PHYSICAL INTER-RELATIONS IN SOILS 41 



these agencies is distinctive in its action and might 

 operate alone. All work together, however, and each 

 undoubtedly modifies to some extent the operation of the 

 others ; this is especially true of the last-named agency. 



60. Relation of soil structure to soil ventilation. 

 An important part of any system of house ventilation 

 is a capacious set of flues, for both the intake and outlet of 

 air. When the flues are insufficient or when stopped or 

 clogged, the system will fail to do proper work. A soil too 

 compact fails to permit the ready and proper passage, in- 

 ward and outward, of air. A soil of open structure possesses 

 a greater capacity for air than does a soil of close or compact 

 structure. The structure of the soil, therefore, has a very 

 direct relation to soil ventilation, and observation reveals 

 the fact that crop-life suffers much from improper and 

 insufficient ventilation. 



61. Effects of life-forms. Ventilation is greatly pro- 

 moted by the burrows of lower animal forms, especially 

 earthworms and ants. These creatures are found in 

 great numbers in soils in which the physical conditions are 

 correct, and burrow to considerable depths. The roots 

 of some plants decompose rather rapidly after the plant 

 has been killed, and thus leave numerous openings or 

 tunnels through the soil. The size of these roots and 

 the depth of penetration depend much on the physical 

 condition of the soil. Every agent active in soil ventila- 

 tion is assisted by the work of these forms of life in the 

 accomplishment of its function. 



62. Influence of soil structure on capillary water. It 

 is a matter of common observation that of two fields of 

 the same soil formation, the same topography, and the 

 same exposure, one will produce a good crop of corn or 

 wheat in a rather dry year, while the other will produce 



