THE SOIL. 29 



other underground channels; but, from whatever 

 source, there is always some circulation of air in 

 the soil from atmospheric changes. 



Then, too, there is a small circulation in the top 

 few inches during warm, sunshiny days, when the 

 soil is cultivated so that it is not too compact. Air 

 has such great power of expansion that, under the 

 added heat of noonday, c ome is forced out; but, 

 cooling and contracting during the night, it 

 returns. 



Soils which are open and well drained, may have 

 too much ventilation, from which injurious results 

 may follow. The air which enters the earth goes 

 there for the purpose of supplying the roots of 

 plants with food directly, or it acts upon the min- 

 erals in the ground and changes them into plant 

 food. If the air circulates too freely in the soil, so 

 much food is formed that the plants cannot use it 

 all immediately, and some of it goes to waste. 

 Some is dissolved and washed downward by the 

 next rain beyond recovery, and some is of such 

 a nature that it soon loses its identity, and thus 

 becomes unfit for use. It should be the aim to 

 keep such soils as moist as possible at the surface, 

 and the pores well filled by the right kind of culti- 

 vation. 



SOIL TEMPERATURE. 



The soil receives its heat in three ways: from 

 the interior of the earth, from decomposition of 



