130 HORTICULTURE FOR SCHOOLS 



the air unites with this new compound to form nitric acid. 

 The nitric acid is then brought in contact with lime and forms 

 a compound of lime, nitrogen, and oxygen called calcium 

 nitrate. This source of nitrogen is certain to become of great 

 commercial importance as other sources fail. 



Thousands of tons of sodium nitrate (Chile saltpeter) are 

 imported to the United States yearly from Chile. 



Slaughter-house by-products, such as dried blood and 

 tankage, furnish nitrogen in large quantities. Analysis of 

 dried blood shows it to contain from 12 to 14 per cent of 

 nitrogen, almost as high a percentage as in nitrate of soda. 

 It differs from the latter in being a little less rapid in its 

 action as a fertilizer, an advantage in some cases and a dis- 

 advantage in others. 



217. Nitrogen and cover-crops. The nitrogen in the soil 

 becomes available under ordinary methods of cultivation at 

 the rate of about 2 per cent each year; that is, if an acre of 

 land to a depth of one foot contains ten thousand pounds of 

 nitrogen, cultivation will enable plants to utilize about two 

 hundred pounds of it yearly. When the winter rains come, 

 this available nitrogen, being soluble, may be washed out of 

 the soil and lost. Especially is this true in sandy or gravelly 

 land, particularly where the precipitation occurs as rain 

 rather than as snow, and comes in large quantities at a time. 

 Since nitrogen is one of the most expensive fertilizers, the loss 

 of it in this way is a serious matter. To prevent this loss in 

 climates sufficiently mild to permit it, winter cover-crops are 

 planted. These take up some of the available nitrogen from 

 the soil. In the spring after danger of washing is over, they 

 are plowed under and the nitrates are then returned to the 

 soil as the plants decay. 



218. Nitrification. The process that these decaying 

 plants undergo is exceedingly complicated and interesting. 

 The active agents are bacteria, which work in groups, each 

 group acting on certain substances. 



