22 THE FOOD OF PLANTS CHAP. 



The Improvement of Clay Soils 



If we take a piece of clay and heat it red hot, 

 when it cools it will no longer knead up with water 

 as before. A common red flower-pot, or a brick, is 

 such a piece of baked clay. 



Pound a piece of brick into a fine powder in a 

 mortar and then mix it with water. It behaves like 

 sand, and allows the water to pass freely through. 



Sometimes, therefore, when the soil contains too 

 much clay, the farmer pares off the surface, gathers 

 the clay of the subsoil into heaps, and burns it, and 

 then returns the baked clay to the soil. 



This baked clay acts like a mixing of sand with 

 the clay, and keeps the soil open, so as to allow 

 water to pass more freely through, and dissolve 

 the food necessary for the plant. 



The Amount of Water in the Soil 



Experiment 17 



Let us now take a moist piece of garden soil and 

 try and find exactly how much water it contains. 



To do this properly, we must make it hot enough 

 to drive off all the water, but at the same time 

 we must not heat it too strongly, as you will see 

 presently. 



