174 THE STUDY OF PLANT COMMUNITIES * Chapter VII 



able water. The instrument is accurate for values ranging from 

 zero tension to approximately 0.85 atmosphere of tension, or from 

 saturated soil to a reduction of 80 or 90 percent of available 

 water. 217 Approaching the wilting percentage, its values cannot be 

 wholly trusted. 



Soil point cones are small, hollow cones of porous porcelain, 

 which can be inserted into the soil with a minimum of disturbance 

 so that each has an equal area of surface in contact with the soil. 

 The amount of water absorbed by the cone is determined by 

 weighing and is taken as a measure of the water supplying power 

 of the soil. In some types of studies, this value alone is sufficient to 

 make comparisons between soils without any further analyses be- 

 ing necessary. It is also indicative of moisture conditions, for, at 

 the wilting percentage, it approximates 0.085 g. in two hours. 



SOIL ATMOSPHERE 



Organisms and Soil Atmosphere.— It was pointed out earlier 

 that air is a component of soil (p. 148). Both the amount and com- 

 position of this air are of importance to plants. Most plants re- 

 quire a well-aerated soil for growth and even for survival. Many 

 seeds will not germinate unless well aerated even though tempera- 

 ture and moisture are favorable. Healthy roots must carry on res- 

 piration continuously, which means that oxygen must be present 

 in the soil. At the same time, their activity produces carbon dioxide 

 and carbonic acid, which tend to accumulate. To some plants, 

 the increase in the proportion of carbon dioxide is more injurious 

 than the decrease of oxygen. Since all microorganisms present are 

 likewise using oxygen and releasing carbon dioxide, the balance of 

 the two cannot be maintained unless there is a free exchange of 

 gases with the air above the soil. If aeration is good, this may be 

 accomplished by diffusion from the air. However, in any soil the 

 proportion of oxygen decreases and that of carbon dioxide in- 

 creases with depth, and the proportion of oxygen is not as great 

 in soil as in air even when conditions are most favorable. 



Relation to Growth and Distribution of Roots.— Since aeration 

 becomes poorer and oxygen decreases with depth of soil, these 

 conditions may limit the depths to which roots can grow. The 

 deepest root penetration is in well-aerated soils. Species growing 



