PHYSICAL PROPERTIES OF SOILS III 



temperature on the south of a garden wall 8 C., higher than on 

 the north side. In cool climates, fruits which refuse to ripen 

 under ordinary conditions may attain perfection when trained 

 against the sunny side of a wall. 



Water Content. The quantity of heat required to raise the 

 temperature of the soil depends upon the materials of which it is 

 composed, but it increases with the quantity of water present. 

 Approximately five times as much heat is required to raise the 

 temperature of one pound of water one degree as for one pound 

 of soil. Since wet soil does not warm up as rapidly as a dry soil, 

 draining a wet soil makes it warmer, as a general rule. Clay 

 soils, since they contain more water, do not warm up as quickly 

 as sandy soils, which retain much less water. This is probably 

 the reason sandy soils are so much better suited to early truck 

 crops than are clay soils. They warm up quicker, and maintain a 

 higher temperature during the early part of the year, thereby 

 forcing the growth of the crop. 



The evaporation of water from wet soils also makes them 

 colder than dry soils, as water in passing into the form of a vapor 

 takes up considerable amounts of heat. King found that a clay 

 soil was 4 to 7 F. lower in temperature than a sandy soil. 



Color. The color of the soil has an effect upon its tempera- 

 ture. A dark soil warms up more rapidly than a light one, pro- 

 vided they contain an equal quantity of water and other con- 

 ditions are equal. Schubler exposed two layers of the same soil 

 to the sun, under the same conditions, making one white by means 

 of a thin layer of magnesia, and the other black with lampblack. 

 The temperature of the blackened soil became 13 to 14 higher 

 than that of the whitened one. Lampadius has given the soil a 

 coating of coal dust an inch thick to aid in ripening melons, and 

 in Belgium and Germany it is found that the grape matures best 

 on certain soils covered with gray slate. These fragments, how- 

 ever, retain the heat through the night. Black soils often contain 

 more water than light soils, and the light soils therefore warm 

 up first. 



Organic matter, in decaying, gives off heat. This heat is 



