Hoagland _108— Plant Nutrition 



The upper limit of the most favorable range of 

 total salt concentration is not very high for many- 

 kinds of plants, perhaps about 2 or 21/2 atmospheres 

 equivalent osmotic pressure in some cases. Therefore, 

 if a large amount of nutrient salt is needed, over a 

 given experimental period, for the growth of a given 

 number of plants, one cannot simply increase the 

 concentration in order to provide at the beginning 

 of the experiment the entire supply of nutrient salts 

 in a small volume of solution. The objective would 

 have to be accomplished by sufficiently increasing the 

 total volume of solution at a favorable initial level of 

 concentration, assuming that it is not desired to add 

 more nutrient salts from time to time, or to change 

 solutions. What constitutes a critically low concen- 

 tration of an ion at a particular period depends upon 

 many factors, including the type of growth the plant 

 makes. Wheat plants, for example, can absorb enough 

 phosphate within a few weeks to insure a reasonably 

 adequate supply stored in the plant for the remainder 

 of the growth cycle. The tomato plant with its in- 

 determinate growth presents a different problem. 



In view of these, as well as many other complica- 

 tions, to decide whether or not a particular salt bal- 

 ance per se is favorable or unfavorable, is indeed very 

 difficult. We can scarcely doubt that unfavorable salt 

 balances can be produced in a nutrient solution irre- 

 spective of the sufficiency of total nutrient supply, but 

 it appears that plants of similar size and similar 

 general characteristics can be grown in solutions of 

 varying salt proportions over a rather wide range of 

 salt proportions, if the supplying power of the medium 

 remains adequate. This is not surprising when we 

 remember that plants may thrive in different soils, 

 not alike in soil solution composition or in proportions 

 of bases held on soil colloids. Further, general ob- 

 servation shows that plants of different species, with 

 different physiological requirements, may thrive in the 



