Chapter 37

• The basic physical properties of soil are texture and composition
• Soil particles are classfied by size: by largest to smallest, they are called sand, silt, and clay.
• Nitrogen, phosphorus, potassium and other minerals are essential for plant growth because of their roles in the structure and function of plant cells
• Topsoil consists of mineral particles, living organisms, and humus, which is decaying organic material
• Loams are the most fertile topsoils and contain equal amounds of sand, silt, and clay.
• A soil's composition refers to its inorganic (mineral) and organic chemical components.
• Many soil particles are negatively charged.
• Anions can be lost from the soil by leaching.
• Cations are prevented from leaching out of the soil through percolating groundwater.
• Humus: dead, organic matter; creates a crumbly soil that retains water but is still porous
• Topsoil contains bacteria, fungi, algae, other protists, insects, earthworms, nematodes, and plant roots
• Agroecosystems occupy more than 40% of the earth's land area and have a great impact on the natural world
• In contrast with natural ecosystems, agriculture depletes the mineral content of soil, taxes water researves, and encourages erosion.
• It can take over 500 years to form one centimeter of topsoil.
• At present, 30% of the world's farmland has reduced productivity because of soil mismanagement
• The goal of sustainable agriculture is to use farming methods that are conservation-minded, environmentally safe, and profitable
• Green Revolution

  • Norman Borlaug
  • began in Mexico in the 1940s
  • developed disease resistant, high-yield varieties of wheat; chemical fertilizers and irrigation techniques developed
  • spread worldwide in the 1950s and 1960s as high-yield varieties of other crops were produced
  • effectively diverted mass famine in some areas
• Irrigation is a huge drain on water resources when used for farming in arid regions
• The primary source of irrigation water is underground water reserves called aquifers
• Approximately 75% of the world's freshwater withdrawals go to irrigation
• More efficient irrigation methods, such as drip irrigation, could minimize evaporation and runoff
• In natural ecosystems, nutrients are recycled, but in agroecosystems, soils can become depleted of nutrients as plants are harvested
• Commercial fertilizers are enriched in nitrogen, phosphorus, and potassium (NPK)
• Excess minerals are often leached from the soil and can cause algal blooms in lakes.
• Sustainable method -> organic fertilizers
• Water and wind erosion are major causes of topsoil removal and loss of soil nutrients
• Sustainable method -> no-till agriculture also reduces erosion by creating furrows for seeds and fertilizer with minimal soil disturbance
• Erosion can be reduced by planting trees as windbreaks, terracing hillside crops, and cultivating in a contour pattern.
• Soil is a carbon sink, but soil disturbance by tilling releases carbon to the atmosphere (no-till techniques minimize this)
• Clearing forests (particularly tropical forests) removes an important carbon sink
• Increase in nitrous oxide (a greenhouse gas) is traced to synthetic fertilizers
• Methane emissions result from livestock and other agricultural practices
• Silent Spring: published in 1962 by Rachel Carson
• Effects of pesticide use on the environment, particularly birds
• The US government banned the use of DDT, beginning in 1972
• Water, air, and soil minerals all contribute to plant growth
• 80-90% of a plant's fresh mass is water
• 96% of a plant's dry mass is from CO2 assimilated into carbohydrates during photosynthesis
• 4% of a plant's dry mass is inorganic substances from soil
• There are 17 essential elements, chemical elements required for a plant to complete its life cycle and reproduce