PROPERTIES AND CONDITIONS OF SOIL 281 



plants to thrive well in it. Nevertheless, some acids are in- 

 dispensable. 



If the raw material must be dissolved in water before 

 entering the plant, it is evident that there must be sufficient 

 water in the soil for this purpose. What is the source of the 

 soil water ? What becomes of the rain that falls upon the 

 earth ? Some runs off into streams, some evaporates, but 

 some soaks into the ground. What becomes of this 

 water ? 



Experiment. — Place a handful of pebbles or gravel in 

 the bottom of a tumbler or, better, a quart jar. Cover 

 the gravel with a piece of wire screening or cheese cloth. 

 Then fill the jar with fine soil from a field or garden. Pour 

 some water on the soil. What does it do? As it soaks 

 into the soil, can you see it between the particles? Pour 

 in more until you can see water standing in the spaces 

 between the pebbles in the bottom of the jar. When rain 

 falls on the ground it does just what this water did. It 

 percolates slowly downward through the soil till it is stopped 

 by an impervious layer, just as the bottom of the jar 

 stopped this. If it keeps on raining, what will happen ? 

 Pour more water into the jar until it stands on top of the 

 soil. Does this ever happen in the fields? Set the jar 

 aside in a warm place till the water stands only a few inches 

 in the bottom. 



The water that completely fills the spaces in the soil 

 and moves slowly downward by the force of gravity is 

 known as free water. Plants do not use this free water. 

 What part of the soil in the jar now has free water? If 

 the top layer of the soil seems very dry, remove it and lay 

 it aside. Take in your hand a little of the moist soil. 



