Agriculture Reference
In-Depth Information
Liebig, discovered in 1847 that of the two elements sodium and potassium, concen-
trations of the former were relatively higher in blood and lymph, whereas concentra-
tions of the latter were relatively higher in soft tissues. J. B. Boussingalt reported
in 1849 that low-sodium diets (apparently sodium deficient) fed to oxen resulted in
signs of illness. In 1881, Sydney Ringer, an English physician, found that a solution
containing the chlorides of sodium, potassium, and calcium was useful in sustain-
ing the function of tissues in vitro, and Ringer's solution is still in use today. Hans
Huebner reported in 1909 that a low-phosphorus diet produced rickets in dogs, and
T. B. Osborne and L. B. Mendel reported in 1918 that dietary phosphorus restriction
retarded growth in rats. The importance of the dietary calcium:phosphorus ratio for
bone formation was demonstrated in rats by H. C. Sherman in 1921. The essentiality
of magnesium was discovered in 1931 by E. V. McCollum, who found that rats fed a
low-magnesium diet exhibited dilation of blood vessels and extreme hyperirritabil-
ity. Low-chloride diets were reported by E. R. Orent-Keiles in 1937 to retard growth
and to cause hypersensitivity in rats.
Discovery of the essentiality of several trace elements awaited development of
analytical techniques that could detect them in low concentrations. Iodine was found
in 1908 to prevent goiter in rat pups when it was given to their mothers. In 1928, E.
B. Hart reported that copper as well as iron is required to prevent anemia in rats fed
a milk-based diet. In 1931, Hart reported that mice failed to grow and ovulate on
a low-manganese diet, whereas McCollum found that rat pups failed to suckle and
survive (due to alterations in normal behavior). Essentiality of zinc was established
in 1934 by Hart, who observed growth retardation and hair loss in rats fed a low-zinc
diet. Cobalt was found in 1935 by E. J. Underwood and H. R. Marston to prevent
anorexia (loss of appetite), anemia, and lethargy in sheep and was ultimately proven
to be essential for synthesis of vitamin B
12
by rumen bacteria. Fluoride, at appropri-
ate concentrations in drinking water, was reported by H. T. Dean in 1938 to inhibit
development of dental caries in children. Molybdenum was found in 1953 by D. A.
Richert and W. W. Westerfield to be a component of the tissue enzyme xanthine oxi-
dase. Selenium was found essential by Klaus Schwarz in 1957 to prevent liver necro-
sis in rats. Walter Mertz reported in 1959 that chromium was a factor in maintenance
of glucose tolerance. Other elements, such as silicon (1972), nickel (1975), arsenic
(1976), lithium (1981), lead (1981), and boron (1981), have been assigned essential
status under special circumstances.
C
a L C i u m
Calcium is the fifth most abundant element on Earth and the most abundant min-
eral element in the body. Over 99% is present in the skeleton and teeth, mainly as
hydroxyapatite [Ca
10
(PO
4
)
6
(OH)
2
]. Thus, the skeleton serves not only as the body's
structural framework but also as a ready calcium reserve. Bone remodeling proceeds
throughout life, with mineral resorption performed by osteoclasts and mineral depo-
sition by osteoblasts. This remodeling is an essential feature of bone growth, repair
of structural damage to bone, and homeostatic maintenance of blood calcium levels.
Mineral deposition exceeds mineral resorption during skeletal growth, whereas min-
eral resorption may predominate during age-related bone loss.
