68 Lewis Knudson 



than the dissolved nutrients (Petermann, 1882). Gourley (1915) reports 

 for certain orchard soils in New Hampshire a soluble organic content 

 varying from ninety to two hundred and fifty parts per million. The 

 fact that there exists in soils such a low concentration of soluble organic 

 substances cannot be a serious argument against a direct nutritional value 

 of the soil organic material. The mineral nutrients of the soil and the 

 nitrates are probably present in concentrations no greater; yet these 

 nutrients are constantly being absorbed, and the same is possible for the 

 soluble organic substances, especially those that can be assimilated. The 

 ability of vetch to remove glucose from weak solutions has been demon- 

 strated, and, as stated at the beginning of this paper, the fact that micro- 

 organisms and saprophytic plants find" in the soil their carbon require- 

 ments lends strength to the argument that the higher plants obtain, 

 to their advantage, organic materials from the soil. 



Schreiner (1911) has called attention to the possibility of the favorable 

 influence on plant growth of nitrogen and phosphorus containing organic 

 compounds of the soil. He states: '' The most beneficial manures under 

 normal circumstances are those of organic origin, and the presence of such 

 directly beneficial compounds, like creatinine, in well-rotted stable manure 

 and in green manures, like cowpeas, goes far toward explaining why these 

 manures are more beneficial to soil as a rule than are equivalent parts of 

 fertilizer in the purely mineral forms." It has been demonstrated, how- 

 ever, that not only are organic nitrogenous substances available for plants, 

 but carbohydrates, alcohols, and organic acids and their salts, can also be 

 absorbed by the roots and assimilated by the plants. 



In view, then, of the established ability of plants to absorb and assimilate 

 organic substances, and in view of the presence in soils of insoluble organic 

 substances which are constantly in a state of transformation to soluble 

 organic compounds, it seems reasonable to conclude, with J. Laurent 

 (1904), that " the organic matter of the soil plays a direct role in the 



nutrition of green plants independently of humus in that 



the roots are able to find in the soil quantities of directly utihzable organic 

 substances which in a weak measure contribute to the carbon nutrition of 

 the plants."^ It seems reasonable to conclude, furthermore, that under 

 certain conditions, especially in greenhouse culture, the soil organic material 

 may play a verj'- important role in the organic nutrition of plants. 



« Translation from the original French. 



