93 



substances, and this, together with the fact of its concentration, makes 

 the sulphate an extremely valuable form of nitrogen. While it is 

 extremely soluble in water, it is not readily removed from the soil by 

 leaching, except in the absence of growing plants, since it is readily 

 absoi'bed by organic and other compounds of the soil. 



Nitrogen as nitrate exists in commercial products as nitrate of soda, 

 nitrate of potash, etc. These, like the ammonia compounds, are ex- 

 tremely soluble, and the nitrogen contained in them is readily avail- 

 able as food for plants. The nitrogen in this form is directly and imme- 

 diately available, no further changes being necessary. 



The chief source of nitrogen as nitrate is nitrate of soda. This salt 

 is rich in nitrogen, showing on the average 16 per cent, and is quite 

 uniform in composition. It is completely soluble in water, diffuses 

 readily throughout the soil, and differs from the ammonia compounds 

 in forming no insoluble compounds with soil constituents. It is, there- 

 fore, liable to be washed out of the soil if applied in too large quanti- 

 ties or when there is an absence of vegetation. Plants that derive 

 their nitrogen from the soil secure it chiefly in the form of a nitrate ; 

 hence nitrate of soda is one of the most directly useful of the nitroge- 

 nous materials. 



As already explained, nitrogen in organic forms is changed into 

 ammonia by the decay or rotting of the substance. Ammonia, while it 

 may nourish plants directly, is usually changed into a nitrate, in which 

 form it is taken up by the plant. An application of nitrogen as nitrates 

 may be completely used by the plant in a very short time ; as ammonia 

 or organic matter it may be partially or wholly used in the course of a 

 season, depending upon whether the conditions are favorable for caus- 

 ing the changes that must take place. 



PHOSPHORIC ACID. 



Phosphoric acid is derived from materials called phosphates, in which it 

 may exist in combination with lime, iron, or alumina as phosphates of lime, 

 iron, or alumina. Phosphate of lime, however, is the form most largely 

 used as a source of phosphoric acid. Phosphoric acid occurs in fertilizers 

 in three forms : That soluble in water and readily taken up by plants ; that 

 insoluble in water but still readily tised by plants, also known as "rever- 

 ted; " and that soluble only in strong acids and consequently very slowly 

 used by the plant. The soluble and " reverted" together constitute the 

 " available phosphoric acid. The phosphoric acid in natural or untreated 

 phosphates isinsoluble in water and not readily available to plants ; that is, 

 the rate of availability depends largely upon the rapidity with .which 

 the substance rots or decays, and the rate of decay again depends upon 

 the character of the substance with which the phosphate is associated. 

 If it is combined with organic substance, as in animal bone, the rate of 

 decay is more rapid than if with purely mineral substances. The inso- 

 luble phosphates are converted into soluble forms by treatment with 

 strong acids, as explained later. Such products are known as acid phos- 

 phates or superphosphates. 



Bone, in its various forms, is the only one of the insoluble phosphates 

 that is now used directly upon the soil, or without other change than is 

 accomplished by mechanical action or grinding. The terms used to in- 

 dicate the character of the bone have reference rather to their mechani- 

 cal form than to the relative availability of the phosphoric acid contain- 



