SOIL TYPE, SOIL WATER REGIME AND GROWTH 321 



This is also a possibility for D, since other experiments have shown that 

 it is an unsuitable growing medium when saturated. The ecological 

 impHcations of the growth attained on Deschampsia humus in this experi- 

 ment are particularly meaningless, since, in all three treatments, appreciable 

 surface drying and shrinkage occurred. The good growth of the seedlings 

 probably resulted from the extensive minerahsation (Birch, 1958). As far 

 as could be ascertained the other humus and soils were not appreciably 

 altered. 



Sands and Rutter (1959) have pointed out the difficulties involved in 

 attempting to apply growth analysis methods to experiments of this sort, 

 in. which a variable number of drying cycles may be included in one samp- 

 ling period. These difficulties are especially serious when soils with different 

 rates of drying-out are to be compared. However, by making several 

 assumptions and taking the growing season as the unit of time, it is possible 

 to calculate approximate figures for net assimilation rate (NAR) (Table 7). 

 These figures can be used to show that some of the differences in RGR 

 result from the effect of water-regime and soil type on NAR. 



Table 7 

 Approximate mean values of NAR (g/m^/week) for a 16-week period in summer 



1959 



* Some leaves lost as the result of infection by oak mildew, i.e. figures probably 

 too high. The values are based on the following assumptions: f leaf area at harvest = 

 mean leaf area; leaf area/leaf wt. ratio is a constant. 



There are several estabhshed examples of excessive irrigation causing 

 reduced growth (Bergman, 1959), but in almost all experiments of this 

 type maximum growth has occurred in the most frequently watered 

 treatment (Stanhill, 1957). An exception to this is an experiment by 

 Kenworthy (1949) with young apple trees. Maximum growth occurred 

 when the plants were watered at c. o-i atm SMT. A 9% reduction in rate 

 of growth occurred when the plants were watered at either 005 or 0-24 atm 

 SMT. Similarly, in heavy soils maximum rates of photosynthesis have 

 been found to occur at SMTs just above field capacity (Schneider and 



