318 Mineral Nutrition of Plants 



grown without organic matter are of inferior nutritive value as food 

 to animals. These claims carry the implication that the metabolism of 

 plants grown in an inorganic medium differs from that of plants grown 

 in the presence of organic matter. Since nutrient solution culture rep- 

 resents an extreme case of an inorganic medium, plants grown in this 

 way and subsequently fed to animals provide a means of testing these 

 contentions. A preliminary investigation was therefore undertaken to 

 compare the nutritive value of grasses grown in water culture and in 

 a soil with a history of organic manuring, using the guinea pig as the 

 test animal. No evidence was found that plants grown in an inorganic 

 medium are deficient in any dietary essentials (<S'). 



If the physiological validity of the nutrient solution technique is ac- 

 cepted, it becomes pertinent to examine its suitability in investigating 

 what inorganic elements are essential for plant growth. The nutrient 

 solution technique is obviously free from the chief inherent shortcom- 

 ings of the soil. Individual nutrients can be added or omitted as desired. 

 Synthetic media are prepared lacking one element at a time, but con- 

 taining all the others accepted as being essential. The growth of plants 

 is then observed. Failure to grow in the absence of the element and 

 resumption of growth upon the addition of the deficient element are 

 taken as evidence of essentiality. It was substantially with this approach 

 that the classical list of essential elements was compiled in the nine- 

 teenth century. Exclusive of carbon, oxygen, and hydrogen, the list 

 included seven elements derived from the soil: nitrogen, phosphorus, 

 potassium, calcium, magnesium, sulfur, and iron. 



Even in the early days of the use of the nutrient culture technique, 

 observations were made by different workers of the "stimulating efTect" 

 from adding elements to the nutrient medium other than the seven 

 recognized as indispensable. But there was no consistent basis for 

 considering them essential. Circumstances were not propitious for ad- 

 vances in this direction. The salts which were used as sources of the 

 seven nutrients contained various impurities unknown to the experi- 

 menter. Water, even of the distilled kind, and containers in which the 

 plants grew, served as other sources of impurities. There was little in 

 the experience of the research workers in those days to enable them to 

 anticipate the low order of concentration of the newer plant nutrients, 



