152 The Department of Plant Physiology [350 



plant water relation, and the near future promises much 

 greater advances. 



The inorganic salt relation of plants. This relation involves 

 the plant responses that result from alterations in the supply 

 and in the consumption or loss of inorganic salts. So far as 

 studies of this relation have progressed, these have dealt 

 mainly with the power of the surroundings to deliver inor- 

 ganic salts (or the ions into which they dissociate) to plant 

 roots, as this power is related to growth. This aspect of this 

 relation has formed the subject of very many experimental 

 investigations during the past century, but the work of this 

 laboratory has approached the problem from a somewhat new 

 point of view. 



The soil presents such a very complicated physical and 

 chemical system that it is quite hopeless, for the present, to 

 attempt to understand the behavior of soil salts in any way 

 adequate to the needs of plant physiology, and our attention 

 has been turned exclusively to the study of plant growth in 

 nutrient solutions and in sand cultures. For the growth of 

 ordinary plants it requires only seven ions of inorganic salts 

 to produce satisfactory growth, these being: potassium (K), 

 calcium (Ca), magnesium (Mg), iron (Fe), nitrate (No 3 ), 

 sulphate (S0 4 ), and phosphate (P0 4 ). Iron is needed in 

 relatively but very small "amount, it being only necessary that 

 the solution bathing the plant roots shall contain a trace of 

 this ion. Variations in the partial concentrations, or in the 

 supply, of the other six ions may produce marked alterations 

 in growth, however, and it is with reference to these that our 

 work was begun. By means of elaborate series of different 

 culture solutions the effects upon the plant, of altering the salt 

 proportions in the nutrient medium, have been experimentally 

 studied. It has been possible to devise a 3-salt nutrient solu- 

 tion for use as a standard, in which the three salts (CaN0 3; 

 MgS0 4 and KH 2 P0 4 ) are present in proper proportions to 

 produce a physiologically balanced solution, producing excel- 

 lent growth. The proper salt proportions for any plant form, 



