CHEMISTRY OF FERTILIZERS 11 



A stimulant or indirect fertilizer is one which 

 does not in itself furnish directly to the soil any 

 needed plant food, but whose chief value depends 

 upon the power it possesses of changing unavail- 

 able into available forms of plant food. The stim- 

 ulant or indirect fertilizers which have been most 

 commonly employed are lime, gypsum, and 

 common salt or sodium chloride. 



Gypsum, or land-plaster, known chemically as 

 calcium sulphate, or sulphate of lime, in some 

 manner aids the process of nitrification, by which 

 ammonia and the nitrogen of organic matter are 

 converted into nitric acid and nitrates. It also 

 acts upon the insoluble forms of potash and other 

 elements of plant food, converting them into 

 soluble and available forms; it is of value on 

 certain soils to certain crops, such as clover, peas, 

 lucerne and similar plants. 



Quick or burnt lime, or calcium oxide, com- 

 monly called lime, produces changes in both the 

 physical and the chemical character of soils. 

 Freshly burned lime acts chemically upon soils 

 by decomposing vegetable and mineral matter 

 present in the soil and changing them into forms 

 which are available as food for plants. Thus 

 lime acts upon insoluble mineral substances con- 

 taining potash, etc., and converts them into 

 soluble forms. Lime aids in the decomposition 

 of animal and vegetable matter, such as vegetable 

 mould, stable-manure, etc., and tends to con- 

 vert them into available plant food. In using 



