122 ELEMENTS OF AGRICULTURE 



crops. 1 It is evident that these soils will become unpro- 

 ductive if the one-crop system continues. 



The better aerated the soil, the warmer the climate, and 

 the more the land is tilled, the more rapidly the humus will 

 be exhausted. The ideal condition is to have the humus 

 decompose just rapidly enough to supply the crop with 

 nitrogen. If it burns out too rapidly, we may keep the 

 land in sod more of the time, apply manure, or plow 

 under crops to keep up the humus supply. 



120. Forms of Nitrogenous Fertilizers. Nitrogen is 

 added to the soil in the form of barnyard manure, sodium 

 nitrate, ammonium sulfate, potassium nitrate, dried 

 blood, tankage, hoof meal, steamed bone, dried fish, 

 linseed-oil meal, cottonseed meal, and in a number of other 

 forms. 



121. Nitrate of Soda (NaNO 3 ). Sodium nitrate, or 

 Chile saltpeter, is the most common nitrogenous fertilizer. 

 Beds of it occur along the western coast of South America, 

 particularly in Peru and Chile. As it is taken from the 

 earth, it contains about 50 per cent of nitrate of soda. 

 This is purified, so that when put on the market it is usually 

 IK} per cent pure. It contains an average of about 15.6 

 per cent of nitrogen, and costs about $60 per ton, or about 

 19 cents per pound for the nitrogen contained. This salt 

 is very soluble and is in a form that plants can take up 

 at once. It should be applied only where plants will 

 soon make use of it; otherwise it may leach out of the soil. 



122. Ammonium Sulfate (NH 4 ) 2 SO 4 ). This sub- 

 stance is a by-product from the manufacture of gas and 

 coke. It contains about 20 per cent of nitrogen. The 



1 Minnesota, Bulletin No. 53 



