Answer: An understanding of our fragile environment can begin with
a recognition of the importance of certain elements, commonly called "mineral
substances" (such as iron and zinc), in the lives of humans and animals and in
the soils that support plants. This recognition is well deserved because these
elements are essential for the life or optimum health of an organism. Some
elements such as carbon, hydrogen, oxygen, magnesium, potassium, and phosphorus
are required in relatively large amounts by organisms. However, others are
required in smaller quantities; these are referred to as trace elements.
Diseases have been related to the deficiency of about 20 elements in animals and
humans and to the deficiency of approximately 13 elements in plants.
At the same time, if these and other elements occur in quantities great enough, toxicity
can result. An element, or any substance, that occurs in the environment and
contains concentrations above what are considered to be background levels may be
considered a contaminant. When contaminants occur at levels that are
potentially harmful to organisms, they are labeled as hazards. Often the
quantitative difference between essential amounts and toxic concentrations of
these elements is very small. For example, the trace element selenium is
required at a level of no less than 0.4 ppm in the diet of cattle but can be
toxic at levels greater than approximately 4 ppm.
Elements that are required
for survival by animals and plant are termed essential while those not
required are nonessential. Trace essential elements such as fluorine,
copper, selenium, molybdenum, can be hazardous to life forms if present at high
levels. Nonessential heavy metals such as a arsenic, lead, mercury, cadmium, and
chromium are usually toxic to organisms at much lover levels than trace essential
elements. (From USGS Circular 1105.)
THE BIOGEOCHEMISTRY OF LIFE -- ESSENTIAL ELEMENTS
Essential to all animals and plants |
Essential to several classes of animals and plants |
Essential to a wide variety of species in one class |
Essential to one or two species only |
Recent work indicates essentiality, but of unknown function |
Hydrogen(H) Carbon (C) Nitrogen (N) |
Silicon (Si) Vanadium (V) Cobalt (Co) |
Boron (B) Fluorine (F) Chromium (Cr) |
Lithium (Li) Aluminum(Al) Nickel (Ni) |
Rubidium (Rb) Tin (Sn)
|
Oxygen (O) Sodium (Na) Magnesium(Mg) |
Molybdenum(Mo) Iodine (I)
|
Bromine (Br)
|
Strontium (Sr) Barium (Ba)
|
|
Phosphorus(P) Sulfur(S) Chlorine (C) |
|
|
|
|
Potassium (K) Calcium (Ca) Manganese (Mn) |
|
|
|
|
Iron (Fe) Copper (Cu) Zinc (Zn) Selenium (Se) |
|
|
|
|
Elements in bold type are generally considered to be trace
elements.
|