Fertilizers are substances applied to agricultural soil to enhance plant growth and improve crop yields, supporting global food production. However, the widespread application of these chemical compounds introduces significant environmental and public health liabilities. The chemicals often escape the field, leading to contamination cycles that affect water, air, and the long-term health of the soil itself.
Contamination of Water Systems and Aquatic Life
Runoff of excess nitrogen and phosphorus is the primary danger from fertilizer use, contaminating surface water and groundwater sources. Crops absorb only a fraction of the applied nutrients, and the unused remainder is susceptible to being washed away. This movement initiates eutrophication.
Eutrophication begins when the oversupply of nitrates and phosphates causes rapid, dense growths of algae known as algal blooms. These blooms block sunlight from reaching deeper aquatic plants, which then die. When the algae and dead plant matter sink, decomposition by microorganisms consumes vast amounts of dissolved oxygen.
This depletion of oxygen creates hypoxic zones where oxygen levels are too low to support most marine life, including fish, crabs, and shellfish. The annual hypoxic zone in the Gulf of Mexico is largely driven by nutrient runoff carried down the Mississippi River watershed. The resulting loss of biodiversity fundamentally alters aquatic ecosystems, favoring tolerant species.
Beyond surface water, excess nitrogen in the form of nitrate (\(\text{NO}_3^-\)) leaches through the soil and contaminates underground aquifers used for drinking water. Nitrate poses a direct health threat when consumed. This contamination requires extensive monitoring and treatment to meet safe drinking water standards.
Direct Risks to Human Health from Handling and Exposure
Synthetic fertilizers present acute health risks during handling and application. Fertilizer dust is an inhalation hazard. Breathing these fine particulates can cause upper respiratory irritation, and prolonged exposure can lead to lung damage.
Nitrogen-based fertilizers, such as anhydrous ammonia, can release irritating gases or fumes that threaten the airways. Ammonia gas is highly water-soluble and causes severe burning and irritation in the eyes, nose, and throat upon exposure. Exposure to these concentrated chemical agents can lead to acute bronchitis or, in severe cases, chemical pneumonitis.
Contact with the skin or eyes can result in chemical irritation or burns due to the high salt index of the compounds. Accidental ingestion, particularly by children or pets, can cause acute poisoning. Ingestion of concentrated chemicals can lead to immediate symptoms such as nausea, vomiting, and severe diarrhea.
The most specific public health danger from contaminated water is methemoglobinemia, known as “Blue Baby Syndrome,” which primarily affects infants under six months old. When infants consume water high in nitrates, bacteria convert the nitrate (\(\text{NO}_3^-\)) into nitrite (\(\text{NO}_2^-\)). This nitrite enters the bloodstream and oxidizes hemoglobin, creating methemoglobin. This altered blood protein cannot transport oxygen, leading to cyanosis (blue skin color), and can be fatal if not treated.
Impact on Atmospheric Quality and Climate Change
Agricultural soil management is a major source of greenhouse gas emissions due to the cycling of applied nitrogen. Denitrification, carried out by soil microbes, converts excess nitrogen compounds into nitrous oxide (\(\text{N}_2\text{O}\)), which escapes into the atmosphere. Nitrous oxide is a powerful greenhouse gas, 265 to 300 times more effective at trapping heat than carbon dioxide.
Another atmospheric concern is ammonia (\(\text{NH}_3\)) volatilization, where nitrogen from fertilizers escapes the soil as a gas. Although ammonia is not a direct greenhouse gas, it forms hazardous air pollution. Once airborne, ammonia reacts with other pollutants to form fine particulate matter (\(\text{PM}_{2.5}\)).
These ammonium salt particles are small enough to penetrate deep into the human lungs, contributing to respiratory diseases, cardiovascular issues, and premature mortality. Furthermore, the deposition of nitrogen compounds contributes to soil acidification and regional smog. Reducing ammonia emissions is an effective way to mitigate this harmful \(\text{PM}_{2.5}\) pollution.
Long-Term Degradation of Soil Structure
Reliance on synthetic fertilizers, while providing immediate nutrient boosts, can cause long-term damage to the soil. Manufactured fertilizers contain high concentrations of soluble salts, and continuous application leads to a damaging buildup called salinization. This high salt index inhibits a plant’s ability to absorb water, which can stunt growth and reduce crop productivity.
The high chemical input disrupts the balance of the soil microbiome, a community of bacteria and fungi. When plants receive abundant synthetic nitrogen, they reduce reliance on symbiotic, nitrogen-fixing bacteria. This reduced demand suppresses the activity and diversity of beneficial soil microorganisms, creating a dependency cycle requiring more chemical input.
Continuous use of certain nitrogen fertilizers contributes to soil acidification, altering the soil’s pH level. This shift in acidity stresses the microbial community and reduces the availability of other nutrients. This degradation of physical and biological properties makes the land less resilient, requiring increased management to maintain agricultural output.