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Photosynthesis

Complex process that takes place in living green plant cells. Radiant energy from the sun is used to combine carbon dioxide (CO2) and water (H2O) to produce oxygen (O2) and simple nutrient molecules, such as glucose (C6HI2O6). See carbon sequestration.

Photovoltaic and solar thermal energy

Energy radiated by the sun as electromagnetic waves (electromagnetic radiation) that is converted into electricity by means of solar (i.e., photovoltaic) cells or useable heat by concentrating (i.e., focusing) collectors.

Phytoremediation

The systematic use of plants to treat environmental contamination. It is being investigated as a potential low-cost technology to help meet environmental regulations. For example, it has been discovered that young seedlings of Indian mustard (Brassica juncea) grown in aerated water are very effective at removing toxic metals from water.

pillow lava

Lava extruded beneath water characterised by pillow-type shapes.

Plant hardiness zones

The USDA has divided North America into 11 hardiness zones based on average annual minimum temperatures. Horticulturalists and nurseries rate plants by their hardiness; the hardiness zone maps can then be used to determine the likely survivability of particular plant species and varieties according to one’s local growing area.

Plate

Rigid parts of the Earth's crust and part of the Earth's upper mantle that move and adjoin eachother along zones of seismic activity.

Plutonic

Applies to igneous rocks formed beneath the surface of the Earth; typically with large crystals due to the slowness of cooling. Synonym of intrusive. Antonym of volcanic.

Point source

A single identifiable source that discharges pollutants into the environment. Examples are smokestack, sewer, ditch, or pipe. See non-point source.

Point source pollution

Pollutants that are discharged or emitted from discrete "point" sources, such as pipes and smokestacks. Both the Clean Water Act and the Clean Air Act focus control requirements on point sources and both require permits for major sources of discharges from point sources. While much agricultural pollution is nonpoint source, some agricultural activities are affected: for example, feedlots of over 1000 animal units are considered point sources requiring permits under the Clean Water Act. However, irrigation return flows, although considered point sources, are expressly exempted from the permit requirement.

Political Economy

Policies that emphasize the interaction between politics and economics and that have political and economic effects.

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