GROWING PLANTS WITHOUT SOIL FOR EXPERIMENTAL USE 



by selecting environmental conditions approxi- 

 mating those under which the plant flourishes in 

 nature. Once established, species from arid regions 

 often grow well under dry conditions and those 

 from nonarid regions often grow well under 

 humid conditions. When the environment can 

 be controlled at least partially, these conditions 

 can often be met and the results are usually 

 gratifying. 



Because there are so many different species 

 and varieties of plants with differing needs for 

 optimum growth, the grower should experiment 

 to find the optimum conditions for each species 

 or variety. 



Light 



Light is required for plant growth. The illumi- 

 nation, light quality, and photoperiod require- 

 ments vary among species (1£). A number of 

 different instruments may be used to measure 

 illumination. In biological research a foot-candle 

 meter is often used to measure illumination. 

 The illumination on a surface that is everywhere 

 1 foot from a uniform point source of light of one 

 candle intensity is equal to 1 lumen per square 

 foot or 1 ft.-c. Although inexpensive foot-candle 

 meters can be obtained, photographic light 

 meters are more generally available and are 

 practical for light measurement. Photographic 

 light-meter readings can be converted to foot- 

 candles by the following formula: 3 



B = 



20(/) 2 

 TS 



Where 



B = illumination in foot-candles 

 / = aperture in / stop 

 T = shutter speed in seconds 

 S = film speed in ASA units 



To measure illumination with a photographic 

 light meter, reflected rather than incident light 

 must be measured. To do this, place a large sheet 

 of white paper on the surface to be measured, set 

 an appropriate ASA film speed on the meter, and 



read the shutter speed required for proper exposure 

 at a given / stop. Values for ASA speeds of 64 to 

 100 at various / stops and shutter speeds are given 

 in table 1. Values for other meter settings may be 

 obtained by solving the equation. The results will 

 be approximate depending on the accuracy of the 

 meter and the cone of light it accepts. For example, 

 false low readings may result if the meter accepts 

 light from an area greater than that of the white 

 paper at which it is directed. Nevertheless the 

 readings can suffice to determine whether illumina- 

 tion is adequate for good plant growth. About 

 1,200 ft.-c. of light in a growth chamber is satis- 

 factory for many plants such as cabbage, carrots, 

 peas, and tobacco, whereas other plants such as 

 corn, cotton, rice, and sorghum grow better when 

 supplied with 1.600 to 1,800 ft.-c. 



Table 1. — Relationship between photographic light 

 meter readings and foot-candles ' 



3 We are indebted to John A. Witz, Department of Elec- 

 trical Engineering, Oklahoma State University, Stillwater, 

 for supplying this formula. 



Film speed 



Aperture 



Shutter 



Illumination 



(ASA) 





speed 







F stop 



Seconds 



Ft.-c. 



100 



16 



1/2 



102 



64 



16 



1/2 



160 



100 



16 



1/5 



256 



100 



11 



1/15 



363 



64 



16 



1/5 



400 



100 



16 



1/10 



512 



100 



11 



1/25 



605 



100 



16 



1/15 



768 



64 



16 



1/10 



800 



100 



16 



1/20 



1,024 



64 



16 



1/15 



1,200 



100 



16 



1/25 



1,280 



100 



11 



1/60 



1,452 



64 



16 



1/20 



1,600 



64 



16 



1/25 



2,000 



100 



16 



1/50 



2 , 560 



100 



16 



1/60 



3,072 



100 



11 



1/150 



3,630 



64 



16 



1/50 



4,000 



64 



16 



1/60 



4,800 



100 



16 



1/100 



5,120 



100 



16 



1/125 



6,400 



64 



16 



1/100 



8,000 



64 



16 



1/125 



10,000 



100 



16 



1/200 



10,240 



1 Measure light reflected from a white paper or other 

 similarly textured surface. 



\ 



