70 



GENERAL BIOLOGY 



m 



to bear on the agriculturist to replace by fertilizers 

 what his crops have drawn out of the soil. 



It is obvious that the materials thus removed 

 depend largely upon the kind of plant that is grow- 

 ing, each kind drawing out certain 

 things for its own particular needs. 

 Yet there are some mineral com- 

 pounds that are demanded by all 

 plants, the absence of which in- 

 terferes with or prevents normal 

 growth. These, as has been noted 

 already, are the metal salts 

 of phosphoric, nitric, and 

 sulphuric acids, usually po- 

 tassium phos- 

 phate, potassium 

 nitrate, and cal- 

 cium and mag- 

 nesium sulphates. 

 It is possible to 

 make a solution 1 

 of a mixture of 

 these salts in 

 which plants will 

 thrive. The ab- 

 sence of any of the above-mentioned elements in 

 such a solution induces marked disturbances in the 

 normal growth of the plant. 



1 The following solution, devised by Schimper, has been much used in 

 experimental work : calcium nitrate, 6 gm. ; potassium nitrate, 15 gm. ; 

 magnesium nitrate, 15 gm. ; potassium phosphate, 15 gm. ; sodium nitrate, 

 1.5 gm. ; distilled water, 600 cc., to which is added a trace of ferric chloride. 



FIG. 23. Cultures of hemp grown in 

 neutral solid substratum. A complete nu- 

 trient solution has been added to /, and the 

 plants have attained a height of 1.5 meters ; 

 a solution lacking potassium nitrate has been 

 added to the substratum in //, and only the 

 sterile substratum placed in the pot in ///. 

 (From MacDougle, after Ville.) 



