Correspondence Courses in Agriculture. 9 



soil, is a layer of black and rich humus. This has resulted from 

 the decomposition of litter and contains large amounts of valuable 

 soil salts. Often this humus is used as a manure for field crops. 

 It is, however, very valuable to the forest and its removal may 

 result in permanent injury through excessive evaporation. As a 

 result of the steady increase in humus over a long period of years, 

 a gradual change takes place in the composition of the forest. 

 Only a few trees of comparatively little value are able to thrive 

 on a sterile soil. Practically all trees, including woods of highest 

 value, flourish in a rich soil. An accumulation of undecayed vege- 

 tation (swamp) results in a sour soil. 



A ground fire often destroys all organic matter, including ground- 

 cover, litter and humus, leaving the soil in an impoverished condi- 

 tion. Many years must elapse before it regains its productive 

 capacity. 



Region of Root Development. This comprises the soil and sub- 

 soil, and the tangled and interlaced root-systems of the forest. The 

 amount of raw food materials required by forest trees from the 

 soil is much less than that necessary to the production of field crops. 

 Provided the roots find moisture, many trees thrive in an almost 

 sterile soil. No soil exists which is too infertile to produce some 

 form of tree growth. The more valuable species (Walnut, Beech, 

 Oak, Hickory), however, demand a deep, fertile soil. 



The fertility of the surface soil has little influence upon the 

 development of trees, after the seedling stage is past. Practically 

 all of the mineral elements are drawn from the sub-soil. As the 

 more valuable elements are stored in the leaves and fruit, and these 

 fall to the ground and decay, it is understood how forest growth 

 results in the improvement of the surface soil. In parts of Europe 

 a crop of trees is often included in the crop rotation for agricultural 

 land. 



The roots extend in all directions in the ground, often farther 

 than the branches. In the forest the roots interlace and the com- 

 petition for soil salts and moisture becomes very great. Here and 

 there a tree dies, the struggle becoming too great. In an old forest 

 the roots drain the soil to such an extent that seedlings cannot be- 

 come established. 



Composition of Woodlot 



Whether composed of one species or several, the stand is known 

 as "pure" or "mixed." It is a pure stand when eighty or more 

 per cent of the trees are of one species; a mixed stand, when two 

 or more species are present in approximately equal mixture. Con- 

 ifers are more apt to occur naturally in pure stand (Longleaf Pine 

 of Georgia and Florida, White Pine of the Lake States, Douglas Fir 

 of the Rocky Mountains), while hardwoods occur usually in mixed 



