INJURIES TO SHADE-TREES 137 



find their specimens gone departed to more hospitable 

 regions. 

 The chief sources of injury to street-trees are : 



POOR SOIL 



The street-soil is generally very poor and the trees con- 

 stantly take the available plant-food out of it. A part of 

 this matter assimilated by the trees is converted into wood ; 

 but by far the larger portion goes into the leaves. In the 

 forest the fallen leaves pile up and form a humus, by means 

 of which the mineral matter contained in the leaves is 

 returned to the soil. In the city, however, the successive 

 crops of leaves are removed and the soil becomes impover- 

 ished. Just as one would not think of success in farming 

 without the yearly use of fertilizers of some sort to enrich 

 the ground, so in the case of shade-trees, the periodic appli- 

 cation of some suitable fertilizer to the soil about them is of 

 the highest importance to aid their growth. 



ROOTS LACK AIR AND WATER 



To insure the proper performance of the tree's functions j 

 its roots must have a supply of air and water. The exclu- 

 sion of either of these requisites from the soil is fatal. It is 

 a matter of common observation that a filling of earth, two 

 or three feet deep, about a thrifty tree will damage or kill it. 

 The covering of earth works this injury simply by excluding 

 air from the active rootlets. Street-trees are greatly limited 

 in their supply of air and moisture by the pavements. 



SALT WATER 



Salt used in freezing ice-cream is sometimes emptied 

 near trees. When it is dissolved by rains and carried to the 

 roots it becomes very injurious. Plate 33, Fig. 6. 



