CHAPTER XXXV 



SOME PHYSICAL PROPERTIES AND CONDITIONS OF SOIL 



In the preceding lessons we have learned that plants 

 use certain elements in manufacturing their foods and 

 products, and that they get these elements from compounds 

 which they obtain from the air, water, and soil. The 

 question now arises, how do plants get hold of this raw 

 material? In order to answer this question, we must 

 understand something about the physical conditions of the 

 soil as well as its chemical composition. In the first place, 

 plants cannot use the materials of the soil unless they are 

 soluble in water. What do we mean by this ? 



Experiment. — Put some salt or sugar into a glass full 

 of water. Drop it in a little at a time, stirring gently until 

 it disappears. Since it dissolves it is said to be soluble in 

 water. Drop a little sulphur into the water and stir. Is 

 it soluble or insoluble? Try some chalk dust. Place a 

 few small pieces of limestone or marble in a tumbler. 

 Cover it with water and stir. Do they dissolve? Drop a 

 little acid into, the tumbler. What is the effect ? Does the 

 acid aid in dissolving the rock? 



We have already learned that acids of various kinds are 

 found in the soil. Some of these acids are valuable in 

 helping to change insoluble substances into soluble ones. 

 Of course we know that a soil may become too sour for 



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