40 LAND DRAINAGE 



upper four inches would be really greater than these 

 figures show. 



These figures are given rather to emphasize the magni- 

 tude of the forces involved, and their possibilities when 

 properly directed, than to arrive at absolutely accurate 

 results in practice. 



58. Capillary water and ventilation. Hellriegell found, 

 as has previously been stated, that when a soil contained 

 more than 60 per cent of the water it was capable of hold- 

 ing, the results in crop-growing were not so satisfactory 

 as when the water content was kept between 50 and 60 

 per cent. The reason offered is that with more than 

 60 per cent of water present, too little room is left within 

 the soil mass for the proper movement of the air, for 

 ventilation. Note that with the pore space of the soil 

 half occupied by water, there is left an equal amount of 

 space for the occupation and circulation of air. 



INFLUENCE OF SOIL STRUCTURE UPON OTHER PHYSICAL 

 CONDITIONS 



While a proper soil structure is greatly dependent 

 on other factors, such as water, organic matter, use of 

 tools, and the like, it in turn becomes a most important 

 factor in the modification and control of other physical 

 conditions. 



59. Agencies active in soil ventilation. Nature em- 

 ploys several agencies by which to cause inward and 

 outward movement of air in the soil, in the process of 

 soil ventilation. The chief agencies concerned in pro- 

 ducing these movements are : (1) diffusion, (2) changing 

 soil temperatures, (3) barometric changes, (4) changing 

 wind velocities, and (5) gravitational water. Each of 



