SOIL AND ITS RELATION TO ROOTS 79 



easy matter to prove that black soil contains organic matter, for if 



an equal weight of carefully dried humus and soil from a sandy road 



is heated red-hot for some time and 



then re weighed, the humus will be 



found to have lost considerably in 



weight, and the sandy soil to have 



lost very little. The material left 



after heating is inorganic material, 



the organic matter having been 



burned out. 



Soil containing organic materials 



holds water much more readily than 



inorganic soil, as a glance at the 



accompanying figure shows. If we 



fill each of the vessels with a given 



weight (say 100 grams each) of 



gravel, sand, barren soil, rich loam, 



leaf mold, and 25 grams of dry, 



pulverized leaves, then pour equal 



amounts of water (100 c.c.) on each 



and measure all that runs through, the water that has been re- 

 tained will represent the water supply that plants could draw on 

 from such soil. 



The Root Hairs take more than Water out of the Soil. — If a 

 root containing a fringe of root hairs is washed carefully, it will be 

 found to have little particles of soil still clinging to it. Examined 

 under the microscope, these particles of soil seem to be cemented 

 to the sticky surface of the root hair. The soil contains, besides 

 a number of chemical compounds of various mineral substances, — 

 lime, potash, iron, silica, and many others, — a considerable amount 

 of organic material. Acids of various kinds are present in the soil. 

 These acids so act upon certain of the mineral substances that 

 they become dissolved in the water which is absorbed by the root 

 hairs. Root hairs also give off small amounts of acid. An in- 

 teresting experiment may be shown (see Figure on page 80) to 

 prove this. A solution of phenolphthalein loses its color when an 

 acid is added to it. If a growing pea be placed in a tube contain- 



Soil particles cling to root hairs. 

 Why? 



