Fertilizer runoff is the movement of excess nutrients, primarily nitrogen and phosphorus, from cultivated land into nearby water bodies. These nutrients are intended to boost crop growth, but when they are not fully utilized by plants, they become pollutants that disrupt natural ecosystems. Only about half of the fertilizer applied to fields is typically taken up by crops. This excess nutrient load makes fertilizer runoff a major contributor to water pollution in rivers, lakes, and coastal areas globally.
The Physical Process of Runoff and Leaching
Excess fertilizer is transported into water systems through two mechanisms: surface runoff and leaching. Surface runoff occurs when heavy rainfall or excessive irrigation causes water to flow over the topsoil. This flow picks up dissolved nutrients and nutrient-rich soil particles, carrying them into streams, rivers, and lakes. This is the primary way phosphorus, which binds tightly to soil particles, enters surface water. Leaching is the downward movement of soluble nutrients through the soil profile into groundwater aquifers. Nitrogen, especially nitrate, is highly water-soluble and does not bind strongly to soil, making it susceptible to leaching. Both processes are influenced by environmental factors like land slope, soil saturation, and the timing of fertilizer application relative to weather events.
Primary Environmental Consequences
The influx of nitrogen and phosphorus into surface waters triggers eutrophication, the excessive enrichment of a water body with nutrients. This overload stimulates the rapid growth of algae, resulting in dense algal blooms. These blooms cover the water surface, blocking sunlight from reaching submerged aquatic plants and disrupting the food web. When the algae die, their decomposition by bacteria consumes vast amounts of dissolved oxygen. This oxygen depletion creates hypoxic conditions, known as “dead zones,” where oxygen levels fall below two parts per million. Aquatic life, such as fish, shrimp, and crabs, cannot survive in these zones and often suffocate, leading to fish kills and biodiversity loss. In freshwater systems, phosphorus is often the nutrient that limits growth, while nitrogen is often the limiting factor in marine environments.
Impact on Drinking Water Quality
Contamination of potable water sources represents a serious health risk from fertilizer runoff. Nitrate, a form of nitrogen that readily leaches into groundwater, is the primary concern. The U.S. Environmental Protection Agency (EPA) set a maximum contaminant level for nitrate in drinking water at 10 parts per million (ppm) to protect public health. This standard guards against methemoglobinemia, or “blue baby syndrome,” a potentially fatal condition in infants where nitrate interferes with the blood’s ability to carry oxygen. Scientific evidence suggests that long-term exposure to nitrate levels below the 10 ppm standard may increase the risk of certain cancers. Removing nitrate requires specialized and expensive treatment processes, placing a financial burden on municipal water systems, particularly smaller, rural communities. This challenge forces some communities to implement temporary restrictions on water usage when nitrate levels spike following heavy rainfall events.
Agricultural Management Strategies
Farmers employ several strategies to minimize nutrient loss and prevent fertilizer movement into water bodies.
Nutrient Management Planning
This begins with regular soil testing to assess existing nutrient levels and determine the precise amount of fertilizer required by the crop. This allows for targeted application that matches the crop’s needs, reducing excess fertilizer available for runoff and leaching. Application timing is adjusted to avoid high-risk periods, such as before heavy rains. Splitting the total fertilizer dose into smaller applications throughout the growing season ensures nutrients are available when the plant needs them most.
Conservation Practices
Conservation tillage methods, like no-till farming, leave crop residue on the soil surface. This improves soil structure, increases water infiltration, and reduces soil erosion that carries phosphorus. Physical controls are also used to intercept the flow of water before it reaches waterways.
Cover Crops and Buffer Strips
Planting cover crops, such as rye or clover, during non-growing seasons helps absorb residual soil nutrients and keeps the ground covered, preventing erosion. Vegetated buffer strips—areas of grass, trees, or shrubs planted along the edges of fields bordering streams—act as natural filters. They slow down surface runoff and allow plants to absorb nutrients before the water enters the stream.