Jan. 2, 1920 Relation of Moisture in Substrata to Salt Balance 371 



spect to the actual root yields produced more than either one resembles 

 the series with medium moisture content. This particular relation 

 between these two series was also apparent but less marked in the graphs 

 representing the dry- weight yields of tops; and, as previously remarked, 

 it is of especial interest in connection with the fact that these two series 

 represent the extremes of moisture content. The graphs of these two 

 series intersect at various points, but there is no marked tendency for 

 the yield values, as a whole, to be either higher or lower with one series 

 than with the other. From an a priori consideration of the problem, 

 however, this is not what might be expected. It appears that the growth 

 rates of both tops and roots are considerably retarded by low moisture 

 content. This is unquestionably the result of greater resistance to water 

 absorption by the plant roots, resulting in an internal water supply 

 deficient for optimum growth. 



It might be expected that with sand cultures such as were here em- 

 ployed, with approximately constant total concentrations of the nutrient 

 media, a progressive increase in the moisture content up to the point of 

 saturation as a limit to the moisture variation would correspondingly 

 accelerate the growth rates of the plants because of a decreased resistance 

 to water absorption. This, however, does not occur, as the graphs of 

 figures 2 and 4 clearly show. It appears that the growth rates are accel- 

 erated by an increase in the moisture content of the sand cultures up to 

 a certain optimum, after which with further increase in the moisture 

 content there is a marked retardation in the rates of growth. This is to 

 be attributed to other factors unfavorable to growth, which are intro- 

 y duced with increased moisture content above the optimum. Whatever 

 the nature of these factors may be, it is clear, as has been brought out, 

 that a sand culture supplied with a well-balanced nutrient solution to 

 give an optimum moisture content is much superior in plant-producing 

 power to a similar sand culture supplied with the same solutions to pro- 

 duce a moisture content closely approaching the point of saturation. 

 In this connection Hall, Brenchley, and Underwood (4) have pointed 

 out that growth in nutrient solutions diffused as films over sand particles 

 is much superior to that in water cultures with the same solutions, but 

 the growth in water cultures is similarly increased when a continuous 

 air current is passed through the solutions. They ascribe enormous 

 advantages to the plants from continuous aeration and attribute to this 

 factor alone the superiority of growth in solid substrata over that in the 

 ordinary water cultures in which aeration is not continuous. 



The fact that the average dry-weight 34elds of both tops and roots 

 from the best nine cultures of the series employing a medium moisture 

 content are always considerably higher than are the corresponding yields 

 from the series with the lowest and highest moisture contents here used 

 clearly shows that well-balanced solutions with optimum total concen- 

 trations for plant growth are not alone sufficient to produce the best 



