CHAPTER XIII 

 SOIL 



Soil is the weathered superficial layer of the earth's surface 

 mingled to a greater or less extent with the remains of plants and 

 animals. It is used by the great majority of plants as the chief 

 source of inorganic food materials and water and for mechanical 

 support. There are some plants, principally the floating water 

 plants, that make no use at all of soil, and most parasites and epi- 

 phytes make no direct use of soil, though, in most cases the plants 

 on which they live must have it. Soils differ very greatly in physical 

 and chemical properties and in various other respects and these 

 differences have very important influences upon plant life. 



92. Soil Formation.— All soils are formed primarily from rocks. 

 Rocks on and near the surface of the earth are constantly being dis- 

 integrated by such forces as alternate freezing and thawdng, erosion 

 by wind and water, and, in some places, surface scouring by glaciers. 

 Plants also play an important part in the formation of soils. This is 

 done in part directly by the disintegration of rocks by the plants 

 growing on them. The only plants that can live on strictly 

 unweathered rocks are certain kinds of lichens. These, however, 

 have a disintegrating effect on the rock surface so that gradually 

 a thin layer of soil is formed and this enables mosses and some other 

 plants to get a start. The disintegration of the rock now goes on 

 more rapidly as the numbers of plants increase. The underlying 

 rocks are split apart and broken into pieces by the roots of plants 

 growing in cracks and crevices (Fig. 64). Thus the building of a 

 soil goes on apace. At the same time whole plants or the deciduous 

 parts of plants are each year dying and adding the material of their 

 bodies to the soil. A deep, black, humus soil, such as is often found 

 in a forest, represents the remains of thousands of generations of 

 plants. 



Another way in which plants add to the soil of a given area is by 

 the accumulation of materials that are being carried by wind or 

 water. This is more readily seen in some places than in others. 

 For example, the flattened leaves of the stag-horn fern accumulate 

 considerable quantities of soil, and a zone of plants such as water 

 lilies along the edge of a glacial lake will often accumulate masses of 

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