PLANT NUTRIENTS AND THEIR ABSORPTION 105 



August. In Ben Davis and Jonathan, the fruit of which is picked the 

 beginning of October, the curve does not rise until November. Spurs 

 in the off year and barren spurs have no such characteristic decrease 

 in ash content during June. 



ABSORPTION 



Mineral constituents and nitrogen are absorbed by the plant mostly 

 through the roots. They are present in the soil as salts in solution and 

 are taken up in large part by osmosis along with the soil water, the 

 osmotic system being the same as that involved in water absorption. 



The Osmotic System. — The soil solution and the cell sap are sepa- 

 rated by a semi-permeable membrane, through which the salts present in 

 the soil solution are able to enter though the organic substances within the 

 cell are, for the most part, incapable of passing in the opposite direction. 

 Inorganic salts dissociate to a considerable degree, so that in a solution of 

 sodium chloride, for example, there are present, besides molecules of 

 salt, ions of sodium and ions of chlorine. These separate ions have the 

 same value in regard to osmotic concentration as entire molecules; 

 consequently a solution of inorganic salts is capable of producing a higher 

 osmotic pressure than a solution of organic compounds having the same 

 number of molecules in a given volume. In order that absorption of the 

 various mineral constituents should take place by osmosis, the concentra- 

 tion of each salt within the plant must be less than its concentration in 

 the soil solution. Though, as previous analyses have shown, plant 

 tissue contains considerable amounts of these mineral elements the plant 

 is still able to absorb material from an exceedingly dilute soil solution 

 which, in many cases, contains a lower percentage of a given constituent 

 than the plant tissue itself. This is possible because the constituents in 

 the plant are insoluble or are combined in an organic form. Since in 

 either case they are removed from the osmotic system, the effective con- 

 centration of inorganic salts within the plant remains less than that of 

 the soil solution. It is evident, though, that a certain concentration of 

 salts in the soil is necessary for osmotic absorption. In other words, the 

 plant is unable to avail itself of all the mineral matter of the soil solution. 

 However, very dilute solutions are often sufficient for ordinary growth. 



Thus "Birner and Lucanus many years ago found that mature crops of good 

 yield could be grown in a well water containing about 18 parts potassium (K) 

 and about 2 parts phosphoric acid (PO4) per million of solution and very satis- 

 factory'' growth of wheat has been obtained in the water from the Potomac 

 River, which contained about 7 parts per million of potassium. "^^ 



When these facts are combined with the conclusions reached by 

 Cameron and Bell,^* that the concentration of the soil solution, with 

 respect to the principal mineral plant nutrients, is sufficient for the growth 



