SOIL TYPE, SOIL WATER REGIME AND GROWTH 317 



Table 3 



The changes in moisUire content of the five soils, for changes in SMT from 0-5 

 atmospheres, expressed on a weight and on a volume basis 



soil volume. On BL about ^ to | of the root system was found at the soil- 

 container junction, and much of the soil was unexploited. 



Relative growth rate (RGR) is calculated from the formula of Fisher 

 (1921): 



\0gelV2-l0geW1 



RGR = 



h-h 



where ^2-^1 is the length of the growing period in weeks, taken here to be 

 the time between appearance of the first whorl of leaves and harvest, i.e. 

 16 weeks. W2 is the mean dry weight per plant at harvest. W^ equals 

 (A — C), where A is the mean dry weight at planting of cotyledon pairs, 

 and C is the mean dry weight per treatment of cotyledon pairs removed 

 at the one whorl stage. 



The results are presented in Table 4, where other indices of performance 

 are also explained. 'Significant' assimilation is of ecological significance, 

 since it does not include the leaf weight, which is not carried forward to 

 the next growing season. 



The significance of the differences between values for certain of the indices 

 given in Table 4 was estimated from analyses of variance. The results are 

 presented in Table 5. Similar analyses could not be applied to the values 

 for aU the indices used because, for some of them, much larger variances 

 occurred in the treatments supporting good growth. 



There is considerable evidence that the growth of many plants is severely 

 reduced by SMTs between 0-5 and i-o atm (for examples see Sands and 

 Rutter, 1959), and studies within single drying-out cycles have shown that 

 for some plants growth ceases completely at SMTs of less than i atm 



