[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS 331 



green plants and in the nutrient media prepared for non-green plants was 

 redistilled several times, until its total metal content was less than 0.0002 

 part per million (0.0002 ppm.). The salts used were recrystallized until 

 the metal impurities in them were less than 0.0001 ppm. 



To cite but one example, it was discovered at the Universitv of Cali- 

 fornia that the symptoms of manganese, zinc, and copper deficiencies in 

 tomato plants disappeared when the culture solution contained 0.5 ppm. 

 of manganese, 0.05 ppm. of zinc, and 0.02 ppm. of copper. The copper 

 deficiency symptoms also disappeared when the leaves were merelv 

 spraved with a verv dilute solution of copper sulfate. Increased amounts 

 of these elements soon result in toxic effects; 2 ppm. of copper in the 

 culture solution was injurious to tomato plants. 



Several botanists have proposed culture solutions in which many kinds 

 of green plants will grow well. The following is an example: 



Water 1 , 000 . 00 cc. 



KH2PO4 0.31 gm. 



Ca(N03)2.4H20 1.04 gm. 



MgS04 . 7H2O . 54 gm. 



(NH4)2S04 0.09 gm. 

 Very small quantities of FeS04, H3BO3J 

 MnS04, ZnS04, and CUSO4. 



Water cultures in tanks. Botanists have studied the growth of plants 

 in water cultures in small laboratory vessels for nearlv a centurv. When 

 the size of the vessel was increased to that of a small tank, the process 

 attracted public attention. It is necessary to stretch "hardware cloth" 

 across the top of the tank and cover it with a thin layer of such substances 

 as peat or excelsior that will keep the seedlings upright. If all the factors 

 are properly adjusted, many kinds of plants will grow and reproduce in 

 these "tank cultures" as well as they do in highlv fertile soils. Tank cul- 

 ture has been commercialized under the name "hydroponics." 



Anyone can play with the idea of tank culture; but the means of main- 

 taining a proper supply of salts and of testing its commercial value are 

 problems that can be solved only by research students. The most modern 

 tank-culture apparatus found in greenhouses consists of a tank filled 

 with cinders or coarse gravel, into which the culture solution is pumped 

 from another reservoir at stated intervals during the day and allowed to 

 drain out immediately. Various modifications of the apparatus may also 

 be used (Fig. 139). The apparatus can be constructed to run auto- 

 matically. 



