ABSORPTIVE PROPERTIES OF SOIL. 



261 



with about a kilogramme of damp soil from a field or garden, and then pouring 

 over it a weak solution of potash salts, phosphates, or ammonia compounds. 

 If the fluid running off below be then examined, only slight traces of the materials 

 named are found in it, or even none at all ; though lime and magnesium sulphate 

 are generally present in large quantities. It 

 is therefore necessary to pour on very con- 

 siderable quantities of pure water, in order 

 to gradually wash out again part of the 

 materials previously held fast in the earth ; 

 the soil of vegetation thus acts on these 

 nutritive materials of plants, as animal 

 charcoal on colouring matters and other 

 chemical compounds. 



:©.< 



FIG. 208. — Seedling of White Mustard.:. A is 

 taken out of the soil and shows the particles of soil 

 clinging to the root hairs : in B these have been re- 

 moved by washing in water. 



Fig. 209. — Root-hair of a seedling ol Wheal 

 closely attached to particles of soil (highly magnified, 

 cf. Fig. 3o6). 



Meanwhile it is of little interest for us to know in how far purely chemical 

 or even molecular forces come into consideration in the absorption of the materials 

 in the soil. So much is at any rate established, that chemical compounds of 

 potash, ammonia, and phosphoric acid, are retained with great force on the 



