PHYSICAL PEOPERTIES OP SOILS. 239 



found also in some soils which are not clays, and hence the 

 term clayey is sometimes loosely applied. Calcareous or lime 

 soils contain calcic carbonate in large amount. To calcareous 

 clay, when the ingredients are in a state of rather fine subdi- 

 vision, the name marl is frequently applied. Peaty or humus 

 soils are those which contain a considerable proportion of par- 

 tially decayed vegetable matter ; when such matter decays under 

 water it becomes peat, or muck ; when it decays without much 

 water it is generalh 7 known as mould. 



642. By mechanical analysis, as by simple washing and sift- 

 ing, it is possible to separate a soil into its mechanical ingre- 

 dients, which are: (1) Gravel; (2) coarse sand; (3) fine sand; 

 (4) clayey sand ; (5) clayey substance, or fine clay. 



The mechanical subdivision of soils has an important bearing 

 upon their physical properties and upon their adaptability to 

 the growth of roots and the sustenance of plants. 1 



From interesting studies by Darwin, 2 it is plain that in some 

 localities earth-worms have exerted, by their burrowing and 

 tunnelling, a vast influence in changing the physical character 

 of the soils in which they thrive. 



643. Physical properties of soils. Of these, the most important 

 to be considered here are those which affect the relations of soils 

 to liquids, to gases, and to heat ; for all of these directly affect 

 the growth and indirectly the nutrition of plants. 



644. Absorption and retention of moisture by soils. It is con- 

 venient to examine the relations of soils both to liquid water 

 and to aqueous vapor. Soils can absorb from the atmosphere 

 and condense upon the surface of their particles, or in their inter- 

 stices, a certain amount of the vapor of water. This property 

 of absorption, known as that of hygroscopicity, is different in 

 different soils, as shown by the following table from Schiibeler. 3 



Five hundred centigrams of each soil carefully dried were 

 spread over a surface of thirty-six thousand square millimeters, 

 and exposed for varying periods to an atmosphere saturated 

 with watery vapor ; the amounts of waters absorbed (in centi- 

 grams) were as follows : 



1 The reader should examine a paper by J. D. Whitney (Plain, Prairie, and 

 Forest), in which is discussed the probable influence of the extreme fineness of 

 prairie soils upon the absence of forests. See American Naturalist, October 

 and November, 1876. 



2 Darwin : The Formation of Vegetable Mould through the Action of 

 Worms. 



8 Knop's Lehrbuch der Agricultur-Chemie, 1868, vol. ii. pp. 13, 14. 



