312 MAN'S USE AND CONSERVATION OF SOILS 



over a Bunsen burner or in a very hot oven. Weigh again. If 

 there is still a loss of weight this must be due to the burning of the 

 organic matter rotten twigs, roots, leaves, etc. which was in 

 the soil. Soils differ greatly in the amount of water they contain 

 and in the amount of organic substance present. 



We have seen from Experiment 91 how the soil takes 

 up water, and how each little particle has a film of water 

 around it. Little hairs on the plant roots are prepared 

 to take up these little films of water which surround the 

 soil particles. These water films have probably dissolved 

 a minute amount of material from the soil particles, and 

 this material enters into the plant and can be used for 

 food. 



Experiment 93. Compute the area of a cubical block of wood 

 four inches on a side. Cut the block in two. Compute the com- 

 bined area of the two pieces. Cut each of these two pieces in two. 

 Compute the combined area of the four pieces. Cut each of the 

 four pieces in two. Compute the combined area of the eight pieces. 

 What effect does dividing the block into smaller and smaller pieces 

 have upon the total surface area? Has the mass or volume of 

 the wood been increased ? 



We found in Experiment 93 that the more we subdivided 

 the block the greater was the combined area of the pieces. 

 This makes clear an important difference between coarse 

 and fine soils. The smaller the particles are in a given volume 

 of soil, the greater is the total surface to be covered by film 

 water. Then too, the smaller the particles, the more readily 

 are they dissolved and the greater is the amount of food 

 within reach of the root hairs of plants. 



Soil air. Experiment 94. Fill an 8-oz. bottle with soil taken 

 from a few inches below the surface. Fit the bottle with a two- 

 holed rubber stopper having the long neck of a three or four-inch 

 funnel pushed as far as possible through one hole and a bent de- 



