Things of Which Soil Is Made 59 



being all that compose the soil. There are still many other 

 things, some of which are invisible to the unaided eye and 

 difficult to find. 



We will next take the clear water that remained after 

 the mud settled. We will ppur it into a dish, place the dish 

 over a fire, and let the water boil slowly until it has all 

 evaporated. There will remain in the bottom of the dish 

 a thin white coating. Moisten this with a drop of vinegar 

 or other weak acid and it will disappear in a mass of little 

 bubbles. Such behavior teaches us that the white sub- 

 stance is probably a mixture of lime and soda. Besides 

 these there are tiny particles of potash and phosphorus, 

 which we cannot distinguish by the means we have used. 



Some soils contain a great deal of lime, and because they 

 have been formed from hmestone, are called limestone soils. 

 Plants need a little soda, but when there is much in the soil 

 it wiU kill them. Soils rich in soda are known as alkali 

 soils. They were formed in the bottom of lakes the waters 

 of which contained soda. Salt is another harmful thing 

 found in the soil. You can- sometimes see faint whitish 

 deposits of soda and other salts on the soil in flower 

 pots. 



There is one more thing that the soil contains that we 

 must not forget, for it is one of the most important of them 

 all. This is a living organism so small that we cannot see 

 it with the unaided eye. Many thousands of these organ- 

 isms are contained in a bit of earth such as you could take 

 up on the point of a small knife blade. We have named 

 them bacteria. 



Plants cannot make use of most of the substances in the 

 soil without the aid of th,ese organisms. The bacteria live 



