WATER-CULTURE METHOD — HOAGLAND AND ARNON 479 



depends on the kind of plant to be grown, the length of the growing 

 period, and the purpose for which the plants are grown. Plate 6 

 shows the varied types of containers for nutrient solutions as employed 

 at the California Agricultural Experiment Station for research pur- 

 poses. Some of the smaller containers illustrated would doubtless 

 be convenient for amateur use, but the importance of the factor of 

 aeration of the solution should be stressed. If small containers are 

 employed and a large root system is to be developed, it may be 

 desirable or necessary to provide for special aeration of the culture 

 solutions. Plants differ greatly in regard to their requirements for 

 aeration of the root system. 



For commercial water culture, long, narrow, shallow tanks have 

 been employed. They may be constructed of wood, cement, black 

 iron coated with asphalt paint, or other sufficiently cheap materials 

 which do not give off toxic substances. In these tanks is placed the 

 nutrient solution in which roots of the plant are immersed. Wire 

 screens are placed over the tops of the tanks, or inside, above the 

 solution. The screens support a layer of bedding of varying thickness 

 (often 3 or 4 inches), according to the kind of plant grown (pi. 7). 

 This technique was first suggested by W. F. Gericke. 12 The bed may 

 be prepared from a number of inexpensive materials — for example, 

 pine shavings, pine excelsior, rice hulls. Some materials, such as 

 redwood shavings or sawdust, may be toxic. Seeds are planted in the 

 moist beds, or young plants from fiats are set in them with their roots 

 in the nutrient solution. Koots may later develop not only in the 

 solutions in the tanks, but also in the beds. 



The shallowness of the tanks and the porous nature of the beds 

 facilitate aeration of the root system — an essential factor — but as 

 already pointed out, such aeration unsupplemented by an additional 

 oxygen supply, does not give the best growth of all kinds of plants. 

 Recently evidence became available that significant improvement of 

 growth and yield of tomato plants resulted from continuous bubbling 

 of air through the nutrient solution, although the yields from un- 

 aerated cultures were at least as large as any previously reported for 

 water culture. 



Chemically pure salts commonly employed in making nutrient solu- 

 tions for scientific experiments would be too expensive for commercial 

 practice, and a number of ordinary fertilizer salts can serve in large- 

 scale production of crops. Recent developments in the fertilizer in- 

 dustry have made available cheap salts of considerable degree of pur- 

 ity. Some commercial salts, however, contain impurities (fluorine, 

 for example, is commonly found in phosphate fertilizers) which may 

 be toxic to plants under water-culture conditions. 



» Gericke, W. F., Aquaculture: A meana of crop production, Amer. Journ. Bot., vol. 16, p. 862, 1929. 



