RELATION OF SOILS TO IRRIGATION. 25 



the moisture adhering to the particles of soil. The lat- 

 ter is the most favorable to the growth of crops. In 

 determining the condition of moisture in the soil in the 

 practical application of water, it is only necessary to 

 take out a handful of earth a few inches below the sur- 

 face. If the earth is of sufficient moisture to ball in the 

 hand irrigation at that time is not needed. This is a 

 simple and inflexible rule. 



Temperature. The relation of soil to heat is 

 largely dependent upon the relation of soil to moisture 

 and the amount of moisture contained in the soil. It 

 takes more heat to raise the temperature of a pound of 

 water one degree than to raise the temperature of a 

 pound of soil the same amount ; so that the more mois- 

 ture there is in a soil the more material there is to be 

 heated, and this added material is more difficult to heat 

 than the substance of the soil itself. The temperature 

 of the soil will depend also upon the amount of evapora- 

 tion of the soil. It has been shown that from this cause 

 alone the temperature of the sandy soil may be much 

 cooler at midday than the temperature of the clay soil. 

 If the soils had been dry this would have been just the 

 reverse, as the substance of the clay is more difficult to 

 heat than the substance of the sand. It has been shown 

 that the mean temperature of a sandy soil is lower than 

 that of an adjacent clay soil, while the sandy soil is 

 drier than the clay soil. These are conditions of a lower 

 temperature and a drier soil, which are used in green- 

 house culture to force the ripening of a plant ; while 

 the higher temperature and the greater moisture content 

 of the clay soil are conditions used in greenhouse culture 

 to produce a leafy development and to retard the ripen- 

 ing of the plant. 



Gravity. The relation of soils to water resolves 

 itself into two lines of investigation, the forces which 

 move the water, and the conditions which determine the 



