18 THE FOOD OF PLANTS CHAP. 



From this it would seem that there was very 

 little store of plant food in the soil ; and it is true 

 that the greater part of the soil is useless as plant 

 food. But there is another reason why so little is 

 dissolved by the water. Some things dissolve easily 

 in water, while others dissolve only to a small extent. 



Experiment 15 



We can certainly dissolve almost as much sugar 

 as we please in water. But supposing instead of 

 sugar we take some gypsum, shake it up with dis- 

 tilled water, and filter. Apparently the gypsum has 

 not dissolved at all. However, we must make sure. 

 Boil the filtered water away, and notice that a slight 

 deposit is left, showing that the water has dissolved 

 a very little, but only a very little, of the gypsum. 

 If we continued to shake up the gypsum, always 

 adding fresh water, each supply of water would dis- 

 solve a little more, until all the gypsum was dissolved. 

 In the same way the soil contains a store of plant 

 food which is only dissolved little by little, year by 

 year. Furthermore, there are slow changes going on 

 in the soil which are changing valuable plant food 

 from an insoluble into a soluble state, and so making 

 it available for use by the plant. 



We can now understand how the roots are 

 supplied with food from the soil. 



The rain soaking into the earth dissolves such 

 soluble substances as it contains, and these are sucked 



