CONDITIONS AFFECTING SOIL FERTILITY 41 



perceptible draft. Wells of this kind are called blowing wells 

 and form very good natural barometers. When a storm ap- 

 proaches, the draft is outward, and when fair weather returns, 

 the air flows back into the ground. The same phenomena aid 

 in the ventilation of the soil, but even without this there is 

 more or less exchange between the gases in the soil and those 

 outside by a process of diffusion, in which different gases tend 

 to mix until all parts of the mixture have equal amounts of 

 each. The ventilation of the soil is also promoted by the air 

 which flows in when the water, after a rain, sinks downward. 



Air in the soil. About half the bulk of dry soil is pore 

 space filled with air. Clay, although apparently more com- 

 pact than sand, has a greater amount of pore space. The 

 spaces, however, are smaller and the air moves more freely 

 in sand. As has been previously stated, air is essential to a 

 fertile soil. It supplies the underground parts of plants with 

 the oxygen necessary for respiration, it makes the soil warmer, 

 promotes the growth of soil bacteria, and aids in weathering. 

 Ordinary plants cannot live in a saturated soil, and a few days' 

 flooding may destroy an entire crop. Certain aquatic plants 

 thrive in such soils, but these have become adapted to their 

 habitat and have other ways of obtaining their oxygen. 



Temperature. All parts of the earth receive the same amount 

 of sunlight in the course of a year, but the shape of our planet 

 makes the distribution of temperature very uneven. The heat 

 is greatest where the sun's rays are vertical and least where 

 they fall obliquely, since in the latter case the same amount of 

 heat is distributed over a greater area. For this reason hill- 

 sides sloping toward the sun are warmer than level fields, 

 while northern slopes are colder. Few realize the enormous 

 heat received from the sun, although familiar with the fact 

 that a lens may bring together the few rays that fall upon it 

 and set fire to paper or wood. It is estimated that the energy 

 received from the sun is equal to one horse power per hour for 



