Daniel I. Arnon 317 



organs remains essentially the same whether plants are grown in soil, 

 sand, or water culture (5). 



Until recently the physiological validity of artificial cultures rested on 

 small-scale experiments with limited opportunities for appraising the 

 crop-producing potentialities of this method of supplying nutrients to 

 plants. Conclusive as the vast number of culture solution experiments 

 was in establishing the physiological suitability of the method, there 

 was until recently no direct comparison of the inherent productive 

 capacity of a fertile soil with a favorable nutrient solution. A special 

 study of this problem with the tomato as the test plant has revealed 

 (6) that the capacity for crop production of sand and water culture 

 media is of the same order of magnitude as that of a highly productive 

 soil. The productivity of artificial nutrient media was even found under 

 certain conditions to surpass that of highly fertile soil. 



The wide use of nutrient solution techniques, which in recent years 

 included a number of commercial ventures, has provided impressive 

 evidence bearing on other fundamental aspects of plant growth in soils, 

 namely, the importance of organic matter and soil colloids. A large 

 number of species of higher plants has been grown successfully in 

 artificial culture with the roots furnished only with a solution of inor- 

 ganic salts, under suitable conditions of root aeration. No factor in- 

 separable from the soil, nor any preformed organic compound exter- 

 nally derived, including vitamins (5), appears to be indispensable to 

 the functions of the plants investigated. Under normal conditions these 

 plants, which include most of the agricultural species, are fully capable 

 of synthesizing the organic substances which they require. The im- 

 portance of organic matter and of clay colloids in soil is of course be- 

 yond question, but these factors may be regarded as operating in a 

 secondary way and should be distinguished from those factors that are 

 indispensable for plant growth in a primary manner. It may be stated 

 that the progress of plant nutrition in the current century has corrob- 

 orated the fundamental concept of the inorganic nature of plant nu- 

 trition as developed in the nineteenth century by De Saussure, Liebig, 

 and Boussingault. 



One final point with regard to the physiological adequacy of nutrient 

 culture techniques: Claims have arisen in some quarters that plants 



