SPECIAL ASPECTS OF NUTRITION OF PLANTS 119 



abundant in some soils are usually soon exhausted or reduced to 

 such a small quantity that the plants indicate the deficiency of 

 the soil in nitrogen, by their poor and often sickly growth. 

 Uncultivated soils, where the plant covering is left to decay, grad- 

 ually become richer in plant foods. Cultivation of crops tends 

 to impoverish the soil, if the plant foods taken out are not re- 

 plenished, since mineral substances and nitrogen are removed in 

 the harvesting of the crop. The most costly plant food is nitrogen, 

 or nitrogenous fertilizers. Stable manure is rich in nitrogen 

 (ammonia compounds) and is also beneficial to soil, since the 

 plant remains in it improve the physical condition of the soil 

 (see nitrification, paragraph 200). Among commercial ferti- 

 lizers nitrogen is obtained in cotton seed meal; guano, the excre- 

 ment of birds found in rich deposits on certain islands near 

 southern sea coasts; Chili saltpeter, a nitrate of soda found in 

 great deposits in Chili and Peru, etc. These latter supplies of 

 nitrogen are becoming exhausted. Indeed were it not for the 

 fact that certain processes in nature are going on by which 

 nitrogen, a gas constituting four-fifths of the air, is fixed, i.e., 

 combined into nitrogenous substances, and made available for 

 plant food, the supplies of nitrogenous food would become 

 exhausted, since in the process' of decay (especially in the 

 absence of air) and by fire, the nitrogen fixed in compounds is 

 set free as a gas. 



NITRIFICATION. 



200. Nitrification. It has been pointed out above that in 

 the fields the combined nitrogen absorbed by plants is usually in 

 the form of nitrates, while in the forest where there is much 



are removed from the soil and concentrated. They are farther concen- 

 trated by animals which feed on plants, as well as by carnivorous animals. 

 On the death of the animals they are again returned to the soil, and often are 

 deposited in considerable quantities in rocks forming extensive deposits 

 known as phosphate rock. Large beds of this phosphate rock are very 

 valuable, and the rock is quarried, ground and sold as a fertilizer for the soil. 

 Extensive beds which have been of great commercial value exist in Eastern 

 South Carolina, but are now nearly exhausted. Other beds of phosphate 

 rock exist in Florida and recently some have been discovered in the West. 



