116 ELEMENTS OF AGRICULTURE 



spent $29,000,000 for fertilizers, in 1889, $38,000,000 and 

 in 1899, $55,000,000. 



NITROGEN 



114. Sources of Soil Nitrogen. All soil nitrogen cornea 

 from the air. There is no nitrogen in the rocks except 

 when these rocks contain the remains of plants and ani- 

 mals. The amount of nitrogen in the soil usually decreases 

 very rapidly with the depth. The great inexhaustible 

 source of nitrogen is the air. Nearly four-fifths of the air 

 is nitrogen. There are about 35,000 tons of this gas over 

 every acre of land. But no farm plants are able to take it 

 from the air above ground. We may have sickly, yellow 

 plants, starving to death for nitrogen while immersed in this 

 inexhaustible supply. Since nitrogen is the most expensive 

 of the fertilizing materials, costing about 18 cents per 

 pound when purchased in commercial fertilizers, we may 

 well be interested in getting the supply in the air into 

 compounds that are available for the growth of crops. 

 At the rate it must be paid for in commercial fertilizers, 

 there are some ten million dollars' worth above each acre 

 of land, if it could be used! 



115. Nitrogen in Rainfall. A small amount of nitrog- 

 enous compounds are brought down with the rain and 

 snow. Usually this does not amount to over two or three 

 pounds per acre per year, while about 40 pounds are re- 

 quired to produce a fair wheat crop. 



116. Nitrogen Fixation by Bacteria on Legumes. Some 

 of the oldest writings refer to the fact that pea-like plants 

 have some effect on the soil that benefits following crops. 

 Only in the last fifty years has this fact been explained. 



