Since the bulk of all mineral substances is alsE deposited in the foliage and not in the wood (see table), the forest trees, every fall, return to the soil, in the form of dead leaves, the greater part of what they have taken up through their roots Thus forest trees, in addition to furnishing their own fertilizer, by bringing up mineral substances from the deeper layers of the soil and depositing them on the surface, accomplish practically the same result that is brought about in farming by deep plowing. Therefore, the soil under the forest (provided the leaf litter is not removed or other• wise destroyed) is constantly gaining in fertility instead of becoming exhausted,—just the reverse of what happens in farming, where every harvest impoverishes the soil by depriving it of a part of its nutritive substances.
While farm land must, of riecessity, be fairly level, since a slope of 20° renders it unfit for till ing, and an incline of 25° unfits it even for pasture, gradients up to 45° are still capable of sustaining tree-growth. On slopes from 5° to 30°, the forest finds its true home, producing there more wood, and often yielding greater revenues than when grown in the valley. The reason for the increased growth of trees on moderate slopes is to be found in the stimulating effect of favorable exposures with their greater amount of light and air, of more perfect drainage, and of greater protection from wind and frost than is usually found on flat ground.
The ability of the forest to grow on situations too poor or otherwise unfit for agriculture led to designating such situations as absolute forest land (Figs. 425-6). To absolute forest land, therefore, belong all territory north of the range of cultivated plants, all steep slopes, gullies, situations too rocky or too dry for agricultural plants, and swamps. It is impossible, of course, always to draw a distinct line of demarcation between absolute forest land and other land, since the soil may be artificially im proved, as, in the case of swamps by drainage, but such improvements are, as a rule, very costly, and in this country, where there is still a comparative abundance of land, the absolute forest soil may be made profitable without improvements, by devoting it to forest growth, for which it is fitted, as it were, by nature itself.
Labor.
The raising of agricultural crops demands a great amount of human effort. The land must be plowed, harrowed, manured or otherwise fertilized, the seeds put in the ground, the harvest gathered and threshed, and all this has to be repeated year after year.
In the growing of wood crops the application of human labor is very limited. The forest provides for the fertilization of its own soil, new crops start, as a rule, from self-sown seeds transported by wind or birds, or from stumps or roots of old trees, and wherever man does undertake to assist nature by sowing or planting cut-over land, the work on the same area has to be done only once in many years.
It is only the harvesting of the timber crops which requires any considerable labor, and this occurs at very long intervals. Tens, often hundreds of years must pass before the new crop becomes ready for the axe.
While farm crops must be harvested as soon as they ripen, a delay of even a few days often caus ing considerable loss, the harvesting of timber crops may be postponed for many years without injury to the crop, and can be done at a time and rate most profitable and convenient to the timber owner.
The relative importance of labor as a factor in the production of timber and farm crops is well shown by the fact that while the 414,000,000 acres of improved farm land, given by the Twelfth Cen sus, engage 10,000,000 men, or one man to every forty acres, the 700,000,000 acres of forests engage only 120,000 men, em ployed in harvesting the timber and getting it out to the nearest points of shipment, or one man to every 5,800 acres. This dif ference is especially large in this country because most of the labor that is employed in our forests is engaged solely with harvesting and transporting the timber crops, and practically none with forest-culture proper, which is still in its inception. But even in forests managed most intensively, only one-tenth to one-thirtieth of the labor required by an acre of farm land is needed per acre of forest land. The different branches of agricultural production may be arranged in the order in which each calls for labor, beginning with that which needs least, as follows: Ranching, wood-grow ing, hay-raising, production of cereals, fruit - growing a n d truck-farming.
At a smaller expenditure of labor, forest land is capable of producing at the same time an equal, if not a greater amount of useful vegetable substance than farm land. Thus, common farm crops yield on an average 3,400 to 4,600 pounds of vegetable substance per acre ; of this, only about one-third (1,000 to 1,500 pounds) is in the form of grain. An acre of forest produces under human care 8,000 to 10,000 pounds of vegetable substance annually, and of this about one-half is in the form of wood, the re mainder being roots (450 pound s) , and leaves (3,000 pounds). Deducting from the wood the amount of water held by it mechanically, there re mains 1,500 to 3,600 pounds (dry weight) of vegetable sub stance, as the product of one acre in one year.