64 FIRST PRINCIPLES OF AGRICULTURE. 



formerly unproductive, chiefly because of physical imper- 

 fections, have been made very productive mainly through 

 the application of marl. 



The use of marl is now less general than when the 

 fertilizing constituents from artificial sources were dearer, 

 and when the labor of the farm was more abundant and 

 cheaper. The quicker effect of more soluble fertilizer 

 constituents has had an influence in reducing the use of 

 marl where quick returns are desirable. Where farmers 

 have deposits upon their own farms, or within short dis- 

 tances of them, and can secure it at a low price per ton, 

 it is a desirable method of improving land. 



The results from the use of marl are frequently due 

 quite as much to the improvement given to the physical 

 condition of soils as to the increase in fertility furnished 

 by the essential mineral constituents. Marls may be 

 carted and spread upon the land when other work of 

 the farm is not pressing, thus making it possible to get 

 a considerable addition of fertility at a small expense. 



Lime. Lime, while an essential constituent of plants, 

 is usually more abundant in soils than the other mineral 

 constituents, phosphoric acid and potash. It is, how- 

 ever, regarded as a direct source of plant-food in a great 

 many cases, though its greatest value lies in its favorable 

 action upon soils. This action is both physical and 

 chemical, and has already been discussed in a previous 

 chapter. 



Lime, as is it generally understood, is an oxide of cal- 

 cium, and is produced by burning limestone, or carbonate 

 of lime. The lime loses the carbonic acid when burned in 

 the kilns, and the oxide of lime remains behind ; this is 



