2l8 ENGINEERING FOR LAND DRAINAGE. 



port of soil bacteria, which are now recognized as 

 playing such an important part in converting soil 

 humus into nitrates. These elements of fertility are 

 absorbed by the soil and held in readiness for the use 

 of plant-roots. The changes wrought by the passage 

 of water through the soil and consequent circulation of 

 air are continuous processes and play no unimportant 

 part in keeping up the productiveness of soils. In 

 other words, well-drained soils do not become ex- 

 hausted as soon as undrained ones, even under the 

 most ordinary treatment. 



Drainage Increases the Temperature of the Soil. — 

 A soil cannot become warm until the water upon its 

 surface is evaporated or thoroughly warmed by the sun. 

 The cause is easily explained. A large amount of 

 heat is used up in evaporating the excess of water. 

 Prof. F. H. King in his work, "The Soil," gives de- 

 ductions from personal experiments which are valuable 

 in showing the effect of drainage upon soil tempera- 

 ture. He says: <* While lOO heat units will raise the 

 temperature of one pound of water through ioo° F., it 

 is necessary to use 966.6 heat units to evaporate one 

 pound of water from the soil ; but this if withdrawn 

 directly from the cubic foot of saturated clay would 

 lower its temperature about 10.3^ F. It must be evi- 

 dent, therefore, that to allow the surplus water to drain 

 away from a field rapidly, rather than to hold it there 

 untiiit has time to evaporate, must greatly favor the 

 warming of the soil. ' * 



He cites the following observations showing the dif- 

 ferences of temperature in the surface inch of well- 



