THE SOIL AS RELATED TO PLANTS. 87 



are obtained from animal, mineral, and vegetable 

 sources, but the source of fertilizers has nothing 

 whatever to do with their value as such. The 

 value depends upon the form in which a fer- 

 tilizer contains the particular plant-food desired. 

 The nitrogen, if wanted for the immediate use 

 of the plant, is best in the form of a nitrate, 

 since it is soluble, and may be better distributed 

 through the soil to the feeding roots, and is 

 readily taken up by them. 



Ammonia is the next nitrogeneous plant-food 

 in order as regards availability. Some plants 

 can use ammonium salts, which are soluble in 

 water, and thus are easily distributed through- 

 out the soil to the roots. As a rule, however, 

 the salts of ammonia are changed into nitrates 

 (see " Nitrifying Bacteria "), which is done very 

 rapidly in the soil before being used by plants. 



Animal or vegetable products cannot furnish 

 available nitrogen to plants until decomposition 

 takes place; hence the more rapid the decay of 

 an organic fertilizer the more readily available 

 is its nitrogen, since it must first be converted 

 into ammonia and then into nitrates. (See 

 "Nitrifying Bacteria.") 



Among the fertilizers of animal origin, which 

 are largely used on account of their rapid decay 

 and comparative inexpensiveness, are : dried 

 blood, dried meat and fish, -hoof-meal, and guano. 

 Others as, wool, hair, and leather decay more 



