38.1 Plants require a variety of nutrients in addition to the direct products of photosynthesis.
Plant Nutrients
• The major source of plant nutrition is fixation of atmospheric carbon dioxide into simple sugar, using energy from the sun. (p. 782)
• Plants require nutrients in both large quantities (macronutrients) and small quantities (micronutrients). (p. 782)
• There are nine macronutrients: carbon, oxygen, hydrogen, nitrogen, potassium, calcium, phosphorus, magnesium, and sulfur. (p. 782)
• There are seven micronutrients: chlorine, iron, manganese, zinc, boron, copper, and molybdenum. (p. 782)
Soil
• Soil is composed of a mixture of sand, rocks, clay, silt, humus, minerals, organic matter, and living organisms. (p. 784)
• About half of the total soil volume is occupied by spaces or pores, which may be filled with either air or water. (p. 784)
• Natural communities recycle nutrients, but cultivated communities are much more exposed to erosion, and thus often must be supplied with mineral nutrients. (p. 784)
• Other solutions to nutrient loss are crop rotation, planting legumes with nitrogen-fixing bacteria, allowing fields to periodically lie fallow, and plowing-under plant material. (p. 784)
• Nitrogen (N), phosphorus (P), and potassium (K) are the most important minerals that usually need to be added to soils. (p. 784)
• Organic fertilizers build up humus, but do not supply any elements to plants that inorganic fertilizers cannot supply. (p. 784)
38.2 Global change could alter the balance among photosynthesis, respiration, and use of nutrients acquired through the soil.
Global Change
• Carbon dioxide may be at its highest concentration in the atmosphere in 20 million years. (p. 785)
• As carbon dioxide levels increase, less nitrogen and other macronutrients may be found in plant leaves, thus causing herbivores to eat more biomass to obtain the same level of nutrients. (p. 786)
38.3 Some plants have novel strategies for obtaining nutrients.
Nitrogen Fixation
• Plants need ammonia (NH3) to build amino acids, but most of the nitrogen in the atmosphere exists as N2, and plants lack the biochemical pathways necessary to convert gaseous nitrogen to ammonia. (p. 787)
• Some bacteria live in close association with plant roots and can fix atmospheric nitrogen. (p. 787)
Nutritional Adaptations
• Some plants can also obtain nitrogen directly from other organisms by adopting a carnivorous feeding strategy. (p. 788)
• Symbiotic associations with mycorrhizal fungi are found in about 90% of vascular plants. (p. 789)
• Parasitic plants exist in photosynthetic and nonphotosynthetic varieties. (p. 789)
38.4 Plants can remove harmful chemicals from the soil.
Phytoremediation
• Phytoremediation is the process of removing contamination from soil or water using chemicals produced by plants. Examples are trichloroethylene (TCE), trinitrotoluene (TNT), and several heavy metals, including arsenic, cadmium, and lead. (p. 790-791)
• The estimated costs of phytoremediation are 50-80% lower than the costs of more conventional cleanup strategies. (p. 791)