SOIL STRUCTURE AND TEXTURE 



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materials of all sorts. But this sort of accumulation is going on more 

 or less wherever plants are growing. 



Rock masses are often built up through the activities of plants. 

 This is done largely by algse which absorb soluble materials from 

 the water and secrete them in an insoluble form. INIarl is built up 

 in this way by Chara while strata of silica are built up by diatoms. 

 A few algse store up lime in a comparable way. In fact limestone 

 is usually built up by some such means and the material of coral 

 reefs, formerly thought to be built up entirely by corals, is now 

 known to be due in part, at least, to the activities of algse. The trav- 

 ertine deposits around hot springs are also built up largely by algse. 



Fig. 64.— Soil formation on sandstone rock. Plants growing in the crevices. 



93. Soil Structure and Texture.— A soil ordinarily consists of a 

 mixture of rock particles, water containing dissolved substances, 

 air, dead organic matter and living plants and animals. The relative 

 amounts of these various components vary greatly in different soils. 

 Leaving out of consideration for the present the living organisms, 

 about 40 per cent, by volume, of a rich garden soil is rock particles; 

 25 per cent is water, 25 per cent air and 10 per cent organic matter. 



The size of the rock particles varies greatly in different soils. 

 They range from coarse gravel to particles that are too small to be 

 seen by the most powerful microscope. Soil particles that are more 

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