Agricultural fertilizer, whether natural or synthetic, provides essential nutrients to soil or plants to support growth and increase crop yields. Water quality describes the chemical, physical, and biological characteristics of water, determining its suitability for various uses like drinking, recreation, or supporting aquatic life. Maintaining good water quality is important for human health, ecosystem stability, and community well-being. This article explores the relationship between agricultural fertilizer use and water quality, detailing how fertilizers applied to crops impact water systems.
Key Nutrients in Fertilizers
Fertilizers primarily supply plants with three major macronutrients: nitrogen (N), phosphorus (P), and potassium (K). Nitrogen promotes leaf and stem growth, phosphorus supports the development of roots, flowers, and fruits, while potassium aids in strong stem growth and water movement within plants. These nutrients are naturally present in the environment from sources like decaying plant and animal matter. However, agricultural practices concentrate and apply these nutrients to soil in forms plants can readily absorb, such as nitrates (a form of nitrogen) and phosphates (a form of phosphorus).
While essential for crop production, an excess of these nutrients, particularly nitrogen and phosphorus, can become problematic for water quality. Unlike some other nutrients, nitrates are not readily adsorbed by soil particles, making them prone to movement through the soil. Similarly, phosphorus, when present in high concentrations in the soil, can also move into water bodies. The increased use of synthetic fertilizers since the 19th century has allowed for larger-scale industrial agriculture, but this has also led to higher concentrations of these nutrients in agricultural landscapes than would naturally occur.
How Fertilizers Reach Water Bodies
Fertilizer nutrients move from agricultural fields into water systems through several primary mechanisms. One significant pathway is surface runoff, which occurs when rainfall or irrigation water flows over the land, picking up dissolved or particulate fertilizer. This water carries the nutrients from fields into nearby streams, rivers, and lakes. For instance, during heavy rains, excess fertilizer can be swept away from the soil and into surface waters.
Another mechanism is leaching, where water percolates downward through the soil profile. As water moves through the soil, it dissolves nutrients, particularly nitrates, and carries them deeper into the ground. This process can transport nutrients into groundwater aquifers, which are underground water sources. Once in groundwater, these nutrients can eventually emerge in surface water bodies or contaminate drinking water wells.
Several factors influence the extent of nutrient movement from fields. Soil type plays a role, as soils with poor drainage or low organic matter content may be more susceptible to runoff and leaching. Topography, or the physical features of the land, also affects nutrient transport; steeper slopes can increase the velocity of runoff, leading to greater nutrient loss. The amount and intensity of rainfall are additional factors, with heavy or prolonged precipitation increasing the likelihood of both surface runoff and leaching.
Consequences for Water Quality and Health
Once excess nitrogen and phosphorus enter water bodies, they can trigger a cascade of negative impacts, notably a process called eutrophication. This occurs when the overload of nutrients stimulates excessive growth of algae and aquatic plants, often resulting in dense algal blooms. While these blooms might initially seem like increased life, they can be ecologically disruptive. As these large masses of algae eventually die and decompose, bacteria consume significant amounts of dissolved oxygen in the water.
This depletion of oxygen can lead to hypoxic conditions, where oxygen levels become too low to support most aquatic life, creating “dead zones.” Fish and other aquatic organisms may suffocate, leading to large-scale fish kills and a reduction in biodiversity. These conditions degrade the health of freshwater and marine environments, impacting aquatic ecosystems.
Beyond ecological impacts, nitrate contamination of drinking water poses direct health risks to humans. High nitrate levels in drinking water are a particular concern for infants, as they can cause a condition known as methemoglobinemia, or “blue baby syndrome.” This condition reduces the blood’s ability to carry oxygen, which can be life-threatening. Exposure to toxins produced by certain harmful algal blooms, through contaminated water or seafood, can also cause a range of human health issues, including skin irritation, respiratory problems, and liver damage.
Managing Fertilizer Use for Water Protection
Minimizing nutrient loss from agricultural fields and protecting water quality involves implementing various management practices. One effective approach is precision application, which involves applying only the necessary amount of fertilizer based on specific crop needs and soil conditions. This can be achieved through regular soil testing to determine existing nutrient levels, preventing over-application. Timing fertilizer applications to avoid periods of heavy rainfall or irrigation also helps reduce runoff and leaching.
Establishing buffer zones, which are vegetated strips of land along the edges of waterways, can filter runoff before it enters streams and rivers. These strips help trap sediment and absorb excess nutrients, preventing them from reaching the water body. The plants in buffer zones act as natural sponges.
Practices like cover cropping and conservation tillage also contribute to water protection. Cover crops, planted between main cash crops, help to keep nutrients in the soil, reduce soil erosion, and improve soil health. Conservation tillage, which involves disturbing the soil less than conventional methods, helps maintain soil structure, increases organic matter, and enhances the soil’s capacity to retain water and nutrients. These methods collectively aim to keep nutrients in the field, where they benefit crops, rather than allowing them to escape into water systems.