Petroleum or crude oil is a natural product which has very wide range of uses. In the oil producing states like Assam (India), different companies are engaged in exploration, production and transportation of crude oil. A large number of contaminants including hydrocarbons and heavy metals enter into the nearby areas of an oil collecting station (gathering station) through spills, leaks as well as through emissions from gas flaring and from effluents which are likely to pollute the environment.

If oil is spilled on the surface of soil during the drilling operation the hydrocarbons gradually penetrate into the soil system. It has been found that the oil hydrocarbons can infiltrate up to a depth of 50-cm. (Ilangovan and Vivekanandan, 1992). It is known that rainfall prior to or during the spills reduces oil infiltration into the soil and washes petroleum components away to runoff waters (Francke and Clark, 1974). The oil concentration in the soil of the contaminated field decreases with time. Initially the oil concentration is high in the upper 1 to 30-cm layer but after six months, the rapid decrease of oil in the upper layer reverses the situation. Biodegradation, evaporation and leaching could be considered as causes for the decrease but leaching has been shown to be not very significant (Raymond et al., 1976; Dibble and Bartha, 1979a and b). Biodegradation is the metabolic activity of microorganisms to transform or mineralize organic contaminants into less harmful, non-hazardous substances, which are then integrated into natural biogeochemical cycles (Margesin and Schinner, 2001). The time required for degradation of petroleum hydrocarbons in soils depends on the chemical compositions of crude oil, the climatic conditions and the properties of soil. Unless all the proper conditions are met for a given compound, biodegradation is not likely to occur (Bitton and Gerba, 1985).

Oil infiltration into the soil modifies the soil properties and hence may deteriorate the natural environment (Kalita et al., 2007). Considerable effort is being made to remediate soils contaminated with petroleum hydrocarbons, heavy metals and other organic and inorganic compounds that have resulted from industrial activities, accidental spills and improper waste disposal practices (Saldana et al.,2005). Remediation can lead to quick recovery of the polluted soils Gradi 1985, Alexander 1978). Biodegradation of oil is one of the most important processes for the eventual removal of petroleum from the environment, particularly for the non-volatile components of petroleum (Albert and Xueqing, 2003).

Soil conditions are often controlled to increase the rate of contaminant degradation (Odu, 1978, Gradi, 1985). In the biodegradation process, pH of the soil plays a predominant role. The growth and activity of soil microorganisms are very much dependant on the soil pH. For example, fungi predominate under acidic conditions (pH<7) while bacteria and actinomycetes predominate near pH 7.0. A soil pH of 7.8 has been shown to be the optimum pH for microorganisms’ growth (Dibble and Bartha, 1979a). The soil pH regulates the solubility, mobility, and the availability of the ionized forms of contaminants (JRB Associates, Inc., 1984).] Changes in salinity may affect oil biodegradation through alteration of the microbial population (Ward and Brock, 1978).

Microbial activity is significantly controlled by the availability of nutrients, especially nitrogen and phosphorus (Margesin and Schinner, 1997). The additions of nutrients are necessary to enhance the biodegradation of oil pollutants (Choi et al., 2002). The provision of limiting nutrients has been shown to be effective in stimulating hydrocarbon degradation after oil spills. One possible explanation may be that the presence of nutrient stimulated the activity of indigenous bacterial populations sufficiently to enhance the degradation of the crude oil (Pitchard et al., 1992; Swannell et al., 1999). In theory, approximately 150mg of nitrogen and 30mg of phosphorus are utilised in the conversion of 1g of hydrocarbon to cell materials (Rosenberg and Ron, 1996).

The results obtained by Coulon et al. (2004). clearly show that fertilizer addition improves the rate of degradation of both n-alkanes and polycyclic aromatic hydrocarbons (PAHs) in sub-antarctic soils contaminated by petroleum hydrocarbons. The use of lipophilic fertilizer to enhance microbial utilization of crude oil has also been investigated by different authors (Olivieri, et al., 1978, Abu and Ogiji, 1996) and it has been found that the efficiency of a fertilizer depends greatly on the environment and design of the treatment protocol (Riser-Roberts, 1998). In a different study Coulon et al. (2005) found significant positive effects of a commercial oleophilic fertilizer containing N and P (and also of temperature) on hydrocarbon-degrading microbial abundance and total petroleum hydrocarbons (TPH) degradation. Chayneau et al. (2005). have also reported the negative effect of a high N-P-K level on the biodegradation of hydrocarbons.

The present study was designed with an aim to investigate the effect of soil pH and addition of N-P-K fertilizer on degradation of hydrocarbons present in crude oil contaminated soil by carrying out detailed laboratory scale study.