Forests and Soils 



air. To some degree, like a living body, 

 it absorbs oxygen and releases carbon 

 dioxide. A soil has characteristic parts 

 in harmony with its environment. Its 

 productivity depends on all of its parts. 



The formation of a soil, a slow proc- 

 ess, doubtless began on the first crust 

 of the earth, when heating and cooling 

 and wetting and drying cracked the 

 surface rocks, made little patches of 

 loose rubble, and caused little pockets 

 of mineral crystals to settle in crannies 

 and depressions, and allowed lichens, 

 mosses, and other simple plants to grow 

 in the thin soil and on porous rocks. 

 Rain dissolved the softer parts of the 

 rocks and made soluble minerals avail- 

 able to plant roots. 



As the soil mantle grew deeper, the 

 soil grains became finer. Water perco- 

 lated through the soil mass, carried fine 

 particles from the surface layer down- 

 ward, and deposited them at lower 

 depths to form subsoil. Plant debris fell 

 on the surface and the micro-organic 

 life appeared. Larger and more com- 

 plex plants appeared until finally trees 

 and forests, as we know them today, 

 emerged with their characteristic soils. 



Each forest soil developed its orderly 

 arrangement of horizons, its porous 

 and absorptive structure, and its bal- 

 anced and active population of bac- 

 teria, molds, fungi, worms, insects, and 

 animals. Roots of the trees anchored 

 the soil in place ; the leaves provided a 

 protective cover of litter and added 

 fertility yearly. 



Soils differ broadly among climatic 

 provinces. Basic differences occur be- 

 cause rainfall, temperature, and rocks 

 are different. Any part of the earth 

 having a characteristic climate and 

 parent-rock material has its special 

 kinds of soil: The gray, desert sage- 

 brush soils of southern Wyoming; the 

 subhumid, chestnut-colored prairie 

 soils of western Nebraska; the black, 

 tall-grass prairie soils of Iowa and Illi- 

 nois; the gray-brown, hardwood-cov- 

 ered soils of Indiana and Illinois; the 

 gray, leached, pine-covered podzol 

 soils of Maine; and the rich-brown, 

 humid, forest soils of the Northwest. 



Even within a climatic province, 

 many differences occur among soils, 

 chiefly because of differences in vege- 

 tation, degree of slope, and the nature 

 of the parent rock. Vegetation alters 

 the surface of soils, but the primary 

 local difference is permeability of the 

 soil to water. Permeability is altered 

 according to changes in coarseness of 

 the soil and is controlled largely by the 

 nature of parent rocks and by the sub- 

 soil density associated with topography. 

 A basic factor in soil formation is the 

 relation of topography to subsoil. 



Rain falling anywhere on bare soil 

 puddles it with muddy water. Such 

 muddy water contains colloidal soil 

 exceedingly small soil particles, some 

 of them almost molecular in size. If 

 the soil surface is sloping, much of the 

 muddy water runs off. If the surface 

 is flat, much of the water seeps into the 

 lower soil, where the colloidal particles 

 are deposited, forming a part of the B 

 horizon. This horizon forms in the 

 lower soil at depths usually ranging 

 from 8 up to 36 inches, depending on 

 height of the water table during the 

 formative period. The thickness of this 

 zone depends on the rate of internal 

 drainage and fluctuation of the water 

 table during the rainy season. Density 

 of the B horizon is affected by the flat- 

 ness of the terrain. In general, the 

 flatter the terrain the denser the B hori- 

 zon. This horizon sometimes called 

 the subsoil, or where very dense, a clay- 

 pan is the key to drainage. 



Soils in any one area are affected by 

 differences in parent-rock material. 

 Coarse sands from sandstone do not 

 puddle as much as clay from shales; 

 hence subsoil formation is less pro- 

 nounced in sandstone- than in shale- 

 derived soils. Differences in the rock 

 composition do not alter the basic soil- 

 forming process, but do affect the rate 

 of soil formation. 



SOILS AFFECT THE TREES principally 

 through soil air and soil moisture. 

 Seasonal available soil moisture com- 

 monly determines what species grow in 

 any forest and their rate of growth. 



