D. R. HOAGLAND 



certain species of plants growing in some types of soil high in organic 

 matter may have lost the power of synthesizing at an adequate rate 

 vitamins or other essential organic units, and that these species depend 

 in part on the absorption of these units from an environment in which 

 they have been synthesized by microorganisms. But most or all species 

 of higher green plants so far intensively studied (these are mainly 

 plants of economic importance) can go through their cycles of growth 

 by virtue of their own synthetic powers, at least so far as can be ascer- 

 tained by the use of purified inorganic media, although usually without 

 complete exclusion of all microorganisms. It follows that the range 

 and diversity of biochemical reactions that need to be investigated 

 is enormous. Correspondingly great are the opportunities for the 

 study of different synthetic processes in a living organism, to the extent 

 that adequate methods can be devised for attacking such complex 

 systems. 



The preoccupation of plant physiologists engaged in agricultural 

 research with the inorganic elements absorbed by crop plants from the 

 soil, and the frequent designation of these elements as "plant foods," 

 tend to subordinate appreciation of the biochemical aspects of plant 

 nutrition. The inorganic elements derived from the soil constitute 

 only a small percentage of the dry weight of a plant, and one of the 

 most important objectives of research in plant nutrition should be an 

 understanding of the mechanisms by which these inorganic com- 

 ponents become directly incorporated into the organic compounds 

 synthesized by the plant, or activate the enzymes which catalyze the 

 syntheses and breakdown of organic compounds. 



We have as a foundation the knowledge that plants of the kind 

 in question absorb from an inorganic medium, and have an essential 

 need for, the elements nitrogen, phosphorus, potassium, calcium, 

 magnesium, sulfur, and iron. Also, as a result of research in plant 

 nutrition during the past decade or two, other elements have been 

 shown to be equally essential though required in only minute quan- 

 tities. These additional elements include boron, manganese, copper, 

 and zinc, with strong but still limited evidence that molybdenum is 

 also essential. There may be, and probably are, still other chemical 

 elements indispensable to plant growth, but conclusive proof of the 

 general indispensability of other elements, over a wide range of plant 

 species, has not yet been obtained. Limitations of technique are soon 



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