Environmental Science Agriculture and Food Production 2 — Questions and Answers
Question 1: The Ogallala Aquifer, which underlies the U.S. Great Plains, is a critical concern for agriculture because:
- It supplies drinking water to New York City and other major eastern cities
- It is being depleted by irrigation much faster than it is naturally recharged (Correct answer)
- It contains high levels of arsenic that contaminate irrigated crops
- It is shrinking due to climate change reducing recharge from glacial melt
Correct answer: It is being depleted by irrigation much faster than it is naturally recharged
The Ogallala is a fossil aquifer recharged over thousands of years; current irrigation withdrawals vastly exceed recharge rates, threatening the agricultural productivity of 8 Great Plains states.
The High Plains (Ogallala) Aquifer underlies about 174,000 square miles of the U.S. Great Plains and is the primary water source for one of the world's most productive agricultural regions. Natural recharge is extremely slow (< 1 inch/year) because most precipitation evapotranspires. Irrigation pumping has depleted saturated thickness by >100 feet in parts of Kansas and Texas. If depleted, restoration would take thousands of years.
Question 2: Terracing as an agricultural technique on steep slopes primarily serves to:
- Increase sunlight exposure to crops growing on the shadowed side of hills
- Create level planting areas that reduce runoff velocity and soil erosion (Correct answer)
- Allow heavier farm machinery to operate safely on slopes
- Improve drainage of waterlogged soils on hillsides
Correct answer: Create level planting areas that reduce runoff velocity and soil erosion
Terraces cut horizontal steps into hillsides, converting steep, erosion-prone slopes into level or nearly level planting areas that hold water and soil.
Terracing involves cutting a series of horizontal platforms (benches) into a hillside. Each terrace level retains soil and water that would otherwise flow downslope. By reducing slope length and gradient, terraces dramatically decrease erosion rates and allow water to infiltrate rather than run off. Terracing has enabled agriculture on steep slopes for millennia - the Inca's Andean terraces and Southeast Asian rice paddies are famous examples.
Question 3: In terms of water use efficiency, which of the following irrigation methods uses water most efficiently?
- Flood irrigation (surface irrigation)
- Sprinkler irrigation
- Drip (micro) irrigation (Correct answer)
- Furrow irrigation
Correct answer: Drip (micro) irrigation
Drip irrigation delivers water directly to the root zone through emitters, minimizing evaporation and runoff; typical efficiency is 85-95% compared to 60-80% for sprinklers and 25-60% for flood irrigation.
Drip or micro-irrigation delivers water slowly at low pressure through emitters placed at or below the soil surface near plant roots. Water use efficiency reaches 85-95%. Sprinkler irrigation (60-80%) loses water to evaporation and wind drift. Surface/flood irrigation (25-60% efficiency) loses large amounts to deep percolation and evaporation. Despite higher upfront capital costs, drip systems reduce pumping energy, limit weed growth, and allow fertigation.
Question 4: Genetically modified (GM) crops engineered with Bt toxin produce an insecticide that:
- Is derived from a naturally occurring soil bacterium and is toxic to specific insect larvae (Correct answer)
- Is a synthetic chemical identical to conventional pesticides but produced inside the plant
- Targets all insects including beneficial pollinators like honeybees
- Kills adult insects by disrupting their reproductive systems
Correct answer: Is derived from a naturally occurring soil bacterium and is toxic to specific insect larvae
Bt toxin is derived from Bacillus thuringiensis, a naturally occurring soil bacterium; its crystal proteins are highly selective, primarily toxic to Lepidoptera (caterpillars) or Coleoptera (beetles) larvae, not to mammals or most non-target organisms.
Bt (Bacillus thuringiensis) produces proteins (Cry proteins) that form pores in the midgut cells of susceptible insect larvae after ingestion, causing death. Different Bt strains are specific to different insect orders: var. kurstaki targets Lepidoptera (caterpillars), var. israelensis targets Diptera (mosquitoes, blackflies), var. tenebrionis targets Coleoptera. Bt-transgenic crops (Bt corn, Bt cotton) express these proteins constitutively, reducing insecticide spraying. Resistance management through refuge planting is essential.
Question 5: Salinization of agricultural soils is primarily caused by:
- Acid rain depositing sulfuric acid compounds in the soil
- Irrigation with saline water or waterlogging that allows dissolved salts to accumulate as water evaporates (Correct answer)
- Excessive use of nitrogen fertilizers that convert to salt compounds
- Mining activities that expose salt-bearing rock layers to farmland
Correct answer: Irrigation with saline water or waterlogging that allows dissolved salts to accumulate as water evaporates
When irrigation water evaporates or transpires, it leaves behind dissolved salts; in waterlogged conditions without adequate drainage, salts accumulate in the root zone to toxic levels.
Soil salinization occurs when salts accumulate in the soil profile to levels that inhibit plant growth. All irrigation water contains dissolved salts; when evapotranspiration removes the water, salts remain. Without leaching and adequate drainage, salts build up. An estimated 20-30% of the world's irrigated land is affected by salinization, causing major yield losses especially in arid regions with high evaporation rates.
Question 6: Agroforestry differs from conventional agriculture in that it:
- Uses hydroponics to grow crops without soil
- Deliberately integrates trees and shrubs with crops and/or livestock to create ecological and economic benefits (Correct answer)
- Restricts farming to naturally forested areas without clearing trees
- Applies forest management techniques like selective logging to annual croplands
Correct answer: Deliberately integrates trees and shrubs with crops and/or livestock to create ecological and economic benefits
Agroforestry intentionally combines perennial woody plants with crops and/or animals to exploit synergies: nitrogen fixation by tree legumes, reduced erosion, diversified income, carbon storage, and microclimate benefits.
Agroforestry systems include alley cropping (crops between rows of trees), silvopasture (trees with livestock), windbreaks, and multi-strata home gardens. Benefits include: nitrogen input from leguminous trees; reduced wind and water erosion; improved microclimate; increased biodiversity; carbon sequestration; and economic diversification. Shade-grown coffee and cacao are well-known examples that support bird habitat while producing export crops.
Environmental Science Agriculture and Food Production 2
The Ogallala Aquifer, which underlies the U.S.
Great Plains, is a critical concern for agriculture because: