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A plant that nourishes itself but grows on the surface of another plant for support, usually on the branches or trunks of tropical trees.

crop rotation

The practice of planting nonlegumes one year and legumes in alternating years to restore concentrations of fixed nitrogen in the soil.


A chemical substance that an organism must obtain in relatively large amounts.


The most fertile soil type, made up of roughly equal amounts of sand, silt, and clay.

sustainable agriculture

Long-term productive farming methods that are environmentally safe


Decomposing organic material that is a component of topsoil.


A swelling on the root of a legume. It is composed of plant cells that contain nitrogen-fixing bacteria of the genus Rhizobium.


The soil region close to plant roots and characterized by a high level of microbiological activity.


Association of a fungus with a plant root system in which the fungus surrounds the roots but does not cause invagination of the host (plant) cells' plasma membranes.

nitrogen fixation

The conversion of atmospheric nitrogen (N2) to ammonia (NH3). Biological nitrogen fixation is carried out by certain prokaryotes, some of which have mutualistic relationships with plants.


An element that an organism needs in very small amounts and that functions as a component or cofactor of enzymes.


An emerging nondestructive biotechnology that seeks to cheaply reclaim contaminated areas by taking advantage of some plant species' ability to extract heavy metals and other pollutants from the soil and to concentrate them in easily harvested portions of the plant.


A form of bacterium Rhizobium contained within the vesicles formed by the root cells of a root nodule.

essential element

In plants, a chemical element required for the plant to grow from a seed and complete its life cycle, producing another generation in the form of seeds.


A soil bacterium whose population size is much enhanced in the rhizosphere, the soil region close to a plant's roots.

nitrogen cycle

The natural process by which nitrogen, either from the atmosphere or from decomposed organic material, is converted by soil bacteria to compounds assimilated by plants. This incorporated nitrogen is then taken in by other organisms and subsequently released, acted on by bacteria, and made available again to the nonliving environment.

cation exchange

A process in which positively charged minerals are made available to a plant when hydrogen ions in the soil displace mineral ions from the clay particles.

hydroponic culture

A method in which plants are grown in mineral solutions rather than in soil.


The process by which an organism takes in and makes use of food substances.

no-till agriculture

A plowing technique that involves creating furrows, resulting in minimal disturbance of the soil.

soil horizon

A soil layer that parallels the land surface and has physical characteristics that differ from those of the layers above and beneath.

arbuscular mycorrhiza

Association of a fungus with a plant root system in which the fungus causes the invagination of the host (plant) cells' plasma membranes


A mixture of particles derived from rock, living organisms, and decaying organic material (humus).


A mutualistic association of plant roots and fungus.


The addition of mineral nutrients to the soil.

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