326 GENERAL SCIENCE 



the ravines will aid in carrying away surface water without 

 damage to the soil. 



In low lands where the slope is not sufficient to carry 

 off the excess water, the farmers have to resort to artificial 

 drainage. Some dig ditches every few rods, but these are 

 a hindrance to extensive and free cultivation. The best 

 method is to lay tile eighteen to twenty-four inches below 

 the surface. The tile drains will carry away the excess 



AN IRRIGATED ALFALFA FIELD 



water and permit the air to enter the soil more freely 

 and thus the soil will become more favorable for the 

 growth of plants. 



Much water is removed from wet soil by evaporation, 

 but this keeps the soil too cold for rapid plant growth, 

 because a large amount of heat is required to make the 

 water evaporate. For this reason wet soils stay cold 

 longer in the spring than soils which are well drained 

 of their excess water. 



225. Irrigation. In regions where the rainfall is not 

 sufficient for farm crops to grow, the farmers use arti- 

 ficial means of moistening the soil if sufficient water can 

 be obtained at reasonable cost. Dams are built in moun- 



