376 Journal of Agricultural Research voi. xvni. No. 7 



of water was required to produce a single gram of dry plant material, 

 and the water requirement ratios are high for this series. With the 

 lowest moisture content, on the other hand, the transpiration rates 

 were retarded proportionately more than were the growth rates. Thus 

 for each gram of dry plant substance produced a relatively small quantity 

 of water was required, and the water-requirement ratios for this series 

 are low. A further comparison of the graphs representing water re- 

 quirement of tops and of roots with the corresponding ones of transpira- 

 tion and yields brings out the fact that the approximately optimum 

 moisture content here employed with the cultures of series B is corre- 

 lated with maximum yields of tops and of roots, with high transpiration 

 rates, and with water-requirement ratios which are intermediate in 

 value between those of the other series (series A and C), having degrees 

 of moisture in the sand cultures considerably below and above that of 

 the optimum. 



SUMMARY 



This paper is a report of studies on the influence of different degrees 

 of moisture in a solid substratum upon the physiological salt balance 

 for young wheat plants and upon the relative plant-producing value 

 of various salt proportions. 



Three different degrees of moisture were maintained in sand cultures: 

 40 per cent, 60 per cent, and 80 per cent of the water-retaining 

 capacity of the sand. Tests were made with 36 different sets of salt 

 proportions of the three salts KH2PO4, Ca(N03)2, and MgS04 in solutions 

 (Shive's optimum 3-salt series) with each of the three different degrees 

 of moisture. The solutions, all having an initial total osmotic concen- 

 tration value of 1.75 atmospheres, were supplied to the sand cultures in 

 such quantities as to produce the different degrees of moisture. All 

 three of the different moisture-content series were conducted simulta- 

 neously and were then repeated. The culture solutions were renewed 

 every third day. The cultures were weighed each day, and the water 

 loss by transpiration was restored through the entire growth period of 

 28 days. The growth period was the same for the first and the repeated 

 series. 



The main results of this study may be summarized as follows: 



(i) For the set of conditions under which these tests were made the 

 physiological balance of the nutrient solutions producing the best yields 

 of wheat tops and roots was not altered by variations in the moisture 

 content of the solid substratum to which the solutions were applied. 

 The physiological balance of salt proportions which was best mth the 

 lowest moisture content was the best also with the medium and the 

 highest degree of moisture. 



(2) A slight shifting of the physiological balance, as this affects the 

 growth of plants, is indicated for the growth of 9 high-yielding cul- 

 tures, as a whole, out of a series of 36, with each increase in the moisture 



