76 AGRICULTURE 



the question of drainage may be shortly set 

 out as follows. When one constructs drains 

 in a field that is more or less water-logged, 

 the withdrawal of the superabundant water 

 at once leaves vacant spaces between the 

 soil particles, and these are immediately 

 occupied by air which enters on the with- 

 drawal of the water. The presence of 

 abundant supplies of air at once creates new 

 conditions in the soil, enabling manures to 

 decay more rapidly and so of becoming 

 more quickly available for the use of plants. 

 All organic substances decay very slowly in 

 a water-logged soil, partly owing to the low 

 temperature of such land, but chiefly owing 

 to the fact that under such circumstances, 

 oxidation is much retarded. Nitrification, 

 too, which is intimately associated with the 

 breaking down of organic substances, is 

 practically at a standstill. 



Wet land is cold, as has previously been 

 explained, because water, having a high 

 specific heat, requires much heat to raise 

 its temperature, that is to say, heat from 

 the sun has a difficulty in making itself felt. 

 Then again, we cannot have water in soil 

 without a certain amount of evaporation, 

 and whenever liquid water takes the form of 

 vapour a great deal of heat, as the term is 



