30 THE FERTILITY OF THE SOIL [CH. 



The constant use of the hoe in the garden reduces 

 considerably the need for watering, and many a good 

 gardener will declare that "the hoe is the best water- 

 ing can." In dry regions the disk-cultivators are 

 brought out as soon as possible after a shower so 

 as to break up any crust that may have formed on 

 the arable land and restore the protective coating of 

 fine soil. 



Air supply. The harm that results from a water- 

 logged soil is not due to the excess of water but to 

 the exclusion of air. The plant roots and the food- 

 making bacteria alike need air, and air must there- 

 fore be allowed access to all parts of the soil. Once 

 the excess of water goes the air comes in: such 

 devices as liming a clay soil, laying drains, and break- 

 ing a pan, therefore have the effect of improving not 

 only the water supply but the air supply as well. 



Temperature. The heat relationships of a soil are 

 also intimately bound up with the water content. Its 

 mean temperature is of course directly dependent on 

 its location, and does not differ greatly from that of 

 the atmosphere. The top half inch of soil is hotter 

 than the air in direct sunshine and colder by night : 

 a little below the surface the fluctuation is no greater 

 than in the air, while six inches below it is much less. 

 Heat only travels slowly through dry soil and does 

 not affect the subsoil till some time after it has 

 reached the surface. But it travels more quickly 



