PLANT NUTRITION 245 



;nul nuihioniiie. It is also a siructiiral c ompoiunl oC biolin and 

 tliianiinc, iwo iinpoiLant plant hormones. Sullur also forms a part 

 of the mustard oil glycosides, which are responsible for imparting 

 characteristic otlor and flavor to the mustartls, onions, and garlic. 



Although sulftn can enter the leaves as SO^ when this gas is in the 

 atmosphere, the usual route of entry for the element is through the 

 roots as the sulfate ion. Before combination in the organic compounds 

 mentioned above, the sidfate ion is reduced, usually to the stdfhydryl 

 ( — SH) group. Once combined in organic form, the sidfur apparently 

 can be recon\erted to inorganic sulfur, usually the sidfate ion, and 

 thus be rcilistributed within the plant and used again as a source of 

 sidfur lor the synthesis of other organic compounds. 



Sulfur is also known to play an important role in the physiology 

 of plants. A deficiency of sulfur interferes with the production of 

 chlorophyll, resulting in the leaves appearing pale green, as in nitrogen 

 deficiency. 



Calcium 



Much of the calcium in most plants is located in the leaves. In 

 contrast to phosphorus, more calcium is generally found in older 

 rather than in vounsrer leaves. It is believed that much of the calcium 



J o 



in plants is permanently fixed in cell walls as the calciimi salt of the 

 pectic substances, or as calcium oxalate. Calcium is also widely 

 distributed in the plant in the form of calcium soaps and is present 

 in vacuolar sap as calcium ions. 



Calcium has been found necessarv for continued mitotic division. 

 It is also known to play an important role in the nitrogen metabolism 

 of some plants, for a deficiency of calcium leads to a poor absorption 

 or assimilation of nitrates in such plants. 



Unlike phosphorus, calcium is not readily redistributed in a plant 

 when the soil is deficient in this element. Although redistribution 

 apparently takes place at a very slow rate, such translocation is not 

 suffic ient to meet the metabolic needs of the younger, rapidly growing 

 tissues. 



Magnesium 



Although seeds contain an apjireciable amount of magnesiinn, the 

 largest proportion of this element present in plants is found in the 

 chlorophyll-bearing tissues. Most soils contain a sufficient amount 

 of magnesium to supply the requirements of plants. In the event 

 that an adequate supply is not available to the plant, the production 



