Chapter 12

Food

Land use has increased exponentially with…

human population

What has happened more recently?

• Traditional agriculture

• Industrialized agriculture

• Green Revolution

Traditional agriculture:

Making use of human and animal labor

Industrialized agriculture (1950s-present):

mass production by machines and technology

Green Revolution (1970s-1990s):

Widespread adoption across the world

Industrialized agriculture

• Yield has increased faster than population

• We currently produce enough food to meet the calorie needs of about 10 billion people

Yield

harvested product / unit land

Farming is the…

largest use of habitable land

Challenges in Agriculture

• Population continues to grow! (expected to surpass 10 billion in 2060)

• We have passed “peak agricultural land”

• Livestock takes up more land than crops

How do we make the best use of the land we have?

• Soils?

• Pests?

• Crops, animals to farm?

Nutrients

Elements and compounds needed by plants come from soils

Biogeochemical cycles:

exchange of elements between different parts of the Earth system (land, water, air)

Macronutrients of soil

nitrogen (N), phosphorus (P), potassium (K)

Other nutrients of soil

calcium (Ca), magnesium (Mg ), iron (Fe), zinc (Zn)

Parent material of soil

P, K, Ca Mg

Organic Material of Soil

N, P, K, Ca, Mg

Atmosphere Material

N

Nitrogen cycle

• Only nitrate (NO3-) and ammonium (NH4+) are useable by plants

• Formed by decomposers and nitrogen-fixing bacteria

• Soluble in water

• Denitrifying bacteria

Denitrifying bacteria

respiration converts nitrate back to N2 gas (often in low oxygen)

Nitrogen depletion

• Harvesting of crops

• Soil erosion

• Leaching

• Low aeration in soils

• Overirrigation or flooded soils

Crop strategies for nitrogen

• Leave fields to fallow

• Crop rotation

• Cover crops

• Fertilizers

Crop rotation

Rotate fields between crops to limit nitrogen depletion

Cover crops

keep bacteria active ; take up nutrients to protect against leaching and soil erosion

Fertilizers

add more nutrients

Nutrients added by humans to supplement the supply in nature

Fertilizers

Organic fertilizer

made from the remains of living things elsewhere (e.g. compost, manure)

Inorganic (‘synthetic’) fertilizer

• made from non-living materials by chemical processes

  • Potassium and phosphorous were available before the Industrial Revolution

  • Nitrates not available in fertilizer until 1913

Haber – Bosch process (1913):

• Synthetic nitrogen

• Industrial process to form ammonia from atmospheric nitrogen

  • A difficult chemical reaction - energy intensive!

  • Needed an iron catalyst at very high pressures

• Enabled mass production of nitrogen fertilizer

Synthetic fertilizers

• Estimated 4 billion people are fed due to synthetic fertilizer!

• Supports growth of high-yield varieties of crops

• Release rapidly into soils

Eutrophication

• Excess fertilizers enters waterways

• Small inputs collect in lakes, estuaries, and coasts until they cause algae blooms

• Rise in decomposer populations as algae die

Eutrophication Impacts

• “Dead zones” with low dissolved oxygen (hypoxia)

• Harmful algal blooms (HABs)

• Impact drinking water resources

Buildup of excess fertilizers in waterways

Eutrophication

Impacts of Eutrophication

• Algal blooms – phytoplankton like dinoflagellates, diatoms,

  cyanobacteria

• Low dissolved oxygen (hypoxia)

• Harmful algal blooms – algae that produce toxic compounds

  • Microcystin in cyanobacteria is a liver toxin!

• Contamination of drinking water

Eutrophication - examples

• Gulf of Mexico “dead zone”

  • Hypoxia impacts

  • Nonpoint source pollution

• Harmful algal blooms

  • Lake Erie

  • Otterbein Lake (cyanobacteria blooms)

Soils are sensitive to chemical changes

• Plants rely on the storage of cation nutrients (+) in soil

• Organic matter and clay tend to have (-) charges, store cations

Cation exchange capacity

Ability of a soil to store cations used by plants

Cation exchange is sensitive to changes in soil

pH

Soil erosion

• Removal of soil by wind or water

• Impacts surface topsoil (O,A horizons) first

Agricultural fields are vulnerable to…

EROSION

• U.S. farmland: average loss of 4 tons/acre/year

Topsoil is eroding…

• more quickly than it is formed

• 1 inch topsoil = at least 100 years to form

• Once depleted, it is difficult for topsoil to regenerate

Tillage agriculture

• Overturning or digging through the top layer of soil to prepare soil for planting

• Promoted in 17th century Europe as a way to “bust up” nutrients in soil

• Widely available machinery and tools

Consequences of tillage agriculture

• Tillage was first promoted in humid, temperate soils of Europe

• Not suited to other biomes (like grasslands!)

Mitigating soil erosion

• Reduced or no-till agriculture

• Cover crops

• Contour farming

Reduced or no-till agriculture

Till in a limited way, use specialized equipment, or simply leave leftover crops to decompose in the field