THE SOIL 71 



plants through the solvent action of the organic acids. 

 By the nitrification process it is converted from the 

 insoluble form to the soluble acid phosphate in which 

 form it may be used by plants. 



These principles find application in practical agri- 

 culture. The results of the Ohio and Illinois stations 

 have shown that the crushed raw rock phosphate ap- 

 plied with liberal amounts of farm manure (or crop 

 residues) gives yields that are equal to the acid phos- 

 phate, and at considerably less expense. The failures to 

 secure results upon soils deficient in phosphorus by 

 the use of raw rock phosphate have been due to the fail- 

 ure to provide decaying organic matter. Nature has 

 wisely locked up the phosphorus in the soil in insoluble 

 form so that it will not be wasted, but has beneficently 

 provided a means whereby it may become available to 

 plants. The "why" in agricultural practice is as im- 

 portant as the "how" ; it is the safest guide to a perman- 

 ent and profitable system. 



50. Parasitism (disease). That the soil harbors many 

 disease-producing microorganisms is now a matter of 

 common knowledge. Among the bacterial plant dis- 

 eases crown gall, pear blight, black leg and soft rot of 

 potatoes, are typical examples, and among the fungous 

 diseases flax wilt, wheat rust, and the smuts. These 

 organisms, however, are not related to the fertility of 

 the soil. 



51. Nitrogen-fixation by Leguminous Plants. Three- 

 fourths of the atmosphere by weight is nitrogen gas ; over 

 one acre of land there are 70 million pounds of nitrogen, 

 enough for nearly a million bushels of wheat, if it were 

 available to that crop. But wheat cannot utilize di- 

 rectly the free nitrogen gas of the air. 



