22 MANUFACTURE OF FERTILIZING MATERIALS 



of attracting and holding water. Its further 

 action has already been explained. 



Phosphorus never occurs in nature in a free 

 state but exists in combination in greater or less 

 quantities in all soils and in many minerals. Its 

 combinations are also found in large deposits of 

 mineral known as phosphorite, apatite and as so- 

 called pebble and phosphate rock. Phosphorus 

 in some sort of combination is one of the most 

 essential elements in animal and plant food. 

 In animals its compounds form almost all of 

 the mineral matter of the bones, and in plants 

 they are important constituents of the ash of 

 seeds. It exists in organic combination both in 

 animal and vegetable tissue as lecithines and 

 other compounds. 



Nitrogen as a mineral constituent of soils, is 

 found chiefly in the form of nitrates, but owing to 

 their solubility, they cannot accumulate in soils 

 exposed to heavy rainfalls. The gaseous nitrogen 

 in the soil is also of some importance, since it is 

 on this material that the organisms which have a 

 symbiotic activity with the rootlets of some plants 

 probably act in the process of the fixation of 

 atmospheric nitrogen in a form accessible to plants. 

 Nitrogen in the free state, it is believed, is not 

 directly absorbed into the tissues of green plants. 

 It is necessary that it be oxidized in some way to 

 nitric acid or some compound containing it before 

 it can be assimilated. The importance of nitro- 

 gen as a plant food has been already described. 



