Influence of Certain Carbohydrates on Green Plants 67 



Does the plant utilize substances from the soil, and, if so, what is their 

 importance in the nutrition of the plant? 



With respect to the absorption of humus and humate compounds, it 

 has aheady been stated that J. Laurent (1904) and Maze (1911) found 

 such absorption. Molliard (1912), on the other hand, as a result of an 

 interesting experiment, came to the conclusion that the humates of the 

 soil are not absorbed. He grew plants under sterile conditions in a closed 

 chamber on a loam rich in humus with no carbon dioxide supplied. The 

 radish showed a slight increase in dry weight, due to the assimilation of 

 carbon dioxide produced from decomposition of the soil humus. From 

 his results Molliard concluded that none of the humus could have been 

 absorbed. 



The organic content of soils is of course extremely variable and few 

 satisfactory data are available as regards the soluble organic material 

 in the soil. According to Schreiner (1911) the organic content in ordinary 

 soils is large. The average content of 237 types of United States soils, 

 determined by analyses of thousands of samples, is 2.06 per cent for the 

 topsoil and 0.83 per cent for the subsoil. In greenhouse practice the 

 soils used have much higher organic content. In the forcing of cucumbers 

 and tomatoes, for example, this organic content may be as high as 25 per 

 cent or even higher. 



As stated at the beginning of this paper, the problem of the relation 

 of organic substances to plant nutrition is not merely that of the relation 

 of humus. It is concerned with all the soluble organic substances that 

 must arise from the decomposition of plant and animal residues. The 

 practicability of the power of the plant to utilize to advantage various 

 organic substances rests on the extent to which these substances are found 

 in the soil and the ability of the plant to remove them from weak solutions. 



There is found in most soils, then, a considerable quantity of organic 

 material, but most of this is in an insoluble state and therefore nonavailable. 

 It is constantly being acted upon, however, by enzymes secreted by micro- 

 organisms ^ and by other agents, and soluble organic substances are pro- 

 duced. The soluble organic substances are present in most soils in ex- 

 tremely low concentration, yet their sum total may be as high as or higher 



' During the summer of 1913 a very marked stimulative effect of the fairy-ring fungus Marasmius 

 oreades Fr. was noted on the growth of lawn grass on the Cornell campus, the grass in the region of the 

 riag being darker in color and more vigorous in growth than that in other places. A possible explanation 

 is the digestion of organic material in the soil by enzymes secreted by the fungus mycelium, and the utili- 

 zation by the grass of the products of digestion. 



