What Will Fertilizer Do to My Asphalt Driveway?

Asphalt Binders

• Asphalt is composed of aggregates held together with a binding agent. Petroleum asphalt is the most commonly used asphalt. Its binding agent is petroleum-based oil. When petroleum asphalt is used on paving surfaces, it takes on a hardened form and is referred to as asphalt cement. Asphalt cement, when exposed to water and compression, breaks down. These two factors, when combined, are responsible for creating potholes in streets. Liquid fertilizers have the potential to break down asphalt because of their water content. Vehicles in driveways cause compression. Liquid fertilizers, combined with vehicle weight, have the potential to damage and degrade asphalt driveways.

Asphalt Aggregate

• Asphalt's second component, aggregate, can come from a range of sources. Sand and gravel often are used, but synthetic aggregates as well as those constructed from recycled glass or automobile tires also are used. Stone and stone dust are used as aggregates. The liquid portion of asphalt hardens as it cools and binds disparate pieces of aggregate into a smooth, solid surface. Fertilizers interact with both aggregate and binding materials. The quality of the aggregate, its size, cleanliness and its source determines how fertilizer chemicals -- both dry and liquid -- react with asphalt.

Fertilizers

• Most fertilizers contain nitrogen, phosphorous and potassium as well as minor nutrients and minerals such as zinc and magnesium. Commercially sold fertilizers provide these nutrients in concentrated chemical formulas. High levels of sulfates and urea have the potential to degrade some binding agents, while other chemicals can stain aggregate materials. Fertilizer should be removed from asphalt walkways and driveways to prevent damage.

Considerations

• Many granular fertilizer mixes include a warning to brush fertilizer off asphalt surfaces such as driveways and paths. Sweeping unused fertilizer off asphalt surfaces helps ensure fertilizer does not run off driveways and enter storm drains. High nitrogen and phosphorous fertilizers damage waterways by providing algae with a highly nutritious food source. Algae grows rapidly and makes aquatic environments unhealthy for fish and other aquatic life. Fertilizers also can contaminate drinking water supplies.