358 Journal of Agricultural Research voi. xviii. no. 7 



It has frequently been observed that plants grow less rapidly in a nu- 

 trient solution of high total concentration (within certain limits) without 

 toxic effects than they do in a less concentrated solution containing the 

 same constituents in the same relative proportions. It is also a fact of 

 common observation that, regardless of the total concentration of the 

 medium, plants grow less rapidly in a solid substratum, such as soil with 

 low moisture content, than they do in the same medium with an optimum 

 supply of moisture. If, now, a given set of salt proportions, in solution 

 with approximately optimum total concentration for plant growth, 

 should be added to an inert solid medium, such as sand, in such quantities 

 as to produce a very low moisture content, it is obvious that plants would 

 grow less rapidly in such a culture than in a similar culture supplied with 

 the same solution to give an optimum moisture content. Thus, with any 

 given set of salt proportions somewhat similar changes in growth or other 

 forms of plant activity may result (i) from growing the plants in solutions 

 with varying total concentrations of the salts in the given proportions, 

 or (2) from growing the plants in an inert solid substratum, such as sand, 

 supplied with the given set of salt proportions in solutions with constant 

 total concentration so as to produce varying degrees of moisture content. 

 Here any changes in growth with alterations in the degree of moisture 

 would take place independently of the total concentration of the soil 

 (sand) solution. 



Since both variations in total concentration of the nutrient medium 

 and alterations in the moisture content of a solid substratum have a 

 somewhat similar influence upon plant activities with respect to retarded 

 or accelerated growth rates, and since it is already well known that the 

 physiological value of any given set of salt proportions is markedly 

 influenced by total concentration, it seemed desirable to determine 

 whether or not the salt balance of nutrient solutions could be similarly 

 influenced or the relative value of a fertilizer treatment be altered by 

 variations m the degree of the moisture content of a solid substratum, 

 such as sand, when the mineral nutrients are diffused as films on the 

 solid particles in the form of solutions of constant total concentrations. 

 The following pages present the result of an experimental study dealing 

 with this question. 



OUTLINE OF EXPERIMENTAL METHODS 



In order to study the effect of differences in the moisture content of a 

 solid substratum upon the physiological salt balance of the nutrient 

 medium, sand to which nutrient solutions were added was here used as 

 the medium in which the plants were grown. The nutrient media added 

 to the sand consisted of the 36 different 3-salt solutions (osmotic con- 

 centration value 1.75 atmospheres) comprised in an optimum series 

 previously used with wheat {12) and with buckwheat {13) in studies of 

 physiological salt balance. Three series of these sand cultures were 



