CHAPTER VIII. 

 FERTILIZERS 



Fertilizing consists in the physical application to the soil of 

 elements which are immediately or mediately available for plant 

 food, or which aid in changing from unavailable to available 

 forms of plant food any elements already existing in the soil. 

 It is meant, of course, to exclude water, the contribution of 

 which is irrigation, but any elements held in suspension or so- 

 lution by irrigation waters, and falling under the conditions of 

 the definition, are fertilizers. 



Historical The Homeric Greeks were familiar with the use 

 of manure as a fertilizer. Cato mentions irrigation, frequent 

 tillage and manuring as means of fertilizing the soil. To these 

 Virgil adds ashes. The ancient Peruvians were skillful in the 

 application of manure, a practice that has existed in parts of 

 Russia from time immemorial. The earliest records on agri- 

 culture show that the value of fertilizing had already been 

 taught by experience. The degree to which intensive cultivation 

 had developed, the natural fertility of the soil, and the inciden- 

 tal occurrence of materials that could be used as fertilizers 

 have always been, in general, the factors determining the extent 

 of the practice. 



NATURAL FERTILITY. 



Soil Composition and its Relation to Plant Life. From a 

 physical point of view the soil of the field may be analyzed as 

 follows: (1) The soil proper, consisting of various sizes and 

 arrangements of grains made up of insoluble or imperfectly 

 soluble minerals; (2) humus, more or less decomposed organic 

 matter derived from the decay of former animal and plant life ; 



(3) the soil moisture, covering the soil grains, and containing 

 in solution a varying amount of the soluble soil constituents; 



(4) the soil atmosphere, differing from air in composition to 

 some extent, and usually saturated with water vapor; and (5) 

 soil ferments, or bacteria, which so permeate the soil that it 



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