Soil and Agriculture: Key Concepts for Environmental Science

Soil Horizons

Layers in mature soil: O (surface litter) , A (topsoil) , B (subsoil) , C (parent material).

Soil Profile

Cross-section showing all soil horizons.

Soil Triangle

Diagram showing % of sand, silt, and clay; used to determine soil texture, porosity, and permeability.

Loam

Ideal soil with equal parts clay, sand, silt, and humus; good fertility and drainage.

Soil Formation

Soils are formed when parent material is weathered, transported, and deposited.

Soil Functions

Filter and clean water, store nutrients.

Water Holding Capacity (Water Retention)

Total water a soil can retain; varies with soil type and contributes to land productivity and fertility.

Soil Testing

Determines porosity, permeability, fertility; informs irrigation/fertilizer needs.

Soil Conservation

Reducing erosion and restoring fertility through terracing, contour farming, strip cropping, alley cropping, no-till, etc.

Minimum-tillage farming

Loosens subsurface soil without overturning topsoil.

No-till farming

Seeds/fertilizer injected directly into unplowed soil to reduce erosion.

Terracing

Broad, nearly level steps cut into steep slopes to prevent erosion.

Contour farming

Plowing and planting crops across, rather than up/down, a gentle slope to reduce runoff.

Strip cropping (intercropping)

Alternating rows of one crop with another to prevent erosion.

Alley cropping (agroforestry)

Alternating rows of trees or shrubs with crops.

Windbreaks (shelterbelts)

Trees planted along cultivated land to reduce wind erosion.

Gully reclamation

Planting quick-growing plants in gullies to catch and hold sediment.

Organic fertilizers

Animal manure, green manure (fresh vegetables), and compost (layers of N/C wastes, and topsoil); restore nutrients naturally.

Inorganic fertilizers

Manmade chemical fertilizers (NPK).

Strategies to Improve Soil Fertility

Includes crop rotation and the addition of green manure and limestone.

Agricultural Impacts (Environmental Damage)

Greater harmful impact than any other human activity; includes biodiversity loss, erosion, desertification, pollution, and aquifer depletion, often caused by tilling, slash-and-burn, and fertilizer use.

Industrialized agriculture

High-input farming using large amounts of fossil fuels, water, commercial fertilizers, pesticides, and monocultures.

Plantation agriculture

Industrialized farming in tropical developing regions (e.g., bananas, coffee).

Traditional subsistence agriculture

Produces only enough crops or livestock for a farm family's survival.

Traditional intensive agriculture

Produces enough food to feed their family and to sell for market income.

Green Revolution

Shift to scientifically bred, high-yield grain crops using heavy input of fertilizers, pesticides, and irrigation; increases productivity but increases reliance on fossil fuels and harms the environment.

Steps of Green Revolution

1. Developing and planting monocultures. 2. Lavishing fertilizer, pesticides, and water on crops. 3. Increasing cropping intensity and frequency.

CAFOs (Concentrated Animal Feeding Operations/Feedlots)

Industrial livestock systems for quickly getting animals to slaughter; tend to be crowded, generate a large amount of organic waste/pollution, but result in lower costs for consumers.

Free-range grazing

Animals graze naturally on grass during their entire lifecycle; meat is often free from antibiotics/chemicals, and waste acts as fertilizer; requires more land and is more expensive for consumers.

Livestock Downsides

Causes 14% topsoil loss , methane emissions (12-15% of all atmospheric methane) , waste (21x U.S. human population waste) , and water pollution (especially hog farms in NC due to high density and flood-prone coastal location).

Rotational Grazing

The regular rotation of livestock between different pastures in order to avoid overgrazing in a particular area.

Overgrazing

Too many animals feed on a particular area, causing loss of vegetation, leading to soil erosion and desertification.

Reducing Meat Benefits

Less CO₂, methane, and N₂O emissions; better water and soil conservation; reduced use of antibiotics and growth hormones.

Undernutrition

Caloric intake below 90% of minimum daily requirements on a long-term basis.

Malnutrition

Insufficient protein/nutrient intake.

Overnutrition

Too many calories leading to obesity, diabetes, heart disease, and high blood pressure.

Pests

Any organism that competes with humans for resources or causes damage where we don't want it.

Pesticides

Chemicals used to kill pests; include insecticides, herbicides, fungicides, nematocides, and rodenticides.

First-gen pesticides

Contain natural toxins (arsenic, lead, mercury).

Second-gen pesticides

Synthetic (DDT, chlordane, methyl bromide).

Pesticide benefits

Save lives (from diseases like malaria), increase food supply, lower food cost, increase profits, and work faster than alternatives.

Ideal pesticide

Targets only pests , breaks down safely/disappears into something harmless , doesn't cause genetic resistance , harms nothing else , is cheaper than doing nothing.

Pesticide problems

Genetic resistance is the number one problem , pollution (at least 95% don't reach target pests) , non-target deaths (killing natural predators) , health threats, ecosystem disruption, and arrival of new pests.

Bhopal disaster (1984)

Union Carbide pesticide leak in India released 43 tons of methyl isocyanate gas, killing up to ~5,000 people.

Pesticide regulation

Managed by EPA, FDA, USDA; focuses mostly on tolerance levels of residue on foods, not environmental effects.

Delaney Clause (1958)

Bans carcinogenic additives in food (part of the Food, Drug and Cosmetic Act).

Pesticide alternatives

GMOs, biocontrol (natural predators), crop rotation, biopesticides (plant toxins), insect birth control, and IPM.

Integrated Pest Management (IPM)

Uses a combination of biological, cultural, and limited chemical methods to effectively control pests while minimizing disruption to the environment. Methods include biocontrol, intercropping, crop rotation, and natural predators.

IPM Benefits/Drawbacks

Reduces the risk that pesticides pose to wildlife, water, and human health, and minimizes environmental disruption , but can be complex and expensive.

Global Water Distribution

97% saltwater , 3% freshwater (mostly locked in ice caps/glaciers) ; only 0.024% usable (in soil moisture, groundwater, water vapor, lakes, and streams).

Surface runoff

Precipitation that doesn't infiltrate or evaporate.

Watershed (drainage basin/river basin)

Area of land where all water drains into a specific water body.

Groundwater

Water that infiltrates into soil/rock layers.

Zone of saturation

Depth where all pores in soil/rock are filled with water.

Water table

Upper boundary of the zone of saturation.

Aquifer

Porous, water-holding rock/sand/gravel layer underground through which groundwater flows.

Recharge area

Land area through which water enters an aquifer.

Aquifer depletion

Severe overuse of groundwater for agricultural irrigation (e.g., Ogallala Aquifer).

Aquifer subsidence

The sinking of land when groundwater is withdrawn, often at 4x the replacement rate in the U.S.

Saltwater intrusion

Seawater enters aquifers near coasts, rendering them unusable for terrestrial organisms.

Freshwater use breakdown

70% irrigation (largest human use) , 20% industry, 10% domestic.

Irrigation methods (most → least efficient)

1. Drip (5% water loss; most efficient, most expensive). 2. Spray (≤25% water loss; more efficient than flood/furrow, but more expensive and requires energy). 3. Furrow (≈33% water loss; inexpensive). 4. Flood (20% water loss; can lead to waterlogging).

Waterlogging

Occurs when too much water is left in the soil, raising the water table and inhibiting a plant's ability to absorb oxygen through its roots.

Salinization

Salts in groundwater remain in the soil after water evaporates; can make soil toxic to plants.

Water scarcity causes

Dry climate, drought, desiccation, and water stress.

Water stress

Too many people relying on limited levels of runoff.

Tragedy of the commons

Shared resource depletion due to overuse (one of three forces, with population growth and unequal access, that lead to unsustainable resource use).

Water conservation methods

Shorter showers, low-flow devices, evening watering (to minimize evaporation), and turning off taps while brushing teeth.

Watershed transfer

Moving water from one basin to another; harms ecosystems downstream by reducing flow (e.g., Cape Fear River) and causing greater flooding upstream (e.g., Neuse River).

Dams and reservoirs

Store water for power, irrigation, flood control, and recreation; disrupt ecosystems, destroy habitats, and interfere with fish migration.

Colorado River case

Overused by southwest US; rarely reaches the Gulf of California due to overexploitation and major dams.

Three Gorges Dam (China)

World's largest dam; produces power and reduces flooding, but displaced 5.4 million people and is built on a seismic fault.

Aral Sea disaster

Once the world's 4th largest freshwater lake; shrank significantly since 1960 due to diversion for irrigation.

Desalinization

Removes salt from ocean/brackish water via distillation or reverse osmosis; energy-intensive, expensive (3-5x the cost of other water sources), and not affordable for most.

Floodplain

Flat area along rivers that floods naturally; people settle there due to fertile soil, good for aquifer recharge, and wetland habitats.

Flood management methods

Channelization (straightening/deepening streams) , levees/dams , wetland restoration , and floodplain management/relocation.

Aquaculture

Fish farming; highly efficient, requires small areas of water, and little fuel . Drawbacks: may spread disease/invasive species, contaminates wastewater, and escaped fish may compete/breed with wild fish .