GROWING PLANTS WITHOUT SOIL FOR EXPERIMENTAL USE 



13 



length of 0.25-mm. bore capillary tubing into 

 the air line for each aerator tube (fig. 3). Aerating 

 can be done continuously or at regular intervals, 

 such as 2 hours on, 2 hours off, by using a time- 

 clock. 



If compressed air is available, this may be 

 more economical than using many small pumps, 

 which require frequent maintenance. Oil vapors 

 from a rotary- type compressor must be trapped 

 out with a charcoal filter before the air reaches 

 the plants. 



Subirrigation 



At the Fargo laboratory the plants grown in 

 vermiculite-filled pots or flats generally receive 

 water and nutrient solution by subirrigation. 

 Saucers or shallow pans are used under the pots 

 and enamel trays under the flats. The amount of 

 water or nutrient solution added is dependent 

 on the plant and growing conditions. Common 

 sense and experience are often required to de- 

 termine how often and how much to water. It 

 is not so easy to overwater plants grown in 

 vermiculite as it is in soil. However, plants that 

 require well-aerated media can become water- 

 logged if the vermiculite is watered excessively. 

 Care must be taken to avoid this. Waterlogged 

 plants will often wilt, although for different 

 reasons than when they are desiccated. When in 

 doubt, consult those who have previously grown 

 these plants, or refer to literature on the specific 

 plant (2). 



Some salt accumulation may occur on the sur- 

 face of the vermiculite after a period of time, 

 and this should be leached out once a month. 

 This is done by applying excess water at the 

 top and letting it drain through the container. 

 Then subirrigation can be resumed on the usual 

 schedule. Salts are leached away very rapidly 

 from vermiculite by this method (2). The pots 

 and flats are watered from the top only at seeding 

 time. When very small seed is placed in flats 

 and left uncovered, subirrigation is used. If they 

 are watered from the top, a fine spray must be 

 used to prevent the seed from being carried too 

 deep into the vermiculite to emerge. 



The following methods can be used to apply 

 nutrients to plants: (1) Mix the nutrients directly 

 into the watering system. (2) Add nutrient solution 

 two or three times per week and water at other 



times. (3) Supply a nutrient solution every day 

 without mixing it in the watering system; the 

 concentration can then be varied as needed. 

 When using method 2, tapwater can often be 

 substituted for distilled or deionized water if 

 the plants are not going to be used for a critical 

 experiment in which nutrition should be carefully 

 controlled. The use of tapwater several times per 

 week has the advantage of reducing the volume 

 of deionized water required. However, it must 

 be employed with caution, since some domestic 

 water supplies are apt to contain levels of salts 

 that will damage plants. Soft water is not neces- 

 sarily low in salt content since the softening 

 process usually only exchanges highly soluble 

 salts for less soluble ones. 



The acidity or alkalinity of the water used may 

 also affect plant growth adversely. Some of the 

 adverse pH effects probably result from changes 

 in the availability of nutrient salts. For example, 

 excessive watering of corn with alkaline (pH 9) 

 tapwater can cause the leaf margins to become 

 chlorotic and torn. These symptoms are remi- 

 niscent of certain nutrient deficiencies. The 

 addition of dilute acids, bases, or buffering salts 

 to the tapwater or nutrient solution can help 

 overcome these difficulties. Care must be taken, 

 however, to insure that neither the nutrient 

 solution nor the tapwater used between additions 

 contains too much salt. 



We emphasize again that it is necessary to 

 experiment in order to determine the optimum 

 amounts of nutrient for individual species. 



Nutrient Solution 



The formula of Hoagland and Arnon (9) is used 

 with a slight modification. Preparation is as 

 follows: 



Preparation of Stock Solutions 



Major Elements. — Individual 1 M stock solu- 

 tions of each major element are made with de- 

 ionized water. 



Grams 

 Chemical Formula per liter 



Potassium dihydrogen phos- 

 phate KH 2 P0 4 131.1 



Potassium nitrate KN0 3 101.1 



Calcium nitrate Ca(N0 3 ) 2 -4H 2 236.2 



Magnesium sulfate MgSCv7H 2 246.5 



^ 



