FOREST GROWTH AND SOIL. 265 



may store in its trees, say, 1,000 pounds of carbon, 15 to 20 pounds of mineral substances, and 

 5,000 pounds of water in a year, it will have transpired — taken up from the soil and returned to 

 the air — from 500,000 to 1,500,000 pounds of water (one-quarter to one-half as much as agricul- 

 tural crops). 



Mineral substances are taken up only in very small quantities, and these are mostly the 

 commoner sorts, such as lime, potash, magnesia, and nitrogen. These are carried in solution to 

 the leaves, where they are used (as also on their passage through the tree), with a part of the 

 water, in food preparation. The main part of the mineral substances taken up remains, however, 

 as the water transpires, in the leaves and young twigs, and is returned to the soil when the leaves 

 are shed or when the tree is cut and the brush left to decompose and make humus. The mineral 

 constituents of the tree remain as ashes when wood is burned, the remaining elements passing 

 into the atmosphere in the form of gas. 



Hence the improvement of the fertility of the soil by wood crops is explained, the minerals 

 being returned in more soluble form to the soil; as also the fact that wood crops do not exhaust 

 the soil of its minerals, provided the leaves and litter are allowed to remain on the ground. 



For this reason there is no necessity of alternating wood crops, as far as their mineral needs are 

 concerned; the same kind of trees can be grown on the same soil continuously, provided the soil 

 is not allowed to deteriorate from other causes. 



As the foliage can perform its woik of food assimilation only when sufficient water is at its 

 disposal, the amount of growth is also dependent not only on the presence of sufficient sources of 

 supply, but also on the opportunity had by the roots to utilize the supply, and this opportunity is 

 dependent upon the condition of the soil. If the soil is compact, so that the rain water can not 

 penetrate readily and runs off superficially, or if it is of coarse grain and so deep that the water 

 rapidly sinks out of reach of the roots and can not be drawn up by capillary action, the water 

 supply is of no avail to the plants; but if the soil is porous and moderately deep (depth being the 

 distance from the suiface to the impenetrable subsoil, rock, or ground water), the water not only 

 can penetrate, but also can readily be reached and taken up by the roots. 



The moistuie of the soil being the most important element in it for tree growth, the greatest 

 attention must be given to its conservation and most advantageous distribution through the soil. 

 Kb trees grow to the best advantage in very dry or very wet soil, although some can live and 

 almost thrive in such unfavorable situations. A moderately but evenly moist soil, porous and 

 deep enough or fissured enough to be well drained, and yet of such a structure that the water 

 supplies from the depths can readily be drawn up and become available to the roots — that is the 

 soil on which all trees grow most thriftily. 



The agriculturist procures this condition of the soil as far as possible by plowing, drainage, 

 and inigation, and he tries by cultivating to keep the soil from compacting again, as it does under 

 the influence of the beating rain and of the drying out of the upper layers by sun and wind. 



The forest grower can not rely upon such methods, because they are either too expensive or 

 entirely impracticable. He may, indeed, plow for his first planting, and cultivate the young trees; 

 but in a few years this last operation will become impossible and the effects of the first operation 

 will be lost. He must, therefore, attain his object in another manner, namely, by shading and 

 mulching the soil. The shading is done at first by planting very closely, so that the ground may 

 be piotected as soon as possible from sun and wind, and by maintaining the shade well throughout 

 the period of growth. This shade is maintained, if necessary, by more planting, and in case the 

 main crop in later life thins out inordinately in the crowns or tops, or by the accidental death of 

 trees, it may even become desirable to introduce an underbrush. 



The mulching is done by allowing the fallen leaves and twigs to remain and decay, and form 

 a cover of rich mold or humus. This protective cover permits the rain and snow waters to pene- 

 trate without at the same time compacting the soil, keeping it granular and in best condition for 

 conducting water, and at the same time preventing evaporation at the surface. 



The soil moisture, therefore, is best maintained by proper soil cover, which, however, is needful 

 only in naturally dry soils. Wet soils, although supporting tree growth, do not, if constantly wet, 

 produce satisfactory wood crops, the growth being very slow. Hence they must be drained and 

 their water level sunk below the depth of the root system. 



