150 Plant Physiology 



the soil, and by root action. The common phosphatic 

 fertilizers are the four phosphates of lime, and the only one 

 of these which is soluble is the saturated, or superphos- 

 phate [Ca(H 2 PO 4 ) 2 + H 2 O], although the reverted or dical- 

 cic phosphate is also readily available. 



Many soluble forms of potash might be used, but the 

 important commercial forms are the sulfate, chloride 

 (muriate), and carbonate. In quantity the chloride is 

 injurious to some crops. The chief sources besides ashes 

 are now the crude products of the German potassic 

 mines. 



81. Plant nutrients in rock. It is more particularly 

 the province of instruction in soils and economic geology 

 to consider the origin of the plant nutrients of the soil. 

 The geological history is, of course, of no physiological 

 significance ; it is information ; so that it is here sufficient, 

 by way of reference, to note some few of the more impor- 

 tant facts. 



The rocks of the earth's crust from the oldest to the most 

 recent, from the hardest to the softest, whatever may have 

 been their origin, are made up of a variety of minerals, 

 some of the chemical constituents of which are the ele- 

 ments previously noted as necessary in the growth of 

 plants. Even the hardest granites, basalts, and lavas 

 contain, in general, a small percentage of potash, soda, 

 lime, magnesia, and iron. A single form of rock, such as 

 one of the red granites, may be deficient in magnesia; 

 another, like a red, soil-forming basalt, may lack in potash ; 

 whilst a limestone may contain no iron. The plant nu- 

 trients form commonly a minor portion of the bulk of the 

 rock, the balance consisting often of silica and alumina. 



