Hydrocarbons in soil may assert acute or chronic impacts to plants, animals and microbial processes if contacted. These have raised political and scientific concerns. Consequential research efforts corroborated that constitutive microorganisms contact the compounds for their metabolic activities. This may result in mineralisation, transformation and/or detoxification (biodegradation) of the compounds. Hydrocarbon biodegradation is relatively cost-effective and ecological, but often marred with limited availability to plant or animal cells (bioavailability) for metabolism. Several authors reported that growth of some plants or administration of requisite rootexudates into soil with hydrocarbons often increases hydrocarbon bioavailability for enhanced biodegradation. However, development of knowledge about this respite from plants is often founded on impacts of plants on single dose or selected mixture of hydrocarbons in soils or culture solutions. These do not; and cannot represent the heterogeneous complex mixture of numerous organic and inorganic compounds in soils where plants grow naturally. In this study, synthetic root-exudates, seedlings of lupin and ryegrass were applied separately into respective soils that were contaminated with aged and/or fresh petroleum hydrocarbons. Individual impacts of the treatments on bulk hydrocarbon concentrations, rate of microbial respiration and total numbers of culturable bacterial colonies in the soils were investigated. Results suggested that application of lupin, ryegrass or synthetic root-exudates into the soils significantly (p ≤ 0.05) induced reduction or upsurge of hydrocarbon biodegradation end-points relative to the type and concentration of hydrocarbons in soil. Thus, it is inferred that growth of plants or administration of root-exudates into hydrocarbon contaminated soils could result in enhanced biodegradation of hydrocarbons in soil.