483 



Even this theory is, however, now abandoned by various well-known 

 investigators who believe that the actual amount of plant food in the 

 soil has little if any influence on soil fertility,— but, on the other hand, 

 there are many equally brilliant investigators who think that the 

 abandonment of this theory is not logical. One has particularly to 

 bear in mind in studying such a complex material as the soil and its 

 fertility — the question of limiting factors, i.e., if one particular in- 

 gredient is entirely absent, the plant starves, however liberally it may 

 be supplied with the others. 



A more recent theory expounded by several investigators, parti- 

 cularly in America, and receiving support from others in various 

 parts of the world, is that fertility is determined by plant excretions, 

 that is, that plants excrete a poison which is injurious to themselves— 

 hence the necessity of crop rotation. 



One investigator in fact claims to have actually isolated com- 

 pounds from such soils, injurious to the same crops which produce 

 them, but the results are far from conclusive. 



Having failed to account for soil fertility by the amounts of plant 

 food actually found in various soils by analysis, we are compelled to 

 search for other causes, and although we have again to return to 

 older theories— as has frequently happened in the advance of science 

 in general, it was not till comparatively recently that such theories — 

 then only advanced as theories— have been found to explain certain 

 facts. 



Recent investigations have shown the presence in soils of bacteria 

 now known as " nitrifying bacteria" which convert nitrogenous mat- ' 

 ter into nitrites and subsequently nitrates by oxidation— and thus 

 supply plants with nitrogen in an assimilable form — since with the ex- 

 ception of a few plants, viz., those of the leguminous order — plants 

 can only assimilate nitrogen after conversion to an oxidized form. 

 Such bacteria are consequently regarded as a great factor in soil 

 fertility. 



The proof of the existence of such oxidizing bacteria explained to 

 a great extent the value of tilth in surface soil, the lack of value of 

 the subsoil in which such bacteria do not exist, and the value of 

 shallow ploughing contrasted with deep subsoil ploughing. 



These discoveries led to othenv, in which two brilliant investiga- 

 tors, Hellreigel and Wilgarth and subsequently others made the in- 

 teresting discovery that certain plants of the leguminous order 

 possessed bacteria growing in symbiosis on the roots, which possessed 

 the power of assimilating nitrogen directly from the atmosphere and 

 converting it into a form suitable for the use of the plant. 



Long before this discoverj^ however, the value of leguminous 

 plants had been realized by the farmer and a system of rotation of 

 crops based on it. Since the discovery, other bacteria possessing 

 similar properties, but not associated with any plant, have been found 

 in the soil. 



