116 THE PRINCIPLES OF AGRICULTURE 



these solutions become a part of the ascending 

 sap, and they are diffused to all parts where 

 assimilation goes on. Plants possess a certain 

 selective absorption, yet soil elements not utilized 

 by the plant are also absorbed in greater or 

 less quantity depending on whether or not de- 

 posited in inert form. Carbonic acid, and per- 

 haps other substances excreted by the root, aid 

 in dissolving some of the mineral salts (30) . 



191. Various substances are taken in with 

 the soil water. Sodium and potassium nitrate 

 (nitre), calcium phosphate (phosphate of lime), 

 and potassium sulfate are well-known ingredients 

 of fertilizers. Chemical analysis and experiments 

 show that from these and allied salts the plant 

 obtains from the soil such necessary elements as 

 nitrogen, potassium, phosphorus, calcium, and 

 sulfur. In addition, plants also secure from the 

 soil traces of iron, and whatever magnesium, sili- 

 con, and other mineral elements maybe necessary. 



192. When a plant is burned in air, the ash 

 contains all of the above-named elements except 

 the nitrogen and a part of the sulfur and phos- 

 phorus. Nitrogen, one of the most important of 

 plant- foods, can be used chiefly in the form of 

 nitrates, except in the case of leguminous plants 

 (110, 138) , in which it is also taken from the air in 

 some obscure way by bacteria of the root tubercles. 



