PHYSICAL CIIAUACTEKS OF THE SOIL. 159 



nourisliment to crops as the latter, since they j)rcsent one- 

 half more absolute weight in a given space. 



Peat soils are light in both senses in Avhich this word 

 is used by agriculturists. 



The Specific Cravity of Soils is the weight of a given 



bulk compared with the same bidk of water. A cubic 

 foot of water weighs 62^ lbs., but comparison of this num- 

 ber with the numbers stated in the last table expressing 

 the weights of a cubic foot of v.u'ious soils does not give 

 us the true specific gravity of the latter, for the reason 

 that these weights are those of the matters of the soil 

 contained in a cubic foot, but not of a cubic foot of these 

 matters themselves exclusive of the air, occupying their 

 innumerable interspaces. When we exclude the air and 

 take account only of the soil, we find that all soils, except 

 those containing very much humus, have nearly the same 

 density. Schone has recently determined with care the 

 specific gravity of 14 soils, and the figures range from 

 2.53 to 2.71. The former density is that of a soil rich in 

 humus, from Orcnbcrg, Tiussia ; the latter of a lime soil 

 from Jena. The density of sandy and clayey soils free from 

 humus is 2.65 to 2.G9. {Bulletin de la Soc. Imp. des 

 JVatitrolistes de Moscou, 1807, p. 404.) This agrees with 

 the density of those minerals which constitute the bulk 

 of most soils, as seen from tlie following statement of their 

 specific gravity, which is, for quartz, 2.65; feldspar, 2.62; 

 mica, 2.75-3.10 ; kaolinite, 2.60. Calcite has a sp. gr. of 

 2.72 ; hence the greater density of calcareous soils. 



§2. 



STATE or DIVISION OF THE SOIL AND ITS INFLUENCE ON 

 FERTILITY. 



On the surface of a block of granite only a few lichens 

 and mosses can exist; crush the block to a coarse powder 

 and a more abundant vegetation can be supported on itj 



