CHAPTER II 



THE FUNCTION OF MANURES AND FERTI- 

 LIZERS, AND THE NEED OF ARTIFICIAL 

 FERTILIZERS 



WHILE in a broad sense a manure or fertilizer may 

 be regarded as anything that will increase the yield of 

 a crop if added to the land, the chief function of ma- 

 nures is to furnish nitrogen, phosphoric acid and potash. 



THE ESSENTIAL ELEMENTS OF FERTILIZERS 



These are called the "essential manurial elements," 

 or "constituents," to distinguish them from the others 

 that are needed by plants, because these three are con- 

 tained in the crops removed in greater amounts than 

 the others, and because they exist in the soil in much 

 smaller amounts than the others. For example, culti- 

 vable soils seldom contain too little iron or sulfur, or 

 magnesium. These elements usually exist in quantities 

 more than sufficient to supply all the needs of the plant 

 for them, and, because they are required in such exceed- 

 ingly small amounts, the soils are seldom exhausted of 

 them. In addition to this property of supplying essen- 

 tial manurial constituents, many substances useful as 

 manures possess, however, a secondary function : they 

 serve to indirectly increase the crop, but do not add directly 

 to the potential fertility of soils. 



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