476 SOILS: PROPERTIES AND MANAGEMENT 



coarse-textured soil, provided the particles were spherical 

 and all of the same size. Under the conditions actually 

 existing in the field, the soils composed of small particles 

 generally possess the larger amount of air space. 



392. Structure. — The volume of air in a water-free 

 soil being identical with the pore space, the formation 

 of aggregates of particles is favorable to a large air volume. 

 The volume of air in any soil, therefore, changes from 

 time to time; and particularly is this true of a fine- 

 grained soil, in which the changes in structure are greater 

 than in a soil with large particles. A change in soil 

 structure may greatly alter the volume of air contained 

 by changing the pore space, thereby influencing the pro- 

 ductiveness. Clay is affected to the greatest extent in 

 this way. 



393. Organic matter. — Since organic matter is more 

 porous than mineral particles of any size or arrangement, 

 the effect of that constituent is always to increase the 

 volume of air. While this is generally beneficial in a 

 humid region, it is often very injurious in an arid region. 

 Unless sufficient water falls on the soil to wash the soil 

 particles around the organic matter and to maintain a 

 supply sufficient to promote decomposition, the presence 

 of vegetable matter leaves the soil so open that the capil- 

 lary rise of moisture is interfered with, and the consider- 

 able movement of air keeps the soil dry, with the result 

 that the portion of the soil layer mixed with and lying 

 above the organic matter is too dry to germinate seeds 

 or to support plant growth. 



394. Moisture content. — It is quite evident that the 

 larger the proportion of the interstitial space filled with 

 water, the smaller will be the quantity of air contained. 

 This does not necessarily mean that the higher the per- 



