482 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1938 



filter paper (or paper toweling), particularly if plants are to be fixed 

 in corks and grown in jars or in tanks with perforated metal or wooden 

 covers. The upper layers of moist paper are removed after seeds 

 begin to germinate. The seedlings are allowed to grow on the moist 

 bed until large enough to place in corks. An excess of water is then 

 added to the moist paper and the young plants removed carefully so 

 as not to damage the roots. 



In transplanting from a flat of soil, the soil is thoroughly soaked 

 with water so that the plants can be removed with the least possible 

 injury to the roots. The roots are then rinsed free of soil with a 

 light stream of water and immediately set out in the beds or corks, 

 with the roots immersed in the solution. When young plants are 

 set out in the beds, the roots are placed in the solution, and at the 

 same time the layer of excelsior is built up over the screen. Then 

 the layer of rice hulls is placed on top of the excelsior (pi. 7). If 

 seeds are to be planted in the bed, the whole bed must be installed 

 and moistened before the seed is planted. 



SPACING OP PLANTS 



In our experiments with tomato plants, they were set close to- 

 gether, in some instances 20 plants to 25 square feet of solution 

 surface. No general advice can be offered as to the best spacing. 

 This depends on the kind of plant and on light conditions. Indi- 

 vidual experience must guide the grower. 



ADDITION OF WATER TO TANKS 



In starting the culture, the tank is filled with solution almost to 

 the level of the lower part of the bed. As the plants grow, water 

 will be absorbed by plants or evaporated from the surface of the 

 solution, and the level of the solution in the tank will fall. The 

 recommendation has generally been made that after the root system 

 is sufficiently developed, the level of the solution should remain from 

 one to several inches below the lower part of the bed, to facilitate 

 aeration. However, since the solution level should not be permitted 

 to fall very far, regular additions of water are required. 15 



As pointed out earlier, when large amounts of water have to be 

 added to a tank, excessive accumulations of certain salts contained 

 in the water may occur, especially if the salt content of the water is 

 high. To avoid this difficulty, the entire solution is changed whenever 

 the salt concentration becomes high enough to influence the plant 



18 Certain methods of circulating culture solutions (such as those described by J. W. Shive and W. R. 

 Robbins, In the citation given in footnote 7, p. 468, New Jersey Agr. Exp. Stat. Bull. 636) may be convenient 

 for maintaining a supply of water and nutrients, as well as assisting in aeration of roots. One commercial 

 greenhouse has utilized on a large scale a method of circulating nutrient solution from a central reservoir 



