[359] SCIENTIFIC MANUAL. 73 



One is special, direct and partial, and designed to affect 

 a particular crop, while the other is general, indirect, and 

 designed to permanently improve the soil, and supply nour- 

 ishment to more than one generation of plants. One sup- 

 plies just those elements of plant- food in ratio to each 

 other, and in aggregate quantity sufficient to nourish par- 

 ticular plants ; the other supplies all the elements of plant- 

 food to the soil as a reservoir from which any plant may 

 select the particular elements necessary for its nutrition. 



One is temporary and looks to immediate returns the 

 other is more permanent, improving the capital invested 

 in land, and increasing the returns through a series of years. 



As population becomes dense, and a higher rate of pro- 

 duction is rendered necessary for its support, as well as to 

 insure a reasonable per cent, upon the enhanced value of 

 lands, soil fertilization is combined with plant fertilization 

 to insure both prompt and continuous returns. 



THE MEANS USED. 



To secure the permanent improvement of a soil, it must 

 not only be abundantly supplied with the various elements 

 of plant-food in forms either available or readily rendered 

 so, but the mechanical condition must be such as to afford 

 the free expansion of the roots of plants, and to freely ab- 

 sorb, and retentively hold, moisture and ammonia. 



It is, therefore, necessary to supply three classes of sub^ 

 stances to the soil to compass these ends, viz : actual plant- 

 food, chemical agents, and mechanical agents. 



Actual plant-food, or fertilizers proper^ embrace all the 

 mineral elements of plant-food, and one volatile nitrogen. 



Phosphoric acid and potash are the only mineral ingre- 

 dients which are so far exhausted as to require artificial ap- 

 plication to most soils, though soda, lime, magnesia, 

 and, for some plants, chlorine in small quantities are 

 sometimes added. Lime, which acts in the triple capacity 

 of fertilizer, chemical agent, and mechanical agent, is pres* 



