Hirnius and Phosphoric Arid 37 



modest iiicomes. Mining- -fanning on virgin, fer- 

 tile, miobstrncted areas can be successfully 

 prosecuted only for a time. 



"The Red river valley native soils contain 

 from .35 to .40 of nitrogen, while the soils 

 which have been under cultivation ( in wheat ) 

 for twelve to fifteen years contain from .2 to 

 .3 of a per cent."* Another important point,: 

 When humus is taken out of the native soil 

 as above, only .02 of a per cent of the phos- 

 phoric acid is soluble by ordinary chemical 

 methods, while in the native soil three or four 

 times as nnich phosphoric acid is soluble and is 

 associated with the humus. Allowing that an 

 acre of soil one foot deep weighs 1,800 tons, 

 the native soil would contain from 12,600 to 

 14,400 pounds of nitrogen per acre, while the 

 cultivated soil would contain from 7,200 to 

 10,800 pounds per acre. If the average amount 

 of nitrogen in native soils ( 13,500 pounds per 

 acre), and the average in the soil after it had 

 been cropped twelve to fifteen years (9,000 

 pounds per acre), are compared, it will be 

 seen that the soil has lost 4,500 pounds of 

 nitrogen per acre, or more than one -third (prob- 

 ably one-half) of the nitrogen which could well 

 be made available, and this in less than a quar- 

 ter of a century. 



♦Henry Snyder, Bulls. :!(), 44, Minn. Exp. Stu. See "Fertility of tlie Land," p. 256. 



