THE DRAINAGE OF FARM SOILS 197 



thermometer is covered with wet muslin, and the 

 bulb of another similar thermometer is left un- 

 covered, the wet thermometer may register as 

 much as 15 degrees cooler when both are swung 

 in dry air. This is due to the cooling effect of the 

 evaporation of the water. Moreover, water is a 

 poor conductor of heat; wet soils warm in the sun 

 slowly, because the water they contain holds 

 down the temperature. There is usually a 

 difference of 5 to 10 degrees between drained and 

 undrained soil in the same field. In fact, the 

 temperature of a soil in summer is very largely 

 determined by the amount of water it contains; 

 the wetter it is the colder it is. Warmth is one of 

 the chief essentials for the germination and growth 

 of farm crops ; it is the coldness of a poorly drained 

 soil, more than the mere excess of water it con- 

 tains, that is responsible for most of the unsatis- 

 factory growth of crops upon it. 



Draining a soil allows the air to enter it more 

 freely. If all the spaces between the soil grains 

 are filled with water air cannot enter. Air is one 

 of the most important agencies that help to make 

 a soil productive. It changes the rock particles 

 of the soil into plant food and is essential to the 

 decay of plants in the soil, making humus. Seeds 

 must have air or they will not germinate. The 

 soil bacteria that make fertility cannot thrive 

 without air; the more thoroughly and the more 

 deeply a soil can be aerated the richer it should be, 

 'and the better should plants grow upon it. The 

 depth to which air penetrates the soil increases 

 when the water-table is lowered by drainage, 

 hence a larger feeding area is presented to the roots. 



Draining a Soil Makes it More Moist. Al- 

 though it may seem a paradox, draining a soil may 



