Fertilizer is a substance containing plant nutrients added to soil to promote growth and boost crop yield. These materials, which can be natural (like manure) or synthetic, replenish essential elements such as nitrogen, phosphorus, and potassium removed from the soil during harvest. Without fertilizer, global food production would be significantly lower, making it a powerful tool for feeding a growing population. However, the misuse and overuse of both synthetic and organic fertilizers carry severe environmental and health consequences that necessitate responsible application practices.
The Threat to Aquatic Ecosystems
Damage occurs when excess nutrients wash from fields into water bodies through runoff or leach into groundwater. This influx of nutrients, primarily nitrogen and phosphorus, triggers eutrophication in rivers, lakes, and coastal zones. The excessive nutrient load fuels rapid growth of algae, leading to dense blooms that cover the water’s surface.
These blooms block sunlight from reaching submerged aquatic vegetation, an important food source and habitat. When the algae die, their decomposition consumes vast amounts of dissolved oxygen. This depletes the oxygen supply, creating hypoxic conditions commonly known as “dead zones.”
Aquatic life, including fish and shellfish, cannot survive in these low-oxygen environments. The Gulf of Mexico dead zone, driven by agricultural runoff from the Mississippi River Basin, is one of the largest in the world. Some algal blooms are also toxic, posing risks to marine life, recreational users, and drinking water sources.
Degradation of Soil Health
Excessive application of synthetic fertilizers compromises the soil they are meant to nourish. Synthetic fertilizers often contain salts that disrupt the natural soil microbiome. This disruption reduces the abundance and diversity of beneficial soil microorganisms, such as nitrogen-fixing bacteria and fungi, which are essential for nutrient cycling and plant health.
Over-reliance on chemical nitrogen fertilizers, particularly ammonium-based types, contributes to soil acidification. The process of nitrification releases hydrogen ions, lowering the soil’s pH and making it too acidic for optimal plant growth and nutrient availability. This alteration can lead to nutrient imbalances, making certain elements unavailable to plants.
Continuous heavy application also leads to salt buildup in the topsoil, known as salinization, which inhibits the plant’s ability to absorb water. These changes reduce the soil’s organic matter content, weaken its structure, and lessen its capacity to retain water. This forces farmers onto a “chemical treadmill” of needing more inputs to achieve the same yield.
Contribution to Atmospheric Pollution
Nitrogen-based fertilizers are a significant source of greenhouse gas emissions. When excess nitrogen fertilizer is applied, microbes convert the nitrogen compounds into gaseous forms through denitrification. The most concerning byproduct of this microbial activity is nitrous oxide (\(\text{N}_2\text{O}\)).
Nitrous oxide is a powerful climate-warming agent, with a heat-trapping capacity 270 to 300 times greater than carbon dioxide over a 100-year period. This gas has a long atmospheric lifetime, persisting for 114 years. Agricultural practices, largely through fertilizer use, are responsible for nearly three-quarters of all human-caused nitrous oxide emissions.
Nitrous oxide also contributes to the depletion of the stratospheric ozone layer, which shields the Earth from harmful ultraviolet radiation. \(\text{N}_2\text{O}\) is a major ozone-depleting substance. The relationship between fertilizer application rate and \(\text{N}_2\text{O}\) emissions means that applying more than a crop needs leads to disproportionately higher gaseous losses.
Protecting Public Health
The consumption of water contaminated by fertilizer runoff poses public health risks. Excess nitrogen from agricultural fields often leaches into groundwater as nitrate. This is a concern for rural communities relying on private wells, which are not subject to the same regular testing as public water systems.
The primary health threat from high nitrate levels in drinking water is methemoglobinemia, commonly known as blue baby syndrome. When infants ingest nitrate, it is converted to nitrite by bacteria in their digestive systems. The nitrite oxidizes the iron in hemoglobin, creating methemoglobin, which cannot carry oxygen.
Infants under four months are especially susceptible due to higher fluid intake relative to body weight and easily oxidized hemoglobin. The federal standard for nitrate in drinking water is 10 milligrams per liter (mg/L) as nitrogen. Exceeding this level is linked to the condition and potential risks for pregnant women, including increased risk of miscarriage or birth defects.