126 GROWTH AND WORK OF PLANTS 



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. Un- 

 cultivated 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 nitro- 

 gen, or nitrogenous fertilizers. Stable manure is rich in nitrog- 

 enous substances and is also beneficial to soil, since the plant 

 remains in it improve the physical condition of the soil (see 

 nitrification, paragraph 202). Among commercial fertilizers 

 nitrogen is obtained from cotton seed meal, fish, and waste animal 

 matter; guano, the excrement 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 of the air is fixed, and made available for 

 plant food, the supplies of nitrogenous material 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. 



202. Nitrification. Nitrification is the conversion of am- 

 monia, or the nitrogen of organic compounds into nitrates. In 

 the fields the combined nitrogen absorbed by plants is usually in 



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 converted into acid phosphate and sold as a fertilizer 

 for the soil. Extensive beds which have been of great commerical 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. 



