FERTILIZERS AND PLANT FOOD. 221 



Fifth. All soils fit for agricultural purposes contain a consider- 

 able amount of the deficient plant food, even when of low crop 

 producing power. This fact gives rise to a classification of plant 

 food as follows : available plant food, or that which is in such a 

 condition that the roots of the plant can take it up in solution ; and 

 unavailable plant food, or that which the roots cannot make use of. 



Sixth. B}' the action of frost, air, water, caibonic acid, etc., 

 changes are brought about in the soil which annually convert a por- 

 tion of the unavailable into available plant food, and this portion 

 sustains the natural crop which all soils will produce. This we 

 term the natural capacity of the soil to produce crops. 



Seventh. If we desire to produce larger crops we must supply a 

 sufficient amount of available plant food in manures or fertilizers to 

 feed the increase of vegetation above what the particular soil would 

 produce, but as only a part of the available plant food is recoverec^ 

 by the crop more must be supplied than the analysis of the increasec^ 

 crop would show. 



In connection with the subject of fertilization the terrns manures, 

 fertilizers, commercial fertilizers, chemical fertilizers, indirect fer- 

 tilizers, natural fertilizers, artificial fertilizers, superphosphate, com- 

 plete and incomplete manures, etc., are used, and as there is often a 

 misunderstanding of the meaning of some of these I will give a few 

 definitions which may help us in the following pages. 



A fertilizer is asy substance which furnishes deficient plant food 

 in an available form. 



Fertilizers are either natural or artificial; the former includino- 

 manures, or the solid and liquid excrement of animals and green 

 crops plowed in to increase fertility. 



The latter, (artificial fertilizers) including commercial fertilizers, 

 sometimes called prepared fertilizers, and chemical fertilizers, or 

 those mixed from crude fertilizing chemicals. 



A fertilizer is complete, sometimes called general, when it contains 

 nitrogen, phosphoric acid and potash, and incomplete, sometimes 

 called s/>ecm/, when furnishing only one or two of these deficient forms 

 of plant food. Mixed animal manures are all complete, or general, 

 fertilizers. Green crops plowed under are complete. The artificial 

 fertilizers, whether commercial or chemical, are complete or incom- 

 plete, according as they are mixed from raw materials containing 

 the three forms of plant food above mentioned, or as they lack one 

 or more of these. A fertilizer is said to be indirect when it does 



