5.4 - Impacts of Agricultural Practices

Learning Objectives

The primary objective is to describe agricultural practices that have environmental impacts. The focus areas are tilling, slash-and-burn agriculture, synthetic fertilizers, and monocropping. The associated skill is describing an environmental concept or process.

I. Monocropping

Monocropping is growing one species in a huge quantity across an extensive field. It’s commonly done with crops like soy, corn, and wheat.

A. Economic Advantage

Monocropping is highly efficient and tends to maximize profits in the short term.

The entire crop can be harvested at the same time, and all the plants have the same needs for irrigation, fertilizer, and pesticides.

B. Environmental Disadvantages

Decreased Biodiversity and Pest Vulnerability: Monocropping greatly decreases biodiversity. With only one species out there, the field is especially likely to fall victim to pests. Pests have nothing but their preferred food source for as far as the eye can see.

Increased Soil Erosion: The whole crop is harvested at once, so the soil sits bare for huge stretches of the year. Bare soil lacks the root structure to hold it together and the vegetation to buffer wind. As a result, the soil is far more likely to erode from wind and rain.

Loss of Habitat: Monocropping removes habitat diversity for native species. That includes removing habitats for natural predators that would otherwise act as pest control. It also cuts down pollinator habitat, which risks losing vital pollinators like bees and moths.

II. Tilling

Tilling is mechanically mixing up the soil. It’s done to make planting seeds easier and to make it easier for plant roots to grow.

Environmental Impacts of Tilling

Erosion and Nutrient Loss: Tilling loosens the soil and destroys the root structure left over from previous crops. That makes topsoil especially prone to erosion. Long-term tilling leads to the loss of organic matter, humus, and key nutrients in the topsoil.

Mechanical Compaction: Repeated tilling leaves the soil hard and compacted, creating a crust on the top layer that’s easily blown away by wind and washed away by rain.

Air Pollution (Particulate Matter): Tilling kicks soil particles up into the air. That increases particulate matter (dust and sediment) in the air, which can worsen asthma and bronchitis in humans.

Water Pollution (Turbidity): Particulate matter from tilling is likely to run off into streams, increasing the water’s turbidity.

Turbidity is the cloudiness of water, or the number of total suspended particles. High turbidity is harmful to fish that need to see to catch prey or need to take in oxygen through their gills.

Real example (the Dust Bowl): In the 1910s and 1920s, farmers on the Great Plains plowed up millions of acres of native prairie grass to plant wheat. When a long drought hit in the 1930s, the loose, bare, tilled soil had no roots holding it in place, and the wind carried it away in huge dust storms. The disaster led the US to create the Soil Conservation Service in 1935 to teach farmers soil conservation methods like contour plowing and windbreaks (5.15).

III. Slash-and-Burn Agriculture

Slash-and-burn agriculture involves cutting down (slashing) vegetation to clear agricultural land and then burning the leftover remains.

A. Advantages and Drawbacks

Benefit: Burning returns key nutrients contained in the plants back to the soil.

Drawback (Deforestation): It causes deforestation and the loss of natural habitats that serve as great reservoirs for biodiversity.

Why the farmers keep moving: Tropical rainforest soils are thin and low in nutrients, because most of the nutrients are stored in the plants themselves. The ash from burning gives the soil a short boost, but heavy rains quickly leach those nutrients away, so crop yields drop within a few years. The farmers then clear and burn a new patch, which spreads the deforestation.

B. Environmental Impacts

Air Pollution and Climate Change: Burning vegetation releases previously sequestered carbon dioxide ( $\text{CO}_2$ ) into the atmosphere, contributing to global warming.

Burning also releases other air pollutants, including carbon monoxide (a toxic gas that harms human health and indirectly adds to warming) and nitrous oxide ( $\text{N}_2\text{O}$ ), which is a really potent greenhouse gas.

It also increases particulate matter in the air, which can worsen asthma and bronchitis.

Loss of Air Filtration: The process eliminates the air pollution filtration the trees were doing.

Increased Temperature (Albedo): The process lowers albedo. The dark, scorched earth absorbs more light than the forested area did, which raises the temperature in the area and dries it out further.

IV. Synthetic (Inorganic) Fertilizers

Synthetic or inorganic fertilizers are man-made chemicals containing essential nutrients like ammonium, phosphates, and nitrates.

A. Shortcomings Compared to Organic Fertilizers

Synthetic fertilizers don’t contribute organic matter to the soil.

Organic fertilizers like compost or manure add organic matter, which increases water holding capacity and provides valuable soil microbes that lead to better rates of decomposition and nutrient recycling.

Synthetic fertilizers may boost short-term plant growth, but they fail to replenish the soil with the organic matter it desperately needs.

B. Environmental Impacts (Leaching and Eutrophication)

Leaching and Runoff: Synthetic nutrients get added in a chemical form that’s really easily leached out of the soil. Rain can wash these chemicals, especially nitrates ( $\text{NO}_3^-$ ):

Down into the groundwater.

As runoff into nearby bodies of water.

Eutrophication: The excess fertilizer washed into nearby bodies of water causes eutrophication, which is excessive growth of algae, or algae blooms.

The algae bloom blocks sunlight, so plants below the surface die.

When the algae and plants die, bacteria break them down and use up a lot of the oxygen in the water.

The eventual result is that fish suffocate and die.

Organic vs. Synthetic Release Rate: Organic fertilizer can still cause eutrophication, but it happens less quickly because the nutrients are released far more slowly as microbes decompose the organic matter. Synthetic fertilizers dump more nutrients than the soil needs, so they easily leach out and contaminate water.

Real example: Fertilizer washed off farms across the Midwest flows down the Mississippi River into the Gulf of Mexico. Every summer it feeds algae blooms that create a large low-oxygen dead zone off the coast of Louisiana, where fish and shrimp can't survive.

Big picture

These agricultural practices represent the environmental cost of maximizing short-term profit and yield. Monocropping centralizes risk by creating a single vulnerability to pests across vast areas. Tilling sacrifices the long-term health of the soil, its organic matter and nutrients, for easier planting. Slash-and-burn gives immediate soil fertility but trades permanent natural filtration systems for a burst of pollution. And synthetic fertilizers are like a quick nutrient boost that, because it isn’t properly absorbed, pollutes downstream ecosystems instead of building soil health.

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