214 INTRODUCTION TO GENERAL SCIENCE 



well not be there. The plowing of land tends to bring up, 

 and expose to the action of the air lower layers of plant food, 

 and for this reason it is advantageous, where the soil is fairly 

 thick, to plow quite deeply. 



Plants are unable to absorb the nitrogen of the air directly, 

 but must obtain it through a solution of some compound, 

 which is taken in through the roots. This is true of all plant 

 food. Compare Leaves, Section 165. There are certain 

 bacteria that grow on the roots of alfalfa, beans, peas, len- 

 tils, cowpeas, and other leguminous plants, which have the 

 power of absorbing the nitrogen of the air and changing it 

 into some nitrate, which is then readily absorbed by the 

 plant. If these bacteria are not present, there must be some 

 nitrate added to the soil, and this is very often sodium 

 nitrate. Barnyard manure is a rich source of nitrogen. 



Phosphoric acid itself is not added to the soil, but some 

 soluble phosphate is used. The plant takes what it needs. 

 The chief source of phosphoric acid is calcium phosphate, 

 which has been rendered soluble by changing it into a lower 

 phosphate by sulphuric acid, and it is the phosphorus part 

 which is made use of by the plant. Other sources are barn- 

 yard manure, bone and refuse from meat-packing houses. 



Potash is necessary for the development of plants, and plays 

 a large part in the formation of seeds. The potassium does 

 not seem to supply material for food directly, but aids in the 

 changes and assimilation of the starches and sugars. Some 

 plants need much more than others, but potash is found 

 throughout the whole of every plant. Thus the ashes of 

 wood is a source of potash. 



The organic acids form salts with the potash acquired from 

 the soil, and the amount formed shows the energy of the plant. 

 Since chlorine seems to be needed in the changing of the hy- 



