SOIL TYPE, SOIL WATER REGIME AND GROWTH 323 



water content and oxygen content are well established (Furr and Aldrich, 

 1943; Boynton andReuther, 1939; Vine, Thompson and Hardy, 1942). 

 Boynton and Reuther were able to relate the performance of apple trees 

 to the influence of pore size on aeration; and Taylor (1949) showed that 

 oxygen diffusion in soil is greatly influenced by the degree of compaction 

 and water content of the soil, through their effects on the available air- 

 fdled pore space. No estimates of pore size were made for these soils, but 

 the estimates of apparent density, oxygen diffusion rate and water avaU- 

 abihty provide some relevant information about their physical properties. 



The apparent densities of the soils BL and BR are close to or above the 

 upper hmit of 1-9, above which Veihmeyer and Hendrickson (1946, 1948) 

 state that, for most soils, no roots of any description are found. Mechanical 

 impedence apparently restricted root growth on BL, since, for all treat- 

 ments on this soil, only a part of the soil volume was exploited. However, 

 theR/S ratio increases with increasing amounts of drying-out, indicating 

 the interaction with aeration. It would also be expected, as noted by 

 Veihmeyer and Hendrickson, that the lower apparent density of the fmer- 

 textured soil SH would have as great an inhibiting effect as the higher 

 apparent densities in the coarser-textured soils. 



It can be seen from Fig. 2 and Table 3 that the three soils BL, SH and BR 

 differ greatly from the humus soils in the amounts of water held between 

 o-i and 1 5 atm SMT. Hence, small reductions in water content will produce 

 large increases in SMT and consequent reductions in growth (Richards and 

 Wadleigh, 1952). However, the very small amounts of water held in these 

 soils may be taken to indicate very small pore volumes and hence deficient 

 aeration in the moist soils. Improvements in aeration as a result of drying- 

 out are apparently more important than the removal of water or increases 

 in SMT. Tliis would be expected if the conditions of aeration were restrict- 

 ing water or nutrient uptake. 



The oxygen diffusion rates (Table 2) were obtained in connection with 

 another experiment and are for soils at 'field-capacity' only. They demon- 

 strate general differences between the soil types, but emphasise that, 

 although it is possible to interpret the results for any one soil in terms of 

 aeration and SMT, the differences in growth between soils result from 

 interactions of physical and nutritional factors. 



It can be concluded from the results on leaf Htter that, when growth is 

 good and not severely restricted by other soil factors, assimilation by 

 seedlings is considerably reduced by low SMTs. RGR is reduced by half 

 by a SMT of 0-5 atm acting for 62% of the growing period (assuming 

 proportionahty of effect). 



