THE SOIL. 11 



bo'dies as food. When they die and decay, this is left 

 as food for other plants. The gray ash left after 

 plants are burned, is the earthy matter. 



As with most things in nature, there is a constant 

 and ceaseless change of activity in the process of 

 soil-building. Plant food, stored in the soil during 

 one season, is partly washed away by rains, partly 

 taken up by plants, and partly left over for future 

 years. This work goes on year in and year out, and 

 must continue to do so as long as we exist upon the 

 earth, for it is the great storehouse from which 

 alone food for plants and animals may be drawn. 



RELATION OF WATER TO SOIL. 



Scattered through the air in all places, and at all 

 times, are countless numbers of very fine dust par- 

 ticles. They are so small that they cannot be seen 

 with the naked eye, yet their presence in the air is 

 probably one of the causes of the blueness of the 

 sky. Every gust of wind catches them up and bears 

 them away high over our heads and over the tops 

 of trees and mountains. The heavier particles fall 

 again when the air becomes still, but the finer and 

 lighter ones float in the air a very long time. This 

 is true not only of fine bits of wood, paper, and 

 such light material, but of rocks, and the heavy 

 metals. When anything is powdered fine enough, 

 each particle has so much surface exposed to the 

 air, compared with its volume, that it will float as 

 lightly as a feather. 



