels in agricultural soils. Although the levels of heavy metals determined in dry season were higher than those of the rainy season, the only seasonal difference was that of Fe in Sawonjo soil significant at p = 0.05. The level of heavy metals in oil palm soils was significantly higher than the level in arable soils (p < 0.05). At present, pollution level of heavy metals in the sampled soils is low and poses no environmental risk, yet, they have to be regularly monitored before they bio-accumulate into toxic. Government should therefore set up soil monitoring agency and provide irrigation facilities to encourage dry season farming.
Heavy metals pollution in agricultural soils has become one of the global challenges facing food production and the sustainability of life. It has been reported that pollution of agricultural soils can be as a result of long-term farming or excessive use of agrochemicals [
In Nigeria, one of the cardinal programmes of the present government is food security and improved agriculture, a drive purported to take the country into the league of the first twenty economies of the world by 2020. Hence, the country is currently witnessing a rapid transformation in her agricultural programmes and policies. But, if the agricultural soils are not well monitored, particularly, in terms of heavy metals level in the soil, there is no way the country would be able to accomplish such a laudable goal by 2020. Unless the country takes into cognizance a soil management practice which is environmental friendly, it will be difficult to effectively address the problematic state of her agricultural soils, particularly, the level of heavy metals contained therein. Therefore, heavy metals in the soil need to be effectively monitored before an irreparable damage is done to farmlands.
A significant proportion of agricultural soil in developing countries like Nigeria, especially for those located in the rural areas, has not been adequately monitored or investigated to check the levels of heavy metals contained in the soil [
The study area is Ogun-State, south-western Nigeria, located within latitude 6˚N and 8˚N and longitude 2.5˚E and 5˚E. This state is predominantly tropical secondary rain forest area with a wooded savannah in the northwest [
Samples were collected in both dry season (December and February) and rainy seasons (June and September), between December 2010 and September 2012. Sampling was done using soil auger and hand trowel, plastic spatula and tape rule.
Randomized Experimental Block Design with two replications was used for this work. Stratified random sampling was used for sampling in which each farmland (arable and oil palm) was divided into plots of size 100 m × 100 m. Five plots were selected and a control site was also selected based on the diversity of farm settlement. Five samples were collected from each plot per sampling-month at 0 - 15 cm soil depth (i.e. top soil) with Geographical Position System (GPS) to record the coordinates.
Soil pH was measured in a 1:2 soil/water suspension. Soil Organic Carbon (OC) was determined by wet oxidation method of Walkley and Black [
very efficient acid mixture Nitric (HNO3), Sulphuric (H2SO4) and Perchloric (HCIO4) acid in a volume ratio of 3:1:1 was used to sufficiently complete the dissolution of 1g of each sample [
Quality assurance and quality control protocol include; procurement of analytical grade chemicals Nitric (HNO3 from Sigma Aldrich), Sulphuric (H2SO4) and Perchloric (HCIO4), recovery studies (spiking), and blanks determination. Glass wares and plastic bottles were taken through the standard analytical processes before use.
Variation in general soil properties between the arable and permanent agro-ecosystems: There are variations in some of the soil properties determined in the soils of the two agro-ecosystems but only variation among organic carbon, organic matter, nitrogen and C/N ratio are significant at p = 0.05 (
On the other hand, phosphorus%, silt% and clay% are higher (even though insignificant) under the arable agro-ecosystem than under the oil palm agro-ecosystem.
The mean pH values of all the soil samples analysed indicated that all are slightly acidic (as reported by ref [
Mean ± S.D | Sig. (2-tailed) | Is Mean Diff sig.? | Range | ||
---|---|---|---|---|---|
pH | Arable | 6.05 ± 0.31 | 0.15 | Not sig | 5.50 - 6.50 |
Oil Palm | 6.18 ± 0.34 | 0.15 | 5.40 - 6.80 | ||
Organic carbon (g/kg) | Arable | 15.9 ± 5.1 | 0.00 | Sig | 9.80 - 26.5 |
Oil Palm | 21.4 ± 6.9 | 0.00 | 12.6 - 36.1 | ||
Organic matter (g/kg) | Arable | 27.6 ± 9.0 | 0.00 | Sig | 16.9 - 45.6 |
Oil Palm | 37.5 ± 12.3 | 0.00 | 22.7 - 63.7 | ||
Phosphorus (mg/kg) | Arable | 19.2 ± 5.9 | 0.08 | Not sig | 8.66 - 29.3 |
Oil Palm | 16.3 ± 4.9 | 0.08 | 3.54 - 23.9 | ||
Nitrogen (g/kg) | Arable | 1.12 ± 0.41 | 0.00 | Sig | 0.68 - 2.20 |
Oil Palm | 1.74 ± 0.64 | 0.00 | 1.06 - 3.60 | ||
CEC (cmol/kg) | Arable | 6.27 ± 3.31 | 0.56 | Not sig | 2.69 - 11.1 |
Oil Palm | 5.73 ± 3.03 | 0.56 | 2.56 - 12.0 | ||
Exchangeable acidity | Arable | 0.23 ± 0.06 | 0.28 | Not sig | 0.15 - 0.31 |
Oil Palm | 0.26 ± 0.12 | 0.29 | 0.10 - 0.60 | ||
% Sand | Arable | 84.1 ± 4.7 | 0.39 | Not sig | 76.4 - 89.4 |
Oil Palm | 85.4 ± 5.3 | 0.04 | 72.4 - 92.6 | ||
% Clay | Arable | 7.83 ± 2.07 | 0.87 | Not sig | 5.40 - 11.0 |
Oil Palm | 7.73 ± 1.76 | 0.87 | 4.80 - 11.0 | ||
% Silt | Arable | 8.08 ± 3.38 | 0.27 | Not sig | 4.00 - 14.4 |
Oil Palm | 6.90 ± 3.83 | 0.27 | 1.40 - 16.6 | ||
C/N | Arable | 14.2 | 0.02 | Sig | 14.4 - 12.0 |
Oil Palm | 12.3 | 0.02 | 7.50 - 17.7 |
a) Seasonal Variation in the Levels of the Heavy Metals in the Soils of the Three Farm Settlements
Seasonal variation was found in the levels of heavy metals extracted from each of the farm settlements in dry and rainy seasons. Levels of most of the heavy metals extracted in dry season were significantly higher than that of rainy season (at p = 0.05) with oil palm soil samples having the highest (
b) Variation in the Levels of Heavy Metals in the Arable and Permanent Oil Palm Plantation
The result of the levels of heavy metals in arable and oil palm soils indicates that variation exist in these levels in the two agro-ecosystem. The mean level of these metals differs significantly (p < 0.05) between the arable and the oil palm plantations (
Sawonjo | Ago-Iwoye | Ajegunle | |||||
---|---|---|---|---|---|---|---|
Season | Mean ± Std. Dev | Sig. | Mean ± Std. Dev | Sig. | Mean ± Std. Dev | Sig. | |
Mn | Rainy | 42.2 ± 2.0 | 0.01 | 44.2 ± 0.8 | 0.00 | 43.1 ± 0.9 | 0.00 |
Dry | 44.9 ± 0.3 | 40.3 ± 0.4 | 47.6 ± 0.9 | ||||
Fe | Rainy | 2262 ± 27 | 0.00 | 1907 ± 96 | 0.00 | 1689 ± 90 | 0.00 |
Dry | 3030 ± 90 | 2007 ± 35 | 1711 ± 10 | ||||
Pb | Rainy | 29.7 ± 0.7 | 0.00 | 15.9 ± 0.3 | 0.00 | 22.0 ± 0.7 | 0.03 |
Dry | 32.5 ± 0.8 | 20.0 ± 0.8 | 23.1 ± 0.8 | ||||
Zn | Rainy | 138 ± 11 | 0.00 | 134 ± 32 | 0.50 | 141 ± 13 | 0.00 |
Dry | 141.1 ± 0.7 | 143 ± 29 | 144 ± 9 | ||||
Ni | Rainy | 15.9 ± 0.5 | 0.00 | 17.0 ± 0.8 | 0.00 | 18.7 ± 0.9 | 0.00 |
Dry | 22.9 ± 0.7 | 20.4 ± 0.8 | 21.4 ± 0.6 |
Sawonjo | Ago-Iwoye | Ajegunle | |||||
---|---|---|---|---|---|---|---|
Season | Mean ± Std. Dev | Sig. | Mean ± Std. Dev | Sig. | Mean ± Std. Dev | Sig. | |
Mn | Rainy | 14.6 ± 1.0 | 0.00 | 24.1 ± 1.0 | 0.00 | 21.0 ± 1.0 | 0.00 |
Dry | 19.4 ± 1.4 | 31.7 ± 1.2 | 32.6 ± 0.8 | ||||
Fe | Rainy | 1763 ± 17 | 0.00 | 1291 ± 12 | 0.00 | 2110 ± 90 | 0.00 |
Dry | 1881 ± 38 | 1395 ± 14 | 2409 ± 10 | ||||
Pb | Rainy | 4.10 ± 0.90 | 0.16 | 6.0 ± 0.8 | 0.97 | 6.7 ± 0.7 | 0.00 |
Dry | 5.00 ± 1.20 | 6.0 ± 1.1 | 9.0 ± 1.0 | ||||
Zn | Rainy | 87.5 ± 1.3 | 0.00 | 67.0 ± 1.1 | 0.00 | 68.0 ± 1.0 | 0.21 |
Dry | 79.6 ± 1.0 | 71.0 ± 0.8 | 67.3 ± 0.7 | ||||
Ni | Rainy | 6.6 ± 0.8 | 0.02 | 10.7 ± 0.9 | 0.02 | 8.7 ± 0.9 | 0.07 |
Dry | 8.0 ± 1.0 | 12.1 ± 0.9 | 9.6 ± 0.7 |
Mean ± S.D (mg/kg) | Range (mg/kg) | Sig. (2-tailed) | Is Mean Diff sig.? | ||
---|---|---|---|---|---|
Mn | Oil palm | 43.7 ± 2.5 | 39.8 - 48.7 | 0.00 | Significant |
Arable | 23.9 ± 6.7 | 13.3 - 33.6 | 0.00 | ||
Fe | Oil palm | 2101 ± 464 | 1687 - 3044 | 0.00 | Significant |
Arable | 1808 ± 392 | 1289 - 2411 | 0.00 | ||
Pb | Oil palm | 23.9 ± 5.7 | 15.5 - 33.1 | 0.00 | Significant |
Arable | 6.14 ± 1.81 | 2.97 - 10.2 | 0.00 | ||
Zn | Oil palm | 140 ± 12 | 120 - 199 | 0.00 | Significant |
Arable | 73.4 ± 7.8 | 65.3 - 89.1 | 0.00 | ||
Ni | Oil palm | 19.4 ± 2.5 | 15.2 - 23.6 | 0.00 | |
Arable | 9.29 ± 1.98 | 5.56 - 13.0 | 0.00 | Significant |
Higher levels of these heavy metals found in oil palm soil in the three farm settlements, may possibly be because crop plantations are often grown with high level of agrochemical (pesticides, herbicides and inorganic fertilizers) which have been reported to contain significant amount of heavy metals [
The rate of nutrient and heavy metals decline under annual (arable) cropping systems are often much higher than that under perennial crops because loss through leaching and soil erosion is smaller in perennial crops plantation than under annual crops [
Nearly all of the components of the photosynthetic apparatus are influenced by these heavy metals, including chlorophyll and carotenoid content, chloroplast membrane structure, light-harvesting and oxygen-evolving complexes, photosystems and constituents of the photosynthetic electron transport chain [
Though, Lead is not considered to be an essential element for plant growth and development, moreover it inhibits growth, reduces photosynthesis, interferes with cell division and respiration, reduces water absorption and transpiration, accelerates abscission or defoliation and pigmentation, and reduces chlorophyll and ATP synthesis [
In contrast, what are harvested in the palm plantations are mostly the palm nuts which do not contain the photosynthetic plasmids, and therefore may not involve translocation of these metals from the farm to the consumers. The palm fronts that drop are largely recycled back into the soil on the farm.
The total levels (in mg/kg) of Mn (30.0 - 48.6), Pb (8.25 - 22.5), Fe (1405.6 - 2243.9), Zn (86.7 - 166.2) and Ni (13.5 - 25.9) and (Tables 2-4) were all within acceptable range. Therefore, they have not yet risen to the level of pollution. The levels of all the metals analysed in the soils were significantly lower (p < 0.05) than the permissible level. This is true for the level in both the rainy and dry seasons, and in all the locations. The permissible level of Mn, Pb and Ni in the soil is between 100 - 300, 15 - 25 and 15 - 30 mg/kg respectively [
Variation found in the levels of the heavy metals determined in the soils from the two agro-ecosystems in the three farm settlements may not only due to anthropogenic source but may be due to the difference in the geological make-up of these soils. Higher level of Fe recorded in all the samples analysed may be as a result of high Fe mineralogical content of these soils that is typical of tropical soil. Abundance of iron oxide has been reported to be present in Ferruginous soil found in Sawonjo area in Ogun State [
Source | Dependent Variable | Type III Sum of Squares | df | Mean Square | F | Sig. |
---|---|---|---|---|---|---|
sampling_loc * land_use | Manganese | 175 | 2 | 87.3 | 2.21 | 0.121 |
Iron | 3,574,378 | 2 | 1,787,189 | 26.7 | 0.000 | |
Lead | 510 | 2 | 255 | 20.6 | 0.000 | |
Zinc | 3765 | 2 | 1882 | 4.64 | 0.015 | |
Nickel | 5.17 | 2 | 2.58 | 0.181 | 0.835 |
Sampling Site | Type of Mineral | Minerals Absorbed | % of the Mineral | Sampling Site | Type of Mineral | Minerals Absorbed | % of the Mineral |
---|---|---|---|---|---|---|---|
Ago-Iwoye Arable | Clay Mineral | Corensite and Kaolinite | 61.3 | Ago Iwoye Oil palm | Carbonate Minerals | Aragonite and Dolomite | 84.2 |
Iron Mineral | Heamatite | 38.5 | Iron Mineral | Siderite | 7.89 | ||
Non Clay Minerals | Quartz | 7.89 | |||||
Ajegunle Arable | Clay Mineral | Montmorillonite and Kaolinite | 23.5 | Ajegunle Oil palm | Clay Mineral | Montmorillonite and Smectite | 27.4 |
Iron Mineral | Siderite, Pyrite and Heametite | 62.3 | Iron Mineral | Lepidogocite | 13.7 | ||
Non Clay Minerals | Calcite | 14.3 | Carbonate Minerals | Dolomite | 43.8 | ||
Sawonjo Arable | Clay Mineral | Montmorrilonite | 10.81 | Sawonjo Oil palm | Clay Mineral | Illite, Chorite and Smectite | 68.7 |
Iron Mineral | Siderite and Pyrite | 75.7 | Iron Mineral | Heamatite | 6.96 | ||
Others | 13.4 | Carbonate Mineral | Aragonite and Dolomite | 15.7 |
High levels of carbonate minerals (aragonite and dolomite) found in oil palm soils of the three farm settlements may also in part, account for the higher levels heavy metals recorded in the oil palm soils of the three farm settlements. Dolomite (limestone mineral), a soil conditional that increase soil alkalinity and reduced soil acidity was indicated in the oil palm plantation soils of all the three farm settlements. This is evident in the pH value in the oil palm plantation soils of all the three farm settlements (
All metals analysed showed a significantly strong positive correlation with one another at 0.05% (
This study examines the effect of season and cropping system on heavy metals (Mn, Fe, Pb, Zn and Ni) pollution level in the farm settlements soil. Although, the pollution level in the studied sites is low, it poses no environmental risk yet; the agricultural soils need to be monitored against any incursion of toxicity. The study establishes that seasons, agronomic practise, and soil geological make-up do affect the level of heavy metals (Mn, Fe, Pb, Zn and Ni) in soils. It was also discovered that heavy metals’ pollution load in oil plantation soils was found to be higher than that of the arable soils and the metal bio-magnification is high in the soil-plant system. In view
Mn | Fe | Pb | Zn | Ni | |
---|---|---|---|---|---|
Mn | 1 | 0.58** | 0.67** | 0.59** | 0.73** |
Fe | 0.58** | 1 | 0.74** | 0.75** | 0.61** |
Pb | 0.67** | 0.74** | 1 | 0.81** | 0.82** |
Zn | 0.58** | 0.75** | 0.81** | 1 | 0.69** |
Ni | 0.74** | 0.61** | 0.82** | 0.69** | 1 |
*Correlation is significant at the 0.05 level (2-tailed). **Correlation is significant at the 0.01 level (2-tailed).
of the higher heavy metal contents in the dry season soil, government should provide facilities for, and encourage dry season irrigation farming at designated places in order to take advantage of the biochemical facilitation of the heavy metals.
Monsurat TemitopeOsobamiro,Gregrory OlufemiAdewuyi, (2015) Levels of Heavy Metals in the Soil: Effects of Season, Agronomic Practice and Soil Geology. Journal of Agricultural Chemistry and Environment,04,109-117. doi: 10.4236/jacen.2015.44012