AP Syllabus focus:
‘Agricultural practices that use fertilizers can cause environmental damage, including nutrient runoff that can contribute to water-quality problems.’
Fertilizers increase crop yields by adding nutrients, but excess nutrients often leave fields in water. Understanding how nutrient runoff and leaching occur is key to explaining major freshwater and coastal pollution problems.
What Fertilizers Add and Why Pollution Happens
Farm fertilizers primarily supply nitrogen (N) and phosphorus (P), the nutrients most likely to limit plant growth. When applied in excess or at the wrong time, nutrients move off-site.
Key nutrient forms in water pollution
Nitrates (): very soluble; readily leach through soil into groundwater and also run off into streams.
Phosphates (): tend to bind to soil particles; often reach water via erosion and sediment runoff.
Pathways from Farms to Water
Runoff (surface transport)
Nutrients dissolve in water or attach to eroded soil and move into:
ditches and storm drains
streams and rivers
lakes, reservoirs, estuaries, and coastal waters
Runoff risk increases with heavy rain, over-irrigation, steep slopes, compacted soils, and bare ground.
Leaching (subsurface transport)
Water infiltrating through soil can carry dissolved nitrate to groundwater, especially where:
soils are sandy or highly permeable
the water table is shallow
fertilizer is applied before major rainfall
fields have tile drains that rapidly route water to streams
Nutrient Runoff and Eutrophication
N and P inputs can shift aquatic ecosystems from nutrient-limited to nutrient-rich, driving rapid algal growth and oxygen depletion.
Conceptual diagram showing how excess nitrogen and phosphorus enter surface waters (runoff, waste inputs, and atmospheric deposition), stimulate rapid algal/plant growth, and then drive oxygen depletion as organic matter decomposes. It visually connects nutrient enrichment to hypoxic “dead zones” and fish mortality—an ecosystem-level consequence of fertilizer pollution. Source
Eutrophication: nutrient enrichment of a water body that increases algal/plant growth and can lead to reduced dissolved oxygen and ecosystem imbalance.
Typical sequence of impacts
Added N/P stimulates algal blooms
Blooms block light, reducing submerged vegetation
Algae die; decomposition increases biological oxygen demand (BOD)
Dissolved oxygen falls, causing hypoxia (low oxygen) or anoxia (no oxygen)
परिणाम: fish kills, loss of sensitive species, altered food webs, reduced biodiversity
Some blooms involve harmful algal blooms (HABs) that produce toxins, closing beaches and shellfish beds and raising drinking-water treatment costs.
Water-Quality Problems Highlighted in APES
Freshwater vs. coastal patterns
Freshwater systems are often strongly affected by phosphorus delivered with eroded soil.
Coastal waters are frequently impacted by nitrogen delivered by rivers, groundwater, and atmospheric deposition, contributing to seasonal “dead zones.”
Nonpoint-source nature of the problem
Agricultural fertilizer pollution is usually diffuse, making it difficult to regulate compared with discharge pipes.
EPA conceptual model diagram mapping nutrient sources (including agriculture) through delivery pathways (surface runoff and subsurface waters) to ecological responses. It emphasizes how excess nutrients indirectly cause stressors such as reduced dissolved oxygen and altered food/habitat structure, which then contribute to biological impairment in aquatic communities. Source
Nonpoint source pollution: pollution from many dispersed sources (e.g., fields and lawns) that is transported by runoff or infiltration rather than a single identifiable outlet.
Because inputs are widespread and episodic (often during storms), monitoring and enforcement are challenging.
Human Health and Economic Effects
Drinking water concerns
Elevated nitrate in wells can make water unsafe, especially for infants (risk of impaired oxygen transport in blood).
Treatment may require expensive nutrient removal, or communities must find alternative water sources.
Ecosystem services and costs
Reduced recreation and tourism (murky water, odors, fish kills)
Increased water-treatment and management costs
Impacts on fisheries and aquatic biodiversity
Conditions That Increase Fertilizer Pollution Risk
Applying fertilizer on frozen or saturated ground
Over-application relative to crop needs
Poor timing (before storms) and poor placement (surface-applied without incorporation)
Lack of vegetated buffers; high erosion rates
Inefficient irrigation that increases runoff and drainage
FAQ
Phosphate ions tend to bind to clay and organic particles. When soil erodes, those particles carry phosphorus into waterways.
This is why reducing sediment loss can also reduce phosphorus loading.
Tile drains rapidly move water from saturated soils into ditches and streams. This can:
bypass natural soil filtration
increase nitrate delivery after fertiliser application
create quick nutrient “spikes” following rainfall
Nutrients can accumulate in soils and sediments and be released later (“legacy nutrients”). Groundwater nitrate can also travel slowly, so improvements may lag by years.
Internal loading is the release of phosphorus from lake sediments back into the water column, often when bottom waters become low in oxygen. It can sustain algal blooms even if external inputs decline.
They use nutrient ratio measurements (often N:P), enrichment bioassays, and monitoring of seasonal changes. Shifts in limitation can occur as land use, runoff timing, and climate-driven stratification patterns change.
Practice Questions
State two ways that fertiliser use on farms can reduce water quality in nearby rivers or lakes. (1–3 marks)
Nutrients (nitrates/phosphates) run off into surface water, increasing nutrient concentrations. (1)
Nutrients cause algal blooms/eutrophication leading to reduced dissolved oxygen (hypoxia/anoxia) and harm to aquatic organisms. (1)
Explain how nitrogen- and phosphorus-based fertilisers applied to cropland can lead to eutrophication and fish kills. In your answer, include (i) one transport pathway, (ii) the role of decomposition, and (iii) one reason the problem is difficult to control. (4–6 marks)
Identifies a transport pathway: runoff to streams/lakes OR leaching/tile drainage to waterways/groundwater. (1)
Notes fertiliser adds nitrogen/phosphorus, increasing nutrient availability in water. (1)
Links nutrient enrichment to algal bloom/rapid primary production. (1)
Explains algae/plant biomass dies and is decomposed by microbes. (1)
Decomposition increases oxygen demand and lowers dissolved oxygen causing hypoxia/anoxia and fish kills. (1)
States a control challenge: nonpoint source/diffuse inputs; storm-driven pulses; difficult monitoring/enforcement. (1)
