Improving the Chemical Nature of the Soil 89 



erable quantities of potash and phosphoric acid. The 

 vegetable matter acts very beneficially, improving the 

 texture and water-retaining property of the soil. An 

 instance of the power of compost to maintain the land 

 at a high state of productiveness has already been 

 given (f 115). Compost should be applied in the fall or 

 early, winter and plowed or harrowed under. Covered 

 barns prevent the loss in value of compost by scattering 

 and leaching. Sometimes the compost is remove4 

 directly to the field. In many cases, where it is stored in 

 bins, sufficient soil should be added from time to time 

 to absorb the ammonia that is formed. When packed 

 down closely to exclude the air, the loss from fermenta- 

 tion will be greatly reduced. 



124. Fixation of Free Nitrogen by the tubercle-forming 

 bacteria, found on the roots of plants belonging to the 

 pea family, is the most important source of nitrogen 

 known. By growing these legumes we add to the supply 

 of combined nitrogen, and thus make the world richer. 

 We do not recover all the nitrogen added to the soil in 

 fertilizing. A part of it is lost by leaching, and a part by 

 the escape of free nitrogen. All combined nitrogen may 

 be used over and over again by plants and animals, but 

 eventually it escapes back to the air as free nitrogen 

 and, in this form, is available only to the bacteria 

 which cause the formation of tubercles on the roots of 

 legumes, and to a low class of microscopic plants. (See 

 H 127, Nitrification.) Without these plants the world's 

 supply of combined nitrogen would become exhausted. 

 In the present state of our knowledge, only the " tubercle 

 bacteria," and one or two other classes of bacteria, 

 whose life-habits are little understood, are known to 

 have the power of fixing free nitrogen. 



