466 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 8 



production. 4 In recent years, great refinements in water-culture 

 technique have made possible the discovery of several new essential 

 elements, which, although required by plants in exceedingly small 

 amounts, often are of definite practical importance in agricultural 

 practice. The elements derived from the nutrient medium that are 

 now considered to be indispensable for the growth of higher green 

 plants are nitrogen, phosphorus, potassium, sulphur, calcium, mag- 

 nesium, iron, boron, manganese, copper, and zinc. New evidence 

 suggests that molybdenum may have to be added to the list. 6 Present 

 indications are that further refinements of technique may lead to the 

 discovery of still other elements, essential in minute quantity for 

 growth. 



In addition to the list of essential elements, which is obviously of 

 first importance in making artificial culture media for growing plants, 

 a large amount of information has been amassed on the desirable pro- 

 portions and concentrations of the essential elements, and on such 

 physical and chemical properties of various culture solutions as 

 acidity, alkalinity, and osmotic characteristics. A most important 

 recent development in water-culture technique has been the recogni- 

 tion of the importance for many plants of special aeration of the 

 nutrient solution, to supplement the oxygen supply normally gaining 

 access to the solution when it is in free contact with the surrounding 

 atmosphere. 



The recently publicized use of the water-culture technique for com- 

 mercial crop production does not rest on any newly discovered 

 principles of plant nutrition other than those discussed above. It 

 involves, rather, the application of a large-scale technique, developed 

 on the basis of an understanding of plant nutrition gained in previous 

 investigations conducted on a laboratory scale. The latter have pro- 

 vided an understanding of the composition of suitable culture solu- 

 tions. Furthermore, methods of controlling the concentration of 

 nutrients and the degree of acidity are, except for modifications 

 imposed by the large scale of operations, similar to those employed in 

 small scale laboratory experiments. 



The selection of a particular type of covering for the tanks adapted 

 to large-scale water-culture operations and of methods for supporting 

 the plants depends on the kind of plant. For example, in growing 

 potatoes by the water-culture method, provision must be made for a 

 suitable bed above the level of the solution, in which tubers can 

 develop. On the other hand, in growing tomatoes it is only necessary 

 to provide adequate support for the aerial portion of the stem, assum- 

 ing that the roots are in a favorable culture-solution medium, ade- 



* However, nutrient solutions such as are employed in water-culture experiments are not applied directly 

 to soils. For discussion of fertilizer problems consult: Hoagland, D. R., Fertilizer problems and analysis 

 of soils in California, Calif. Agr. Exp. Stat. Circ. 317, pp. 1-15. revised 1938. 



• Unpublished data of D. I. Arnon and P. R Stout. 



