​Impact Assessment of Oil Exploration Activities on Soil Fertility in Delta State, Nigeria

Soil is one of the most critical natural components used for cultivation but worst affected by human activities. Human utilization of land negatively affects the quality by changing its biological, chemical and physical properties. Fertile lands used to produce enough food crops have become infertile due to the oil exploration and this had made farming to be very difficult (Godson-ibeji et al., 2016), threatened the communities’ potential for sustainable production of food (O Rourke and Connolly, 2003). Large quantities of oil are transported by pipelines that pass through farming communities and there are oil leakages. The oil can clogged soil particles thereby reducing water infiltration and increasing bulk density which hinder root penetration (Ewetola, 2013). It can form film on planting materials which retard water and oxygen absorption and also inhibit the activities of starch phosphorylase that reduces assimilation of starch (Oyem and Oyem, 2013).
       
Gas flaring, one of the stages in oil exploration generates enormous heat that make the surrounding soils to get scorched and the vegetation looking parched (Giwa et al., 2014). Crops grown in this type of environment will definitely show symptoms of leaves chlorosis and plant dehydration. According to Ukegbu and Okeke (2007), growth and yield of crops reduced due to flow station located in the areas. Substances that are introduced into the surrounding environment during oil exploration can change soil compositions and this can drastically reduce its productive capability (Achi, 2003). Crude oil can impair metabolic processes by reducing carbon-mineralizing capacity of micro-organisms. It can alter soil chemical properties by increasing acidity and in this condition, nitrogen fixation and organic matter decomposition are hindered (Osuji and Nwoye, 2007).
               
The revenue generated from crude oil had improves the nation economy and this had increased budgetary allocations (Nwilo, 1998). The dwindling prices recently made the government to look at how agricultural productivity can be improve to eradicate hunger in Nigeria. Efforts made so far are in areas of provision of improve varieties, fertilizers and training of Agricultural Extension Agent (Irhivben and Omonona, 2013). However, not much work has been done on soil fertility assessment in spite of fact that it is the bed rock of agriculture. It is now pertinent to evaluate the soil fertility status in oil producing communities. The finding will give an insight to the extent oil exploration has depletion the soil for remedial action to be taken. Hence this study, investigated the impact of oil exploration on soil fertility in some farming communities in Delta State, Nigeria.

Study site
 
The study was conducted in four local government areas [Aniocha South, Ika South (non oil producing), Ukwani and Ndokwa (oil producing)], Delta State in 2020. Oil producing State in Nigeria with estimated population of 193,392,500 and land area of 17,698 sq. kilometers (NPC, 2006). It lies within latitude 5°00 and 6°30' North and longitude 5°00 and 6°45' East and characterized by two seasons: Dry season (November - April) and rainy season (May - October). Annual mean rain fall ranged from 1850 - 2250 mm, temperature ranges between 25.7 and 28.3°C. Daily relative humidity varied from 63 to 76% with evaporation of about 1450 mm/y (NIMET, 2017). The soils generally were Entisols and of the Suborder Eutric Tropofluvent (FDALR, 1985). The vegetation is unique characterized by grasses, shrubs, timber species and oil palm and rubber plantations were the major cash crops. The relief is gentle undulating plain to nearly level topography in some cases. The drainage could be well drained or poorly drained depending on the time of the year. Land use is based on rain fed agriculture and crops cultivated include cassava, yam, maize, plantain including okra, pepper in large quantities.
 
Soil sampling and parameters measured
 
Three communities in each local government area were selected and cassava farm that was largely cultivated was assessed. Auger point investigation was carried out at 10 meters intervals along transverses cut at 100 m apart. Mini soil profile at depth of 30 cm was dug randomly for soil samples collection. Soil pH, organic matter (OM), total nitrogen (TN), available phosphorus (P), calcium (Ca), magnesium (Mg), potassium (K) and sodium (Na) in the soil were measured.
 
Laboratory procedures
 
Soil pH was on a ratio of 1:2 soil/water suspensions. Soil organic matter was analyzed by Walkley and Black method (Nelson and Sommers, 1982). Total nitrogen was determined by micro - kjeldahl digestion method (Jackson, 1962). Available phosphorus was measured with Bray (II) (Olsen and Sommers, 1982). Exchangeable bases were extracted with 1N NH₄OAC, Ca and Mg were read with ethylene diamine tetra-acetic acid titration method while K and Na were read with flame photometry (Jackson, 1964).
 
Statistical analysis
 
Data generated were subjected to descriptive statistics and analysis of variance (ANOVA) while least significance difference (LSD) at 5% probability level was used to separate means. Correlation analysis was used to show relationship among nutrient parameters with SAS software version 9.0.
 
Nutrient index determination
 
Parker’s nutrient index (Table 1) modifies by Kumar et al., (2013) was used to rate soil fertility level in the LGAs with reference to Chude et al., (2011) soil fertility chart (Table 2). 
 
Where,
a = Number of samples in lower category.
b = Number of samples in medium category.
c = Number of samples in higher category.
ns = Number of samples.
 

 

Soil fertility status of the farming communities in Ukwani and Ndokwa West
 
Soil fertility indices in the oil producing communities are presented in Table 3. All the parameters measured were not significantly different except available P in Ukwani whereas, organic matter, total N and exchangeable K were significantly different in Ndokwa. Soil pH ranged from 5.27 to 5.33±0.09 with a mean of 5.30±0.08 in Ukwani, 5.70 to 5.90±0.22 with a mean of 5.83±0.08 in Ndokwa. Organic  matter was lowest in Ebedei and highest in Abi. It ranged from 1.42 to 1.64±0.12% with a mean of 1.49±0.11% in Ukwani, 1.56 to 2.02±0.13% with a mean of 1.74±0.11% in Ndokwa. Total N was highest Abi in Ndokwa. Available P ranged between 9.57-11.50±0.47 mg/kg with a mean of 10.14±2.27 mg/kg Ukwani and ranged between 10.83-14.90±1.25 mg/kg with mean of 12.52±0.81 mg/kg in Ndokwa. The values Ca ranged from 0.18-0.58±0.12 in Ukwani and ranged from 1.14-1.41±0.13 in Ndokwa. Magnesium, K and Na content of soils were generally high except in Ukwani LGA.
 

 
Soil fertility status of the farming communities in Ika South and Aniocha South LGA
 
Soil pH values were not significantly different at Ika South but significant differences in Aniocha South (Table 4). The values ranged from 6.53 to 6.70±0.1 with a mean of 6.61±0.08 in Ika South and 5.97 to 6.70±0.05 with a mean of 6.27 in Aniocha South. Organic matter was significantly different in both LGAs. It ranged from 1.90 to 2.97±0.26% with a mean of 2.54±0.11% in Ika South and 2.10 to 3.00±0.17% with a mean of 2.48±0.11% in Aniocha South. Total N was not significantly different at Ika but was significantly different in Aniocha South. Available P was not significantly different in both LGAs. Calcium was generally high in both LGAs while potassium and sodium were generally low.
 

 
Soil fertility status at a glance in the four LGAs
 
Soil pH values in Ika and Aniocha were higher than Ndokwa and Ukwani (Table 5). Organic matter was moderate in Ika and Aniocha but low Ika and Aniocha. Total N was high Ika and Aniocha whereas it was low Ndokwa and Ukwani. Available P was moderate in all the LGAs. Calcium was high in Ika and Aniocha while it was low Ndokwa and Ukwani.
 

 
Soil nutrient variability in the LGAs
 
Soil pH was less variable in the LGAs while OM was less variable in Ukwani and moderately variable in Ndokwa, Ika South and Aniocha South (Table 6). Total N was moderately variable in Ukwani and Udokwa LGA and highly variable in Ika South and Aniocha South. Available P was less variable in Ukwani and Ika South LGA while moderately variable in Ndokwa and Aniocha South. Calcium was highly variable in Ukwani, Ika South and Aniocha South whiles it was moderately variable in Ndokwa. Magnesium and K were highly variable in Ndokwa, Ika South and Aniocha South and moderately variable in Ukwani while Ca was moderately variable in all the LGAs.
 

 
Soil nutrients index in the LGA
 
Soil fertility index are presented in (Table 7). Though, the study areas were generally acidic, Ukwani was more acidic (Strong acidity) while Ndokwa was moderate acidity. Ika South and Aniocha South were slightly acidity. Out of the 7 soil fertility indicators measured, 5 were high and 2 were medium in Ika South. In Aniocha South, 4 were high and 3 were moderate. Contrarily, only Mg was high while other parameters were low and medium in Ndokwa. In Ukwani, 4 parameters were low, 1 was moderate while 2 were high.
 
  
       
Soil pH in the four LGAs is rated as strongly to slightly acidic. The pH range (5.25 - 5.90) in Ukwani and Ndokwa may probably affects nutrient availability leading to the lower level of nutrient recorded. Strong acidic soil conditions hindered organic matter decomposition and nitrogen fixation in soils (Osuji and Nwoye, 2007). The low pH in some communities may affect soil microbial behavior and slow  the breakdown of organic material that releases plant nutrients (Oyem and Oyem 2013) causing the lower nitrogen contents in oil producing communities. The nutrient contents in non oil producing LGAs were higher than oil producing areas. This may be linked to the oil exploration activities that reduced the activities of soil microorganisms.
       
Organic matter in oil producing communities was lower than the non oil producing communities. Chemicals released into the soil during oil exploration might impair metabolic processes which add organic matter (Osuji and Nwoye, 2007), leading to carbon-nitrogen ratio imbalance in the soil (Nkwopara et al., 2012). This could cause nitrogen depletion in oil producing communities. The lower organic matter in oil producing communities compared to non oil producing with similar temperature can be attributed to oil exploration activities.
       
Available phosphorus measured in non oil producing communities were higher this might be attributed to acidic nature in the oil producing community soils that can cause phosphorus fixation (Nkwopara et al., 2012). It has been recorded that phosphorus solubility is maximized at pH of about 6.5 (Wang et al., 2013) so, the lower pH values found in oil producing communities could also reduce available phosphorus. The microbes which utilize total petroleum hydrocarbon as carbon source can also utilize P when degrading hydrocarbons in the soil (Wang et al., 2010).
       
The level of calcium and magnesium in non oil producing communities invariably implied that oil exploration negatively impacted the elements.  It was observed that basic cations can lost in soil with higher acidity (Abii and Nwosu, 2009). The effects of oil exploration activities can be seen from the significant differences between the nutrient in oil producing and non oil producing communities’ soil. Oil exploration had no effect on K and Na though, Na is not an essential element so it is not an index for optimum crop production (Abii and Nwosu, 2009).

The nutrient contents in Ika south and Aniocha South (non oil producing LGAs) were significantly higher than those in Ukwani and Ndokwa (oil producing LGAs). The correlation studies depict that organic matter and total nitrogen negatively correlated with soil pH. The oil producing communities need nutrients amendment most especially nitrogen, phosphorus and potassium to prevent the soil from further degradation.