32 



An Account of the Soil 



PT. 1 



water and attached to an aspirator. A similar tube also 

 containing 20 c.c. of bar3^ta water but open to the air is 

 attached to the same aspirator. Set the aspirator work- 

 ing and arrange the connections so that bubbles pass at 

 the same rate through the two lots of baryta water. The 

 one connected with the soil speedily becomes turbid, in- 

 dicating the presence of carbon dioxide, the other, open 

 to the air, however, only shows turbidity later on 

 (Fig. 11). 



The water is held by physical forces in the pores and 

 the amount present depends on the rainfall, the evapora- 

 tion and the drainage. In the Rothamsted measure- 

 ments the sandy soils were generally found to contain 

 about 9 per cent., the loams about 12 per cent., and the 

 clays about 27 per cent, by weight; a better idea, how- 

 ever, is furnished by taking the proportions by volume, 

 which varj' from 20 to 40 per cent. The following are 

 the figures for some of the Rothamsted soils : 



The water is not pure but contains various salts in 

 solution, the most important of which are nitrates and 

 bicarbonates (p. 25). 



The subsoil. The lower portion of the soil differs so 

 much from the surface layer that it receives a separate 



