PART FOURTH 



MISCELLANEOUS FERTILIZERS AND INSECTICIDES 



498. Classification. Nitrogen, phosphoric acid, and potash are 

 the most important of the plant foods both from a commercial and 

 physiological point of view. They are the chief constituents of 

 the most important fertilizers and manures, but are by no means 

 the sole essential elements of plant nutrition. Lime, magnesia, 

 soda, sulfur, chlorin and many other elements are found constant- 

 ly in plants and must be regarded as normal constituents there- 

 of. It is the purpose here, however, to speak only of those sub- 

 stances which are used as fertilizers and which constantly or occa- 

 sionally are subjected to chemical examination for determining 

 their commercial or agronomic value. These bodies may be con- 

 veniently divided into two classes ; viz., mineral and organic. 

 Among those of mineral nature may be mentioned lime, gypsum, 

 marls, wood ashes, common salt, and ferrous sulfate ; among those 

 of organic nature may be included guano, hen manure, stall man- 

 ure, composts, and muck or peat. 



499. Forms of Lime. By the term lime is meant the prod- 

 uct obtained by subjecting limestone or other lime carbonates to 

 the action of heat until the carbon dioxid contained therein is 

 expelled. The resulting lime, CaO, when exposed for some time 

 to the air, or at once on the addition of water, is converted into 

 the hydrate CaO 2 H 2 , known as slaked lime. On longer exposure 

 to the air, the hydrate gradually absorbs carbon dioxid and be- 

 comes converted into carbonate. In whatever form lime is ap- 

 plied to the soil, it is found in the end, as carbonate. A dis- 

 tinction should also be made between lime obtained from min- 

 eral substances and that got from organic products such as 

 shells. Strictly speaking this is not a weighty matter, inasmuch 

 as limestones are sometimes but little more than aggregations of 

 fossil shells. Practically, the distinction is made, and some farm- 

 ers prefer shell lime to that of any other kind. Gas lime, that is 



