108 ELEMENTS OF AGRICULTURE 



crops of tobacco or cotton are removed year after year 

 from the same field and no organic matter is added, the 

 soil's store of nitrogen soon becomes exhausted. 



106. Denitrification. — "But the loss of nitrogen in 

 soils occurs not only from a decrease in the supply 

 of organic matter. The influence of moisture on nitro- 

 gen compounds in soils has already been mentioned. 

 Nitrification takes place only in moist soils; in very 

 dry soils not only does nitrification cease, but there is 

 actually a loss of nitrogen or what is called denitri- 

 fication. By a process of dry decay the nitrogen- 

 compounds in the soil are broken up and the nitrogen 

 passes into the air as free nitrogen gas. . A loss of 

 nitrogen also takes place from very wet soils % and from 

 soils poorly supplied with air. The water in wet soils 

 drives out the air, and without a supply of oxygen no 

 nitrates can be formed from the decaying organic 

 matter. Some nitrogen compounds are also washed 

 from the soil in the drainage water, and some escape 

 as ammonia gas. And while the loss in this way from 

 ordinary soils is not great, still it is enough to affect 

 the total supply. 



Is it any wonder, then, that the cotton and tobacco 

 lands become poor, and fail to produce crops ? Washed 

 and leached by the winter rains, scorched by the sun, 

 parched by the winds, robbed of their water and or- 

 ganic matter, how can they produce crops ? 



Questions 



1. How does the loss of organic matter affect soils? 

 2. Name two very important plant foods. 3. How does 



