THE FERTILITY OF THE SOIL 233 



dizing atmospheric nitrogen and thus increasing in the 

 soil the stores of available nitrogenous foods for plants. 

 This discovery, so important to agriculture, is all 

 the more to be prized because in the purchase of com- 

 mercial fertilizers those containing available nitrogen 

 are the highest priced. The cost of phosphoric acid 

 and potash is comparatively low when considered in 

 connection with the cost of nitric acid or nitrate of soda, 

 the form in which, as Chile saltpeter, the available 

 nitrogen of commercial fertilizers is chiefly furnished. 

 The farmer who wishes, therefore, to increase the fer- 

 tility of his soils, may well begin on this theory, namely, 

 the great majority of soils of the United States are es- 

 sentially acid, being composed largely of silica or silicic 

 acid. Further, when soils have been long in cultiva- 

 tion their acidity undoubtedly increases, due to the de- 

 velopment of acids from the decay of organic matter 

 and the removal of bases from the soil for the purposes 

 of plant growth. 



BENEFITS OF LIMB. 



These old acid soils no longer are capable of produc- 

 ing leguminous crops, but when they are treated with 

 lime they again, under proper conditions, grow abun- 

 dant crops of clover, alfalfa, cowpeas, and other legu- 

 minous plants. By plowing these crops under when 

 near maturity, not only is an abundance of available 

 nitrogen returned to the soil, but also large quantities 

 of humus, which improve the physical condition of the 

 fields. The farmer thus needs only to buy the cheaper 

 potash and phosphoric acid salts in order to restore his 

 fields to the maximum of fertility. It may be possible 

 to find supplies of potash in the United States. All 

 other kinds of fertilizer we have. 



