372 



Journal of Agricultural Research voi. xviii. no. 7 



yields of which these solutions are capable when they are diffused as films 

 on the particles of a solid substratum, such as sand, and in quantities in 

 excess of the requirements of the plants. An optimum degree of moisture 

 under such conditions is essential to impart to the soil (sand) solution 

 its maximum physiological value. It appears that the actual plant- 

 producing power of any given set of salt proportions or of any fertilizer 

 treatment is largely determined by the moisture conditions of the sub- 

 stratum. 



INFLUENCE OF MOISTURE CONTENT OF SAND CULTURES UPON TRANSPIRA- 

 TION AND WATER REQUIREMENT 



Throughout the growth period the transpirational water loss from each 

 culture during the intervals between successive weighings was recorded. 

 The total water loss from each culture was then determined by summing 

 the partial losses thus recorded for the entire growth period. The ratio 



8-6 66456344218437711221222331261513341 

 C-2423333645412E127342636764811512611i 



Fig. s. — Amounts of water lost by transpiration from wheat plants grown in sand cultures with low, 

 mediuta, and high moisture content. 



between the amount of water lost by transpiration during the entire 

 growth period and the dry weight of tops and of roots produced during 

 the same period were calculated for each culture. These ratios, which 

 are quantitative measures (expressed in cubic centimeters) of the water 

 lost by transpiration during the production of a single gram of dry tops 

 or dry roots, as well as the total water loss from each culture of the three 

 series, are presented in Table III. Each value in the table represents 

 the average of corresponding data obtained from duplicate pairs of 

 cultures of the repeated series. 



