RAINFALL AND STRUCTURE 119 



"casts" which they deposit. Their action is frequently 

 quite important. (See page 28.) Insects, especially 

 ants and other burrowing creatures, aid in this and 

 other ways. 



51. Rainfall. Rain storms compact the surface soil 

 by washing the fine particles into the interstitial spaces, 

 and by the actual pressure of the rain-drops. The result 

 is to form a surface layer having the separate-grain 

 structure, and which when dry, forms a crust, some- 

 times capable of preventing germinating plants from 

 reaching the surface of the ground, and which is con- 

 ducive to the loss of moisture by evaporation. Some 

 clay soils are very susceptible to a change of structure 

 in this way. A heavy thunder-storm may entirely 

 change the structure of a clay soil to the depth of sev- 

 eral inches, in a short time. This is due both to the im- 

 pact of the rain-drops and the saturated condition of 

 the surface layer. 



Surface covering mulches, sod or any kind of 

 covering during the summer season serves to protect 

 the surface soil from the compacting effect of rain. The 

 volume weight of a mulched soil will generally be found 

 to be less, at the end of the growing season, than that 

 of a well-cultivated one. The benefit to be derived 

 from sod has already been mentioned. 



II. ORGANIC CONSTITUENTS OF THE SOIL 



Examination of almost any soil shows it to contain 

 not only mineral particles but also plant and animal 

 remains. The forest soil contains in the surface layer 



