470 Introduction to the Study of Science 



the total air capacity. From these data may be computed the per- 

 centage of air capacity of each. 



Relation of soil air to plants. Water may occupy all the air 

 space in the soil. But plants which are thus deprived of 

 soil air quickly fail. To exclude the soil air from a potted 

 plant, cover the surface of the soil with paraffin, or wrap the 

 pot in air-tight material, closing carefully about the stalk of 

 the plant. Observe the result. Other conditions may be kept 

 most favorable for plant life, but exclusion of soil air is fatal. 



Many instances of the effect of water in shutting off the 

 soil air supply of plants may be found in nature. Such plants 

 are said to be drowned ; but they are in fact suffocated. Trees 

 whose roots may be supplied with nutrient soil, which is covered 

 with air-tight material, as concrete or a hard, impervious clay, 

 do not thrive, and eventually perish from lack of soil air. Many 

 plants, however, as rice, taro, marsh grasses, water lilies, kelp 

 and other seaweeds, grow luxuriantly in water-soaked and 

 water-covered soil; but they are specially adapted to get 

 the necessary air from water. 



Agents of soil aeration. How is the soil aerated? Men 

 cultivate it, loosen it about the roots of trees and around cer- 

 tain farm and garden crops. But nature works to the same 

 end in different ways. The activities of earthworms, ground 

 moles, field mice, and ground squirrels are well known. Grasses 

 and large plants loosen the ground with their roots and allow 

 the air to permeate the spaces. Frost is an important factor. 

 The expansion of freezing moisture in the ground widens the 

 interstices between particles of soil; then, with the thawing 

 and escape of the water, the enlarged spaces are occupied by 

 air. Note the condition of ground which has just thawed out, 

 especially where it was hard packed before freezing began. 



Importance of soil air. Soil air is indispensable to the ex- 

 istence of bacteria and other forms of microscopic plant and 

 animal life, which are necessary to insure a fertile or productive 

 soil. Some plants of economic value, such as the clovers, 



