ON SOILS AND SUBSOILS fiUITABLE FOR PLANTING. 71 



5. The quantity of liumus contained in a soil, plays an important part in 

 regard to the growth of trees ; for by the decomposition of the lei;ves, 

 carbon and various chemical compounds forming ashes, are returned to 

 the soil. Carbonic acid, formed by the decomposition of humus, affords 

 indispensable nourishment to trees, and decomposes such minerals as 

 occur, so that they become soluble in water and are absorbed by the 

 rootlets in the form of alkalies. Humus also absorbs from the atmos- 

 phere ammonia (N H3), oxygen, moisture, &c., all necessary for the 

 nourishment of plants. It has a high degree of power for absorbing 

 and retaining moisture, and is only moderately binding ; it renders more 

 loose a binding clayey soil, and binds a loose sandy one. 



6. The nature of the subsoil affects the quality and quantity of the 

 timber produced, since it materially affects the quantity of moisture 

 contained in any soil. It may be impervious, or cleft in horizontal or 

 inclined layers. Subsoils of plastic clay, or of quartz conglomerate, are 

 the most unfavorable. Loams, marls, or peat, are more suitable (when 

 the latter are not too wet), and such rocks as are moderately cleft, so 

 that the upper soil can be only moderately moist. 



Concerning the situation of the soil, we must consider — 



1. Height above the level of the sea. 



2. Aspect or inclination toward one of the points of the compass. 



3. Angle of inclination toward the horizontal. 



1. Height above sea-level. — Trees reach their limit in a vertical direc- 

 tion very soon, as the mean temperature diminishes about 1° F. for each 

 450 feet of elevation. Trees growing on suitable soils at the sea-level, 

 produce the greatest quantity of timber, and most seed, but the quality 

 is not so good, owing to rapid growth. The following may be accepted 

 as average heights of forest trees in Germany : 



1,200 to 1,300 feet, — hornbeam and Scotch fir, the latter in mixed forests 

 growing much higher, but it is comparatively inelastic, and often suf- 

 fers from snow-break. 



1,600 to 1,700 feet, — birch, elm, and in favorable circumstances, oak. 



2,200 to 2,400 feet, — beech, maple, ash, and birch {Betula pubescens). 



3,000 feet (and often higher), — silver fir {Abies pectinata), spruce fir {Picea 

 excelsa), and larch. 



The heights to which these trees ascend mountain slopes," are modi- 

 fied by the following circumstances : 



2. Inclination of the hillside toward one of the points of the compass. — 

 The greatest quantity of timber is produced on gently sloping eastern, 

 northeastern, and northern hillsides, which are moist and cool. The 

 more southern aspects, however, produce the most durable timber; but 

 young plantations, or natural undergrowth are exposed to the burning 

 rays of the sun and partly to dangerous, late frosts. The northeast, 

 east, and southeast aspects are most exposed to late frosts. The west 

 aspect, when exposed, produces the least quantity of timber, for it is from 

 this point that violent winter storms generally come. 



3. Angle of inclination. — This has a most powerful influence on the 

 growth of trees, for the greater the angle, the more marked are the differ- 

 ences in the quantity of the timber produced. An inclined plane should, 

 coiteris paribus, yield more timber than a horizontal plane. 



a. Because it is greater in the ratio of the secant to the radius, and 

 the trees do not stand quite vertical, and 



b. Because each tree receives more light on a gradual slope. 



In reality, however, this is not the case, for on the hillsides the soil is 

 usually wanting in depth and also in moisture. After heavy ruins some 



