PHYSICAL PKOPERTIES OF SOILS. 47 



prevented by burying the putrid matter in the soil. 

 The efficiency of the earth-closet system of disposing 

 of human excrement largely depends on this power. 

 As vegetable or animal matter decays in the soil the 

 latter absorbs and holds much of the ammonia and 

 carbonic acid given off, and also absorbs these as they 

 are brought to and into the soil by the rain and 

 winds. Clayey and humus soils can do this to a 

 greater extent than can sandy soils. 



Relations of 8oil to Heat. — The temperature of 

 the soil near the surface changes as that of the air 

 varies, although more slowly. During the day, es- 

 pecially if the sun shines brightly, the soil becomes 

 warmer but not so rapidly as does the air, chiefly be- 

 cause of the cooling effect of the evaporation of moist- 

 ure from the surface of the soil. At night the air 

 first cools, then the heat gained by the soil is radiated 

 into the air; the» evaporation of water is checked 

 and often there is a considerable deposit of dew on 

 the surface and an increase of moisture in the air 

 held in the pores of the soil. A wet soil is cooler 

 than the same soil would be if dry. Usually a dark- 

 colored soil is warmer' than a light- colored one. A 

 soil with much sand or gravel will become warm more 

 slowly and will retain the heat longer than an equal- 

 ly dry clay or humus soil. A plant surrounded with 

 stone or growing near a gravel walk may be uninjured 

 by frost when those growing in a clay soil may be 

 seriously injured. A soil covered with a sod is usual- 

 ly cooler during the day but does hot cool so rapidly 

 at night as does one bare of vegetation. A soil which 

 slopes southward will be warmer than one with a 

 northward inclination. 



