PLANT NUTRIENTS 3 



to have no role to play in the nutrition of plants; although silicon 

 is always present in plant ash and sodium salts are found in small 

 quantities in all parts of practically all plants. Nearly all species 

 of plants can be grown to full maturity in the entire absence of 

 these elements from their culture medium. Occasional excep- 

 tions to this statement in the case of special types of plants are 

 known, and are of interest in special studies of plant adaptations, 

 but need not be considered here. 



The second group includes iron, calcium, magnesium, and, 

 generally, sulfur. All of these elements are essential for plant 

 growth, but are usually present hi the soil in ample quantities 

 to insure a sufficient supply in available form for all plant needs. 

 Recent investigations have shown, however, that there are many 

 soils in which sulfur is present in such limited quantities that 

 many agricultural crops, when grown on these soils, respond 

 favorably to the application of sulfur-containing fertilizers. In 

 such cases, sulfur is a " critical " element. 



The " critical " elements are those which are essential to the 

 growth of all plants and which are present in most soils in rela- 

 tively small proportions and any one may, therefore, be the limiting 

 factor in plant growth so far as plant food is concerned. These 

 are nitrogen, phosphorus, potassium, and (possibly) sulfur. 



ROLE OF PLANT FOOD ELEMENTS IN PLANT GROWTH 



The use which a plant makes of the elements which come to it 

 from the soil has been studied with great persistency and care by 

 many plant physiologists and chemists. Many of the reactions 

 which take place in a plant cell are extremely complicated, and the 

 relation of the different chemical elements to these is not easily 

 ascertained. It is probable that the same element may play a 

 somewhat different role in different species of plants, in different 

 organs of the same plant, or at different stages of the plant's 

 development. But the usual and most important offices of each 

 element are now fairly well understood, and are briefly summarized 

 in the following paragraphs. It should be understood that a 

 thorough and detailed discussion of these matters, such as would 

 be included in an advanced study of plant nutrition, would reveal 

 other functions than those which are presented here and would 

 require a more careful and more exact method of statement than 



