196 CULTIVATION. 



quantity of heat is abstracted, and the soil becomes 

 cold. 



This cooling of the soil by the evaporation of 

 water, is of very great injury to its power of pro- 

 ducing crops, and the fact that under-drains lessen 

 it, is one of the best arguments in favor of their 

 use. Some idea may, perhaps, be formed of the 

 amount of heat taken from the soil in this way, from 

 the fact that, in midsummer, twenty-five hogsheads 

 of water may be evaporated from a single acre in 

 twelve hours. 



1 7. When not saturated with water the soil ad- 

 mits the water of rains, etc., which bring with them 

 fertilising gases from the atmosphere, to be deposit- 

 ed among the absorbent parts of the soil, and given 

 up for the necessities of the plant. When this rain 

 falls on lands already saturated, it cannot enter the 

 soil, but must run off from the surface, or be re- 

 moved by evaporation, either of which is injurious. 

 The first, because fertilizing matter is washed away. 

 The second, because the soil is deprived of necessary 

 heat. 



18.' The formation of crust on the surface of the 

 soil is due to the evaporation of the water of the soil. 

 It arises partly from the fact that the water in the 

 soil is saturated with mineral substances, which it 

 leaves at its point of evaporation at the surface. 

 This soluble matter often forms a very hard crust, 

 which is a complete shield to prevent the admission of 

 air with its ameliorating effects, and should, as far 

 as possible, be avoided. Under-draining is the best 



