Wad lei gh and Richards 433 



films on nutrient availability, and (c) the effect of variations in soil 

 moisture tension upon microbiological activity. 



The availability of soil moisture 

 within the available range 



It is at once obvious that regardless of how nearly optimal the level 

 of mineral nutrients and other growth factors may be, growth will be 

 limited by the extent to which water supply to the plant is limited. 

 Hence, the question arises: is water ever limiting to growth within the 

 bounds of the available range of soil moisture? 



Veihmeyer and Hendrickson {122, 121) have carried out extensive 

 investigations on the availability of soil moisture to tree fruits growing 

 on deep alluvial soils in California. Their observations indicated that 

 for all practical purposes, soil moisture between field capacity and 

 nearly down to the wilting percentage is essentially of equal availability 

 to the plant. It is evident, a priori, that more osmotic work is required 

 for the entry of water into a plant when the water is restrained by a 

 force of 15 atmospheres (approximate retentive force at wilting per- 

 centage) as compared to a retentive force of only 0.1 atmosphere; i.e., 

 ease of entry of water into a plant may change markedly over the 

 available range. The apparent contradiction between the two previous 

 statements is partially ameliorated by the fact that the relationship 

 between moisture content and moisture retention is invariably hyper- 

 bolic (Figure 1); that is, most of the available water is removed from 

 the soil before a marked increase in soil moisture tension develops. 

 Furthermore, whether or not modifications in growth response will 

 be observed with variations in depletion of soil moisture within the 

 available range will depend on (a) the nature of the soil, (b) prevail- 

 ing weather conditions, (c) kind of plant being studied, and (d) the 

 criterion of growth being used. 



Deep alluvial soils that permit deep root penetration must be con- 

 trasted with shallow soils or dense, impervious soils. Thus, Boynton 

 (12) found a definite decrease in growth of apples on a shallow soil in 

 New York, if the soil moisture content of the surface two feet decreased 

 to the wilting percentage. That is, the moisture reservoir for these 

 shallow-rooted apple trees was virtually exhausted under the stated 



