## 2 Effects of PH on the Growing Environment and Plants  
The behaviour of PH in the environment determines where they are likely to be found, such as in air, water, soil, sediment, food, or other media that people might come in contact with [27]. In the view of [PERSON] et al. [28], the factors governing the spread/migration of spilled oil in soil include the amount of oil spilled, physical properties of the spilled oil (density and viscosity), and physical properties of the soil medium itself, particularly its porosity. Although the introduction of PH affects the physical, chemical, and biological properties of soil [7], these factors (i.e., the soil type, quantity, and composition of the spilled hydrocarbon) also determine the level of alteration of the soil properties [27; 29]. On agricultural lands, [PERSON] [30] and also [PERSON] [31], reported that oil spills can result in reduced plant growth for some time. PH contamination which is associated with an increase in carbon/nitrogen (C/N) ratio and nitrogen deficiency [32], may also lead to a reduction in plant stem height, stem density, aboveground biomass, and include death of the plant [33; 34; 35; 36; 37]. Meanwhile, the response of plants to the presence of PH in soil varies [20; 38; 39] and is associated with the plant age [40], species of the plant involved, type and concentration of the petroleum, time of exposure to the contaminant [20; 29; 38] as well as the season [36; 41], among other factors. [PERSON] [31], referring to the experiment of [PERSON] [42], indicated the possibility of a low concentration of crude oil in the environment supporting plant growth. Although this test was conducted in duplicate, [PERSON] [42] found out that at 0.75% _w/w_ of crude oil addition to the soil, the growth of soybeans was improved. [PERSON] et al. [19] also recorded an increase in shoot height and weight of two of the plant species examined at 1% _w/w_ oil-in-soil. This stimulating effect according to [PERSON] [30] may be as a result of the bacterial breakdown of the hydrocarbons. Other possible reasons for plant growth-stimulation,  
Figure 1: Chemical classification of petroleum hydrocarbons. (MAH = Monoaromatic hydrocarbons; PAH = Polycyclic aromatic hydrocarbons). Source: [PERSON] and [PERSON] [26]. as suggested by [PERSON] [38], although subject to further investigation, could be attributed to the release of nutrients from the oil, oil-killed vegetation, or hormonal influence. Be that as it may, there are claims that this occurrence in plants is a result of 'petroleum auxin' identified as naphthenic acids, which (i) improve the yield of different varieties of crops; (ii) stimulates photosynthesis; and (iii) increases protein nitrogen [38]. Notwithstanding possible positive effects, most studies regarding the growth of plants in soil containing PH have reported negative effects [20] including on the germination of seeds [18; 20]. Reductions in the plant growth could be a result of (1) the direct toxic effect of oil on plants [31; 38]; (2) absence of viable seeds leading to lack of germination; (3) reduced germination; (4) unfavourable soil conditions [31]; and (5) inhibition of bacterial decomposition of the soil organic matter and associated nutrient remineralization by the toxic components of oil [40]. The inhibition of germination may be linked to oil entering the seed and killing the embryo, or oil coating the seed and hindering the uptake of oxygen and water required for germination [38]. [PERSON] et al. [32] added that reduced seedling emergence could be as a result of toxic effects of the oil or from the adverse soil moisture conditions. Other effects of oil pollution on plants as stated by [PERSON] [38] include yellowing and death of oiled leaves, varying sensitivity, and recovery rates of perennials, among others, while a complete elimination may occur at chronic levels. Heavier oils compared to lighter oils present less immediate toxic impact on plants and other organisms [40].