SHADE TREES. 151 



The moisture content of a relatively dry soil may be greatly modified 

 by the addition of organic matter, which increases the water-retaining 

 capacity and makes the soil more suitable to swamp-loving species. But 

 swamp trees that make excellent growth in dry soil need to be supplied 

 with water during drought periods. 



There are other factors than those of soil texture, water supply, etc., 

 that influence the distribution of plants. The chemical composition of 

 the soil affects the habitat of trees, and is capable of modif ying to some 

 extent their mode of growth. Many plants are restricted in their range 

 owing to differences in the chemical composition of the soil. Certain 

 species are practically confined to the seacoast, where the percentage of 

 chlorine in the soil is greater than it is inland; but these species may be 

 grown successfully elsewhere. The amount of humus in the soil affects the 

 growth of trees materially. While 20 or 30 per cent, of organic matter 

 was formerly contained in the upper strata of our soils, now not more 

 than 2 to 5 per cent, may be found in a large portion of it. Organic matter 

 has ;i vital effect not only on the physical properties of soils, but on their 

 chemical and biological properties, influencing the development of mi- 

 corhiza (beneficial root fungi) that are intimately associated with the 

 roots of some of our shade trees. Soils also contain toxic elements that 

 are often found in sufficient abundance to make it difficult to establish 

 certain species in the desired location. 



It is desirable in all cases when planting trees to give them conditions 

 closely approximating their requirements as determined by their natural 

 habitat. Elm trees often grow in swamps, as well as in dry and sandy 

 soils, but both of these habitats produce poor specimens. The swamp 

 tree is usually of inferior shape, and sandy soil as a rule produces a lank, 

 spindling growth, with inferior foliage. Even the best type of elm, if 

 planted under uncongenial conditions, will make poor development 

 regardless of its inherent qualifications. The elm, therefore, should never 

 be planted in dry, gravelly soil without being supplied with a large amount 

 of good loam of the proper texture. The rock maple, on the other hand, 

 will endure a dry soil much better than the elm, although if too dry borers 

 may affect the tree. The scarlet and black oaks will thrive in such a soil. 



In general, the texture of the soil in most towns is fairly well suited to 

 the growth of a large variety of trees. The soils often lack organic matter, 

 hence the application of loam is advantageous. On the other hand, some 

 of our New England river valleys are particularly adapted to the growth 

 of elms and maples, and the addition of loam in such cases is not so 

 necessary. 



Street trees are too often forced to exist under extremely unfavorable 

 conditions; therefore they require different consideration from those more 

 favorably located. Many city trees are planted in made soil, and some 

 of the refuse found in these fillings is hardly adapted to tree growth. Such 

 soils are, moreover, likely to be deficient in organic matter and plant food, 

 and are often in such poor mechanical condition that the soil capillarity 

 is of little account. 



