322 P. G. JARVIS 



Childers, 1941, with apple; Negisi and Satoo, 1954, with Pinus densi flora). 

 These results are, however, rather less striking than those described here. 



Considering performance on the five soils in this experiment, it can be 

 seen that seedlings respond similarly, and in an inverse direction to that 

 usually reported, on soils BL and SH. On both these soils RGR and NAR 

 (and on BL,R/S also) increase with increasing drying-out, demonstrating 

 the greater suitability for growth on the drier soil. Seedling performance 

 on soil H is of the expected pattern, i.e. RGR and NAR decrease with in- 

 creasing drying-out, but the figures forR/S ratio are irregular. It is possible 

 that a 'small amount' of drying-out (treatment 2) stimulated root produc- 

 tion at the expense of the shoot. Performance on BR also varies in the 

 expected direction, but the differences are mostly small and not significant : 

 NAR, RGR and R/S tend to decrease with increasing drying-out. Per- 

 formance on D is remarkable in that RGR decreases and NAR increases 

 with increasing drying-out. This difference may be related to the occur- 

 rence of mineraHsation, or to some other factor such as the stability of 

 toxic substances in the different moisture treatments. 



Part of the differences in growth, between the different soils, in response 

 to the water regimes may result from apparently similar water regimes 

 causing different degrees of water stress in the different soils. This could 

 be caused by differences in the osmotic pressure of the soil solution, and in 

 the permeabihty of the soil. 



The osmotic component of moisture stress will raise the total soil 

 moisture stress above the SMT by a characteristic amount for each soil. 

 Such an effect would tend to accentuate responses, of the type occurring on 

 H, to water stress in nutrient-rich soils, but could not account for the pattern 

 of response on soils BL, SH and BR, which are anyway low in nutrients. 



The method used for measuring SMT did not measure tensions develop- 

 ing at the root surface. When plants are transpiring rapidly, these tensions 

 may be greatly in excess of those in the soil short distances away. The 

 magnitude of tliis difference in tension will partly depend on the rate of 

 movement of water to the root, i.e. on the permeabihty of the soil (Slatyer, 

 i960). Hence, similar SMTs measured in soils of different permeabihties 

 do not necessarily indicate that the plants are experiencing similar moisture 

 stress. However, the greatest difference between actual stress and measured 

 stress should occur in the least permeable soils, and hence this effect cannot 

 be responsible for the pattern of response on soils BL, SH and BR. 



Since, on soils BL and SH, RGR, NAR and R/S (on BL) increase with 

 increasing drying-out of the soil, it seems probable that the conditions of 

 aeration in these soils are hmiting growth. Inverse relations between soil 



