52 Studies in Forestry [CHAP. m. 



On the whole, the conifers are more moderate in their 

 demands on soil-moisture than the broad-leaved genera of 

 trees. But the limits within which the different trees can thrive 

 vary greatly according to the species ; or in other words, the 

 various kinds of woodland trees exhibit great differences as to 

 their accommodative power with regard to the degree of 

 moisture contained in the soil. 



Although trees derive the greater part of their nutriment 

 from the carbonic acid which they absorb from the atmosphere, 

 still they are also .dependent on the soil for supplies of mineral 

 nutrients; and these can only be taken up when held in solution 

 by the soil-moisture. Whether the soil be loose or binding is 

 also a matter of very considerable importance to the thriving of 

 the various woodland trees. As a rule the broad-leaved species 

 do better than conifers on the stiffer classes of land, although 

 soils of merely average tenacity are on the whole most suitable 

 for all kind of trees, owing to their better endowment as regards 

 physical properties. The chief constituents of soils are clay, 

 lime, and sand ; and as clay yields the most valued nutrients 

 for plants, the qualities of soils are often determinable to some 

 extent by the quantity of clay found in them. The effects of 

 the mineral constituency of soils are, however, very greatly 

 modified by the physical properties inherent in them, and 

 these become prejudiced whenever any one of those three 

 most important constituents occurs in undue excess. Clay 

 soils are tenacious and interfere with the movement of soil- 

 moisture ; sands are too porous, and limes are too easily heated ; 

 hence on the average, the mixed classes of loamy soils are in 

 general most fertile. The absolute demands of the various 

 species of trees on the mineral strength or fertility of soils is not 

 yet fully known, although they can to a certain extent be 

 estimated from the weight and analysis of the ash left after 

 reducing the different kinds of timber by combustion. 



On this point, however, sylvicultural experience shows that Elm, Maple, 

 Sycamore, and Ash, make the greatest demands ; and that Oak, Beech, 



