320 



p. G. JARVIS 



1A0^ 



20 40 60 



% Atmosphere days 



80 



Fig. 3. The effect of the five soils on the relation between the average relative growth 

 rate (of seedlings of sessile oak over a 16-week growing season) and the amount of 

 drying-out of the soil (summed as percentage atmosphere days, up to a SMT of 0-5 

 atm) which was allowed to develop. 



DISCUSSION 



Large differences occur between the maximum RGRs of seedlings on the 

 different soils. Since only five soils are involved, it is not possible to make 

 any far-reaching correlations between plant performance and a particular 

 soil character. However, it may be noted that better growth is associated 

 with the following soil characters: higher humus content, higher base 

 status, higher oxygen diffusion rate, lower apparent density, and greater 

 moisture content (gm/ioo cc of soil) held between 0-5 atm SMT, i.e. 

 larger pore volume. 



The response to the different soil water regimes is very different on the 

 different soil types. Only on leaf litter (H) and Deschampsia humus (D), the 

 soils which supported the highest RGRs, are there marked decreases in 

 RGR in response to the drying-out treatments. On both the soil from the 

 bleached horizon (BL), and the clayey soil from the shale band (SH), the 

 RGRs show significant positive increases with drying-out. The brown 

 earth soil (BR) shows no significant response in RGR; it is possible, for this 

 soil, that the optimum soil-moisture regime Hes between treatments i and 2. 



