GROWTH RESPONSE OF SUGAR BEET 343 



Table 2 



Mean Regression of radiation on time 



radiation , — ■ '^ -■ — ^ 



Year Cal/cm^day Cal/cm^/day/day %/day 



1958 317-4 -1-75 -0-55 



1959 401-2 -1-75 -0-44 

 i960 ii3"i —2-25 —2-00 



measured the radiation incident on the glasshouse, and the figures are not 

 corrected for the transmission of the structure (about 70%). The daily- 

 radiation fell significantly throughout each experiment. 



During 1959 the experimental period had the greatest mean daily radia- 

 tion and the smallest relative rate of decrease; 1958 had a smaller mean 

 radiation and the same absolute rate of change as 1959. Although these 

 time changes were significant they were not large and did not greatly 

 change the growing conditions during the experiment. In i960 the experi- 

 ment was done later in the year when the mean radiation was still less, and 

 the change with time was greater; the growing conditions therefore 

 changed appreciably during the experiment. In the fmal sampHng interval 

 temperature and illumination was unfavourable for growth. 



Soil Moisture 



As only the top 12 in. of soil was sampled, the change in soil water content 

 does not represent the total water loss during drought, when the relation- 

 ship between the various soil layers was unpredictable. Under treatment A, 

 soil water moves more freely, so that when the 0-12 in. layer is at field 

 capacity the entire profile will usually be equally moist and changes in the 

 0-12 in. layer can then be used as a rehable guide to water requirements. 



The water consumption of the A plots, which can be used as an estimate 

 of the maximum deficit in the B and C plots, was about twice the amount 

 of water required to regain field capacity in Exps. i and 2, but in Exp. 3 

 the amounts agreed to within 2%. At least part of the discrepancy was 

 caused by restriction of transpiration during drought on B and C plots. 

 In i960 transpiration was already low and therefore insensitive to drought. 

 The average moisture content of the A plots was higher in 1958 than in 

 1959 so that there was more risk of drainage and the difference in water 

 used (Table i) may underestimate the difference in transpiration. 



23 



