THK REI.ATION OF SOILS TO IRRIGATION. 29 



necessary to take out a handful of earth a few inches 

 below the surface. 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 

 moisture 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 tem- 

 perature of the soil will depend also upon the amount 

 of evaporation 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, and 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 greenhouse culture to 

 force the ripening of a plant; while the higher tem- 

 perature and the greater moisture content of the clay 

 soil are conditions used in greenhouse culture to pro- 

 duce a leafy development and to retard the ripening of 

 the plant. 



Gravity. — The relation of soils to water resolves 

 itself into two lines of investigation — the forces 



