The exact effects on plants growing in an area contaminated by spilled oil vary from plant to plant and even by the type of motor oil spilled due to the differing chemical composition of oils or the concentration of specific contaminants in used motor oil. However, at a minimum, plants coated in oil or growing in soils contaminated with oil will become stunted; unless plants are moved from the site or cleaned, they will rapidly wither and die.
Effects on Plant Growth
Because of its viscous, water-repelling nature, an oil coating on plants' leaves directly interferes with plant respiration processes. Most types of plants have pores on the undersides of their leaves, called stomata, which allow the plant to absorb the carbon dioxide necessary for photosynthesis and other plant metabolic activities, and release oxygen and other byproducts. Plants also absorb hydrocarbon components of oil through membranes covering the leaves and stems; poisoning occurs from oil interfering with metabolic processes. Another cause of plant wilting, yellowing and withering due to oil contamination is by affecting the roots' ability to absorb water and other necessary nutrients from the soil.
Unless the spills are large in volume, motor oil spills remain relatively confined to the area of ground where they initially fell. Oil gradually filters throughout the local area due to water movement and natural soil capillary action. The main effects of oil contamination in soil are to reduce the oxygen available to plants' roots embedded in the soil. If motor oil spills are limited and small in volume, bacterial cultures may degrade a portion of the oil over time. Repeated spills or spills of greater volume may persist for long periods of time, resulting in the area's reduced ability to support vigorous plant life. Plants usually will still germinate and grow, but much more slowly and to a much smaller mature size.
One gallon of motor oil, when dumped directly into water, can contaminate up to 750,000 gallons of water, with contamination defined to mean that petroleum is detectable to human taste buds. When present in groundwater, and that water is used to irrigate plants, the oil's effects may not be readily diagnosed as stemming from a petroleum-based source. Plants may begin to wither and look sickly, with yellowed foliage or a generally mottled appearance--symptoms which are often attributable to soil nutrient deficiencies. Once nutrient deficiencies have been ruled out as a cause, laboratory testing of groundwater sources is the next viable option for determining if poisoning may be caused by oil.
Certain plants, such as the Chinese brake fern (Pteris vittata) are known for their resistance to heavy metals frequently found in used motor oil, such as arsenic, lead, barium and other contaminants. Several strains of soil-borne bacteria species are also known to gradually break down oil present in soils. In the case of the Chinese brake fern, the plant's unusual ability to absorb high concentrations of arsenic from soil and store it in its leaves, without harm to the plant, makes it a target of study for phytoremediation, or the use of plants and other organisms to speed the degradation of oil in soil.
For smaller areas, the main methods of treating soils contaminated by spilled motor oil is to apply fertilizer, which increases the load of nitrogen and phosphorous in the soil. The additional nutrients will encourage a flourish of native oil-eating soil bacteria, known as hydrocarbon degraders. Consistent tilling and turning of soil will introduce higher levels of oxygen to support the growth of bacteria cultures. Tilling also allows better water penetration, which has the dual purpose of aiding bacterial growth as well as facilitating toxin dispersal and dilution.