102 HIGHER PLANTS AND SOIL MICROORGANISMS 



tion may be favorable to certain phases of plant nutrition. Iron 

 is slightly soluble in a strictly aerobic environment at reactions 

 close to neutrality, but becomes much more soluble as it is reduced. 

 Its reduction about plant roots may be important in supplying 

 available iron to the plants in most soils. 



Influence of Root Excretions. — The roots of higher plants 

 are organs not only of absorption but also of excretion. The sub- 

 stances that are excreted and the amounts eliminated at any one 

 period of growth vary with the kind of plant, its stage of develop- 

 ment, and soil conditions. Although it is apparent that higher 

 plants must absorb more of the inorganic substances than they 

 excrete, still at certain stages of growth there is appreciable 

 elimination of various substances, both organic and inorganic, 

 nitrogenous and non-nitrogenous. In studies carried out with 

 plants growing in sterile culture solutions, it was noted that from 

 1.5 to 3.0 per cent as much organic matter was present in solution 

 (excreted by the plant) as was contained in the developed plant. 

 About one-fifth of this organic matter in solution was nitrogenous 

 in nature. A certain amount of the insoluble organic matter 

 which was eliminated from the plants consisted of cellular material 

 sloughed off from the roots. 



The amounts of substances entering the soil solution during 

 growth of certain plants may be sufficiently abundant to modify 

 greatly the development of other plants which are growing in close 

 proximity. This is of considerable importance in the growth of 

 mixed stands of legumes and non-legumes. In such mixtures the 

 non-legumes may profit from the nitrogen which is fixed through 

 the agency of nitrogen-fixing bacteria in the roots of legumes. 

 Certain nitrogenous substances enter the soil solution from the 

 roots of the legumes, and probably through the action of soil 

 microbes become modified and are then absorbed by the non- 

 legumes. Such effects are shown in Fig. 74. The pots were pre- 

 pared in the following manner. Two large pots were filled with 

 sand. In the center of one large pot a smaller unglazed pot was 

 placed. In the other pot a glazed, impervious pot was introduced. 

 Both small pots contained the same kind of sand as the larger pots. 

 The unglazed pot permitted diffusion of substances back and forth 

 between the contents of the small pot and the larger pot, while the 

 glazed pot prevented such movement. All nutrients required for 

 plant development with the exception of nitrogen were added 



