220 Bureau of Farmeks' Institutes. 



but by the action of air, water, carbonic acid, etc., it is gradually 

 changed to soluble or available forms, which the plant can take 

 up and use. As will be noticed later, phosphoric acid in the form 

 of insoluble calcium phosphate or phosphate of lime is unavailable 

 as plant food, but when converted into a superphosphate or soluble 

 calcium phosphate, it becomes available. Organic nitrogen is 

 unavailable until it is changed into nitrate nitrogen. Unavailable 

 plant food is potential food or food in reserve. 



An Indirect Fertilizer is one which does not furnish to the 

 soil any needed plant food and which may not be plant food at all, 

 but which is characterized by the way in which it acts on the matter 

 already in the soil, changing more or less of it from unavailable 

 plant food to an available form, or producing some other beneficial 

 effect in the soil. For example, lime, gypsum, salt, etc., are in- 

 direct fertilizers, as they are generally used by farmers. Later 

 some attention will be given to the action of some of the most 

 familiar indirect fertilizers. They are commonly used by far- 

 mers, not because the elements they furnish are lacking in the soil, 

 but because they can act upon unavailable plant food and render 

 it available, or because they may have some beneficial influence 

 upon the mechanical condition of the soil. 



Natural Fertilizers include the solid and liquid excrement of 

 animals, all kinds of vegetable refuse, green crops for plowing 

 under, cotton seed, mucks, marls, refuse animal matter, etc. 



Artificial Fertilizers are also known by such names as com- 

 mercial fertilizers, chemical fertilizers, etc., and are artificial 

 preparations or mixtures of fertilizing materials sold under trade 

 names. The fertilizing materials used in making these mixtures 

 include the substances found in natural deposits and by-products 

 of numerous industries, which are obtainable by farmers only 

 through the channels of trade. Some substances which might be 

 classed as natural fertilizers, such as cotton seed meal and tobacco 

 stems, are also included among the materials of artificial fertilizers. 



Complete Fertilizers, known also as general fertilizers, are 

 those which contain nitrogen, phosphoric acid and potash. 



Incomplete Fertilizers, also called special fertilizers, arc those 

 which contain only one or two of the three constituents, nitrogen, 

 phosphoric acid and potash. 



There is a common practice among farmers and dealers of call- 

 ing all commercial fertilizers "phosphates," regardless of whether 

 they contain any phosphates at all or not. The practice is clearly 

 objectionable, because a phosphate is not the only fertilizing con- 



