GROWING PLANTS WITHOUT SOIL FOR EXPERIMENTAL USE 



ters. Since they primarily supplement natural day- 

 light in the greenhouse, we employ old lamps that 

 have been used in growth-chamber illumination. 



With an overcast sky during the day the natural 

 illumination in the greenhouse is 900 to 1,100 ft.-c, 

 and on a bright day it may reach 5,000 to 7,000 

 ft.-c. 



The ideal situation is to grow each species of 

 plant under the specific conditions in which it 

 grows best. However, each species has require- 

 ments different from the next. Because of limita- 

 tions in cost, space, and time, it is necessary at the 

 Fargo laboratory to grow several species in the 

 same greenhouse section or growth chamber. 

 Therefore compromises have to be worked out. For 

 example, peas 2 and barley may have to be grown 

 next to each other. Peas grow best with cool tem- 

 peratures and short days, whereas barley requires 

 higher temperatures and longer days. When these 

 two crops are grown side by side year around, we 

 expect that neither will respond optimally to a 

 compromise growth condition. The plants look 

 healthy under these compromise conditions even 

 though better growth could be obtained. We make 

 every attempt to grow plants together that have 

 similar growth requirements. For example, only if 

 absolutely necessary do we attempt to grow peas 

 and corn under the same conditions. 



Under varying environments the water and 

 nutritional requirements of plants will differ. 

 During the summer it may be necessary to water 

 the plants twice daily, whereas once a day may 

 be satisfactory in the winter. The same is true 

 when nutrient solution is applied two or three 

 times a week, alternating with water only. If the 

 growth rate of the plants increases, additional 

 nutrients are necessary. A decreased growth rate is 

 accompanied by reduced nutrient requirements. 

 As the plants approach maturity, their nutrient 

 requirements gradually decrease. 



For certain species of plants, the flowering 

 stage may be delayed or hastened according to 

 the photoperiod they receive (4). The growth 

 rates of the plants discussed here and that are 

 listed in table 2 were obtained under average 

 conditions in the greenhouse. The photoperiods, 

 light intensities, temperatures, and humidities 

 listed in tables 2 and 3 are those found to be 

 satisfactory in growth chambers or incubators. 



2 For scientific names, see the index. 



Growth Chambers 



Many types of controlled environment systems 

 for raising plants are available commercially and 

 their use in research is increasing. The systems 

 vary considerably in size and capability, ranging 

 from simple lighted cabinets to those providing 

 rigorous control of several environmental pa- 

 rameters. Growth chambers are used primarily 

 (1) to obtain uniform plant material, (2) to permit 

 selection of environmental conditions appropriate 

 for a given plant species without regard to season, 

 and (3) to permit environmental manipulation as 

 an experimental variable. If more than one type 

 of growth chamber is available, the selection of 

 the most appropriate one will depend on its 

 intended use. 



In general, all growth chambers contain me- 

 chanical, electrical, and perhaps electronic com- 

 ponents and controls, all of which are subject to 

 breakdown and require maintenance. The more 

 sophisticated chambers generally provide greater 

 flexibility of operation but often require more 

 frequent or more complicated maintenance. 

 Usually the simplest chamber that will meet the 

 required environmental conditions is the best 

 and often the least expensive. 



Several kinds of growth chambers are used for 

 different types of experiments at the Fargo 

 laboratory. Since all the environmental factors 

 are interrelated in their effect on plant growth 

 (10, 16), none can be considered independently 

 of the others. However, satisfactory control of a 

 few major parameters will result in uniform and 

 reproducible plant growth. 



Temperature control is usually achieved through 

 the combined use of refrigeration and heating 

 systems that are thermostatically controlled. 

 It is most useful to have two thermostats, one 

 to control day temperature and the other for 

 night temperature, with a timeclock to switch 

 from one to the other. Switching need not coin- 

 cide with the light-dark cycle. Temperature 

 control of ±2.0° C. is adequate for many pur- 

 poses. Continuous temperature programing is 

 provided in some growth chambers, but it is 

 not needed for most experimental work. The 

 degree of temperature control provided in an 

 ordinary laboratory or classroom may be adequate 

 for many purposes, and the on-off cycling of 



