48 LAND DRAINAGE 



69. Gravitational water may interfere with ventilation. 

 -The effect of gravitational water upon soil ventilation 



may be direct or indirect. When water occupies all the 

 pore-space within a soil, practically all air is thereby ex- 

 cluded, a direct effect. Seeds cannot germinate under 

 such a condition and crops previously occupying the soil 

 quickly die. 



Whenever the physical structure of a soil is affected by 

 the presence of gravitational water to the extent of 

 compacting the surface or reducing the pore-space, ven- 

 tilation is thereby restricted. This restriction of ven- 

 tilation may be sufficient to interfere with all those proc- 

 esses dependent upon soil ventilation. These effects 

 would be called indirect effects. 



70. Gravitational water and food losses. Gravita- 

 tional water, as it moves downward through the soil, 

 dissolves and carries away more or less of the soluble plant- 

 foods that have been prepared within the soils. The more 

 slowly gravitational water moves downward, the larger 

 the amount of soluble plant-food it takes with it. The 

 converse is equally true. 



Indirectly, the presence of gravitational water causes 

 the destruction and therefore the loss of prepared or 

 partially prepared nitrogen compounds, by the process 

 of denitrification, already described and several times 

 alluded to. Denitrification takes place when air is ex- 

 cluded from the soils. 



71. Gravitational water and soil temperature. The 

 presence of gravitational water in a soil directly affects 

 its temperature in two ways : (1) it increases the amount 

 of heat required to warm the soil; (2) it causes great 

 losses of heat that might otherwise be used in warming 

 the soil. Indirectly, there results to the soil later great 



