44 LANDSCAPE-GARDENING 



Some Norway maples in a row next to these elms, 

 on the other hand, were very thrifty in the sand, but 

 stunted in the soil that would commonly be called 

 rich. 



Another important fact concerning land intended 

 for certain plants is the existence of microscopic 

 organisms, bacteria and protozoa, in the soil, which 

 obtain nitrogen directly from the air and supply it 

 to plants. It is not at all unlikely that these or- 

 ganisms may affect the growth of certain plants in 

 other ways, and that there is still much to be learned 

 about these minute organisms and their relation to 

 the growth of trees, bushes, and other vegetation. 



The effect of the mechanical condition of land is 

 indicated by the rapidity in growth of trees and of 

 other forms of vegetation in newly filled ground. 

 Such ground may settle from ten to twenty per cent. 

 This percentage in the newly filled ground must, 

 therefore, be taken by air, and would indicate that 

 an abundance of air in the soil is of great advantage 

 to vegetation. The larger growth upon land that 

 has been shaken by the explosion of dynamite cor- 

 roborates this idea. When plants do not thrive, it 

 is often assumed that the land in which they grow 

 is too poor, and, therefore, rich black ground is added 



