30 Nature-Study Agriculture 







plowing is to be recommended ; but the plow should 

 be allowed to go only a little deeper each year, so that 

 not enough subsoil will be brought to the surface at 

 once to injure the crop. The subsoil when exposed to 

 the air gradually becomes good soil. 



Why dry In arid regions there is usually little difference in the 

 no l subsoii e s il f r several feet down, for there is little clay and 

 the soil, being dry, admits air. The West has, as a 

 rule, no subsoil within reach of the plow. Sometimes 

 soil that is taken from a deep excavation in a dry region 

 is spread upon the surface, where it will support good 

 crops even during the first season. 



Soil water. The presence of water in the soil is one 

 of the most important factors with which the farmer 

 has to deal. As rain falls, it is absorbed by the soil; 

 Rainfall but different soils absorb different amounts (Fig. 19). 

 By " annual rainfall " is meant the depth to which the 

 ground in a particular locality would be covered in a 

 year if none of the rain water ran off or soaked in or 

 evaporated. (Exp. 8.) By adding the numbers repre- 

 senting annual rainfall for a series of years and dividing 

 the sum by the number of years, we obtain what is 

 Abilities of called the " average annual rainfall." A sandy soil 

 absorb and absorbs the rainfall more readily than does a clay soil ; 

 hold water but clay can hold considerably more water than sand 

 can. (Exp. 9.) The more humus any soil contains, 

 the greater the quantity of water it can hold, and there- 

 fore the longer it will keep moist in dry weather. Sandy 

 soil with little humus dries out very quickly. We shall 

 now see why these soils act so differently. 



If a small stone is immersed in water and then lifted 



