217 



only slightly inclined layer of subsoil, forms an impervious soil. If the subsoil present a 

 vertical or strongly inclined stratification an unfavourably quick percolation of the water 

 may be the consequence, whilst a horizontal stratification of impervious subsoil may cause 

 detrimental stagnation. 



According to the depth of surface soil we discriminate a deep or shallow soil, which 

 properties, however, must be considered relative to the species of tree which is to grow 

 on it ; for a soil may be too shallow for oak, which is sufficiently deep for beech and fir. 



Depth of soil, i. e., a deep surface soil is especially desirable and important for such 

 species, as, like oak and spruce, form a taproot and which do not attain considerable 

 height if the development of this root is impeded, especially when the lack of depth is 

 not counterbalanced by extra humidity. The stunted growth of a forest will speedily 

 indicate such a locality. Depth of soil favours the growth for the reason that it provides 

 a continual reservoir of moisture. Thex-efore even those trees with shallow tracing roots 

 prosper best in a deep soil. 



The shallowest soil is generally due to plastic clay, the tenacious quality of which 

 prevents the roots from penetrating deeply, consequently it is not fit for Oak timber for- 

 ests. The same disadvantages are cavised by bog iron stone, which consisting of a mixture 

 of hydrous protoxide of iron, oxide of manganese, phosphoric acid, sand and organic com- 

 pounds, forms continuous slabs not far below the surface, presenting an impenetrable 

 barrier to the roots. In the province of Hanover, since its accession to Prussia, large 

 areas of the Luneburg heath have been inforested by breaking up this stone with subsoil 

 plows, and planting pine seedlings in the furrows, thus converting lands so long unprofit- 

 able into a source of wealth. 



Looseness of soil, when accompanied by sufficient humidity, tends to produce a maxi- 

 mum of fibrous roots, which provide the plants with water and inorganic substances. 

 Consequently the largest yields and especially the greatest height of growths, are to be 

 found on alluvial soil, which from the manner of its formation must be very finely divid- 

 ed. From this cause too results the fertility of the inundation soils of ponds and of the 

 marshes near the sea coasts. The superior growth on the fresh sweating sand is due to 

 its looseness and depth. 



Though the looseness of the soil is most conducive to largest yields, yet a soil will pro- 

 duce a r/ooc/ yield, if it is only sufficiently deep and moist, such as that formed by disen- 

 tegrated rock. 



In regard to humiditi/ the diiferent species require a different degree of moisture. 

 Most of the forest trees require only a "fresh" soil (which when pressed leaves traces of 

 moisture in the hand) ; some species, like the ash, prosper still in "moist" soil (which, 

 when pressed, drop water) ; and the elder requires even a "wet" soil (which drops water 

 without being jDressed. ) A dry soil is a faA^ourite with no species, though birch and pine 

 can best exist on it. 



Two circumstances determine the degree of moisture in the soil, its character and its 

 position. The soil in valleys, river places, near lakes and seas and on northern exposures 

 contains more humidity than that of the mountain ridges, on eastern and southern expo- 

 sures. One quantitative determination of this diflference is known to the writer, where 

 the accretion of a beech growth, fifty -nine years old, on one of the Hessian mountains was 

 measured, and it was found that the accretion on the southern aspect bore the proportion 

 to that of a dell and to that of a northern aspect as 16 : 39: 48. 



The favourable appearance of tree growth on northern aspects may be considered due- 

 to the greater depth of soil generally found in svich localities, and this again is due to the 

 fact that the moisture, which promotes and expedites the disentegration of rocks, is not 

 as quickly absorbed there as on other exposures more subject to the drying influence of 

 sun and wind. 



The lacking humidity of soil may be compensated by the humidity of the atmosphere, 

 especially for such species as, on account of their dense foliage like the beech, evaporate 

 profusely. This accounts for the excellence of the growth in higher mountainous regions 

 where the atmosphere is generally moister than on lower levels. 



Considering that thirty to fifty per cent, is constituted of water, and of the dry sub- 

 stance forming the wood, called cellulose and ligniue, forty-seven per cent, is composed of 



