Agricultural pollution

Management practices, or ignorance of them, play a crucial role in the amount and impact of these pollutants. Management techniques range from animal management and housing to the spread of pesticides and fertilizers in global agricultural practices, which can have major environmental impacts. Bad management practices include poorly managed animal feeding operations, overgrazing, plowing, fertilizer, and improper, excessive, or badly timed use of pesticides.

Pollutants from agriculture greatly affect water quality and can be found in lakes, rivers, wetlands, estuaries, and groundwater. Pollutants from farming include sediments, nutrients, pathogens, pesticides, metals, and salts. Animal agriculture has an outsized impact on pollutants that enter the environment. Bacteria and pathogens in manure can make their way into streams and groundwater if grazing, storing manure in lagoons and applying manure to fields is not properly managed. Air pollution caused by agriculture through land use changes and animal agriculture practices have an outsized impact on climate change. Addressing these concerns was a central part of the IPCC Special Report on Climate Change and Land as well as in the 2024 UNEP Actions on Air Quality report. Mitigation of agricultural pollution is a key component in the development of a sustainable food system.

Abiotic sources

Pesticides

It has been conceded that in the absence of pest control measures, crop losses before harvesting would typically amount to 40 percent. Pesticide persistence is nonetheless a major issue. For example 2,4-D and atrazine have with lifetimes up to 20 years. DDT, aldrin, dieldrin, endrin, heptachlor, and toxaphene are also long-lived. The persistence of pesticides and herbicides persist depends on the compound's unique chemistry. Pesticides can also accumulate in animals that eat contaminated pests and soil organisms. The primary danger associated with pesticide application lies in its impact on non-target organisms.

In principle, biopesticides, derived from natural sources, could reduce overall agricultural pollution. Their utilization is modest. Furthermore, biopesticides often suffer the same negative impacts as synthetic pesticides. In the United States, biopesticides are subject to fewer environmental regulations. Many biopesticides are permitted under the National Organic Program, United States Department of Agriculture, standards for organic crop production.

Pesticide leaching occurs when aqueous solutions of pesticides migrate to off-target sites. Leaching is a major source of groundwater pollution. Leaching is affected by the soil, the pesticide, and rainfall and irrigation. Leaching is more problematic with water-soluble pesticides and when the soil tends to be sandy in texture; if excessive watering occurs just after pesticide application; if the adsorption ability of the pesticide to the soil is low. In addition to treated fields, Leaching may also originate from pesticide mixing areas, pesticide application machinery washing sites, or disposal areas.

Fertilizers

While providing crops with nutrients to increase crop yields, fertilizers can pose risks to human and ecological health. A typical problem is eutrophication.

Most common nitrogen sources are NO₃⁻ (nitrate) and NH₄⁺ (ammonium). These fertilizers have greatly increased the productivity of agricultural land:

With average crop yields remaining at the 1900 level the crop harvest in the year 2000 would have required nearly four times more land and the cultivated area would have claimed nearly half of all ice-free continents, rather than under 15% of the total land area that is required today.

— Vaclav Smil, Nitrogen cycle and world food production, Volume 2, pages 9–13

Although leading to increased crop yield, nitrogen fertilizers can also negatively affect groundwater and surface waters, pollute the atmosphere, and degrade soil health. Not all nutrient applied through fertilizer are taken up by the crops, and the remainder accumulates in the soil or is lost as runoff. Nitrate fertilizers are much more likely to be lost to the soil profile through runoff because of its high solubility and like charges between the molecule and negatively charged clay particles. High application rates of nitrogen-containing fertilizers combined with the high water-solubility of nitrate leads to increased runoff into surface water as well as leaching into groundwater, thereby causing groundwater pollution. Nitrate levels above 10 mg/L (10 ppm) in groundwater can cause "blue baby syndrome" (acquired methemoglobinemia) in infants and possibly thyroid disease and various types of cancer. Nitrogen fixation, which converts atmospheric nitrogen (N₂) to ammonia, and denitrification, which converts biologically available nitrogen compounds to N₂ and N₂O, are two of the most important metabolic processes involved in the nitrogen cycle because they are the largest inputs and outputs of nitrogen to ecosystems. They allow nitrogen to flow between the atmosphere, which is around 78% nitrogen) and the biosphere. Other significant processes in the nitrogen cycle are nitrification and ammonification which convert ammonium to nitrate or nitrite and organic matter to ammonia respectively. Because these processes keep nitrogen concentrations relatively stable in most ecosystems, a large influx of nitrogen from agricultural runoff can cause serious disruption. A common result of this in aquatic ecosystems is eutrophication, which in turn creates hypoxic and anoxic conditions – both of which are deadly and/or damaging to many species.. Beyond eutrophication, nitrogen pollution—specifically in the form of nitrite (NO₂⁻)—acts as a significant driver of species turnover in riverine ecosystems. Research indicates that aquatic invertebrates exhibit a wide range of sensitivities; while molluscs are relatively tolerant, aquatic insects such as mayflies and stoneflies are highly sensitive to nitrite exposure. In groups like amphipods, sensitivity varies by habitat, with upstream species (e.g., Gammarus fossarum) being more vulnerable than midstream or invasive downstream species (e.g., Gammarus roeselii). This differential sensitivity can lead to the replacement of native sensitive taxa with more tolerant ones, potentially altering ecosystem functions like leaf litter decomposition and nutrient cycling. Nitrogen fertilization can also release NH₃ gases into the atmosphere which can then be converted into NO_x compounds. A greater amount of NO_x compounds in the atmosphere can result in the acidification of aquatic ecosystems and cause various respiratory issues in humans. Fertilization can also release N₂O which is a greenhouse gas and can facilitate the destruction of ozone (O₃) in the stratosphere. Soils that receive nitrogen fertilizers can also be damaged. An increase in plant available nitrogen will increase a crop's net primary production, and eventually, soil microbial activity will increase as a result of the larger inputs of nitrogen from fertilizers and carbon compounds through decomposed biomass. Excess nitrogen can disrupt mutualisms; for example, in the legumes-rhizobia resource mutualism nitrogen deposition results in the evolution of less-cooperative rhizobia. Because of the increase in decomposition in the soil, its organic matter content will be depleted which results in lower overall soil health.

Phosphorus

The most common form of phosphorus fertilizer is phosphate (PO₄^3-). It is applied as synthetic phosphate salts as well as manure and compost. Phosphorus is essential for all life. Most agricultural crops only require a small amount of phosphorus, as such, the amount of phosphorus supplied is often excessive. Excess phosphorus typically enters surface waters. The amount that enters surface waters is relatively low compared to the amount applied as fertilizer, but phosphorus often acts as a limiting nutrient in most environments by restricting the growth of algae and aquatic plants, even small amount can disrupt an ecosystem's natural phosphorus biogeochemical cycles.

Phosphorus is largest contributor to eutrophication, especially in freshwaters. In addition to depleting oxygen levels in surface waters, algae and cyanobacteria blooms can produce cyanotoxins which are harmful to human and animal health as well as many aquatic organisms.

Cadmium is common in phosphorus-containing fertilizers varies considerably and can be problematic.

The phosphate rock used in their manufacture can contain as much as 188 mg/kg cadmium (examples are deposits on Nauru and the Christmas islands). Continuous use of high-cadmium fertilizer can contaminate soil and plants. Limits to the cadmium content of phosphate fertilizers has been considered by the European Commission. Producers of phosphorus-containing fertilizers now select phosphate rock based on the cadmium content. Phosphate rocks can also contain high levels of fluoride. Consequently, the widespread use of phosphate fertilizers has increased soil fluoride concentrations.

It has been found that food contamination from fertilizer is of little concern as plants accumulate little fluoride from the soil; of greater concern is the possibility of fluoride toxicity to livestock that ingest contaminated soils. Also of possible concern are the effects of fluoride on soil microorganisms.

Radioactive elements

From equipment and agricultural plastics

Farm machinery and equipment emitting substantial quantities of harmful gases, and operation of the machinery can result in other chemical contamination, such as oil spills, hydraulic fluid leakage, and tire particulates.

Other equipment and materials used in agriculture can introduce pollutants into the soil, and subsequently the environment. For example, the widespread use of plastic sheeting for greenhouses, plastic storage for applications like animal feed and seed bags and sillage, and plastic mulch means that both microplastic and macroplastic are increasingly introduced into farmed landscapes. A 2021 study by FAO of plastic waste in agriculture, found an estimated 10 million tons of plastic waste produced each year. Collection of agricultural plastics is very low globally, with much of never getting collected.

Land management

Soil erosion and sedimentation

Agriculture contributes greatly to soil erosion and sediment deposition through intensive management or inefficient land cover. It is estimated that agricultural land degradation is leading to an irreversible decline in fertility on about 6 million ha of fertile land each year. The accumulation of sediments (i.e. sedimentation) in runoff water affects water quality in various ways. Sedimentation can decrease the transport capacity of ditches, streams, rivers, and navigation channels. It can also limit the amount of light penetrating the water, which affects aquatic biota. The resulting turbidity from sedimentation can interfere with feeding habits of fishes, affecting population dynamics. Sedimentation also affects the transport and accumulation of pollutants, including phosphorus and various pesticides.

Tillage and nitrous oxide emissions

Natural soil biogeochemical processes result in the emission of various greenhouse gases, including nitrous oxide. Agricultural management practices can affect emission levels. For example, tillage levels have also been shown to affect nitrous oxide emissions.

Organic farming and conservation agriculture in mitigation

Organic farming

From an environmental perspective, fertilizing, overproduction and the use of pesticides in conventional farming has caused, and is causing, enormous damage worldwide to local ecosystems, soil health, biodiversity, groundwater and drinking water supplies, and sometimes farmers' health and fertility.

Organic farming typically reduces some environmental impact relative to conventional farming, but the scale of reduction can be difficult to quantify and varies depending on farming methods. In some cases, reducing food waste and dietary changes might provide greater benefits. A 2020 study at the Technical University of Munich found that the greenhouse gas emissions of organically farmed plant-based food were lower than conventionally farmed plant-based food. The greenhouse gas costs of organically produced meat were approximately the same as non-organically produced meat. However, the same paper noted that a shift from conventional to organic practices would likely be beneficial for long-term efficiency and ecosystem services, and probably improve soil over time.

Recent research has suggested that the usage of organic fertilizers and cover crops can enhance soil organic carbon, nutrient cycling, and biodiversity. Simultaneously, it could also reduce greenhouse gas emissions when it is managed effectively. These practices contribute to climate change mitigation and sustainable agriculture when improving soil resilience and long-term fertility. [Khan et al., 2024]

A 2019 life-cycle assessment study found that converting the total agricultural sector (both crop and livestock production) for England and Wales to organic farming methods would result in a net increase in greenhouse gas emissions as increased overseas land use for production and import of crops would be needed to make up for lower organic yields domestically.

Conservation agriculture

Conservation agriculture relies on principles of minimal soil disturbance, the use of mulch and/or cover crops as soil cover, and crop species diversification. It enables the reduction of fertilizers, which in turn reduces ammonia emissions and greenhouse gas emissions. It also stabilizes soil, which slows down the release of carbon into the atmosphere.