8 



possess of changing the insoluble and unavailable potash and phosphoric 

 acid, int_> available forms; and, second, those which furnish directly to 

 the soil th-j more important plant food constituents. Among the com- 

 mon materials of the first class are gypsum, lime, and common salt. 



Indirect Fertilizers. 



Gypsum, or land^plaster, is sulphate of calcium, and has a limited 

 action. It does furnish calcium and some sulphur, which are both re- 

 quired in considerable quantities by such crops as clover and turnips ; but 

 its chief action is in aiding the process of nitrification, by which ammonia 

 and the nitrogen of organic matter are converted into forms which are 

 readily assimilated by the plant, and in liberating potash and other ele- 

 ments of plant food from insoluble forms of combination and making them 

 available. 



Lime, like gypsum, aids nitrification and liberates plant food from 

 insoluble forms of combination ; but it is more powerful in its action. 

 Heavy clays, which are rich in insoluble forms of potash, and soils con- 

 taining large quantities of humus, are those most benefited by lime. In 

 reclaiming swamp lands, the acid humic matter of the peat is neutralized 

 by the lime, and the conditions thus made suitable for the oxidation of the 

 nitrogenous organic matter and the production of ammonia and nitrates. 

 Lime has also a very beneficial influence on the physical condition of the 

 soil. 



Common Salt supplies no essential ingredient of plant food. The 

 little value which it possesses is probably due to its action in the soil, 

 where it helps to set free more important constituents, particularly potash. 



It is important to bear in mind that these indirect fertilizers do 

 not add plant food to the soil, but that their chief value lies in the fact 

 that they liberate plant food from insoluble forms of combination. Hence, 

 if crops are not growing on the land to make use of the liberated food, 

 or if the soil has been over stimulated by a large and frequent application 

 of lime, gypsum, or salt, loss of nitrogen, potash, and phosphoric acid 

 will occur. Consequently, these stimulants should be used in moderation. 

 On soils not acid in nature, one to one and a-half tons per acre of lime 

 at intervals of five or six years would be a safe application. 



Direct Fertilizers. 



Direct fertilizers contain forms of plant food which contribute 

 directly to the growth of plants. Such materials may contain either 

 nitrogen, potash, or phosphoric acid compounds, or any two, or all three 

 of these forms of plant nutrients. 



Nitrogen. The more important purely nitrogenous fertilizers are 

 nitrate of soda, sulphate of ammonia, and dried blood. A new one, 



