152 The Department of Plant Phystology [350 
plant water relation, and the near future promises much 
greater advances. ane 
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 (IX), 
calcium (Ca), magnesium (Mg), iron (Fe), nitrate (No,), 
sulphate (SO,), and phosphate (PO,). 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 (CaNO,, 
MgSO, and KH,PO,) are present in proper proportions to 
produce a physiologically balanced solution, producing excel- 
‘lent growth. The proper salt proportions for any plant form, 
