THE PROPERTIES OF SOILS 67 



74. Soil temperature. Few seeds will germinate at a tem- 

 perature under 40 degrees F. ; most seeds require a tempera- 

 ture above 60 degrees F. Most of the beneficial soil bacteria 

 reach their maximum activity and usefulness between 80 de- 

 grees and 100 degrees F. Thus the importance of a warm soil 

 is apparent. 



The main source of a soil's warmth is the sun. Soils which 

 contain much water warm slowly, because water has a high 

 specific heat 1 and because the evaporation of water from the 

 surface of the soil requires heat. Aside from the season, the 

 water content of a soil is the most important factor which deter- 

 mines its temperature. Wet soils are said to be cold ; dry soils 

 are said to be warm. Wet soils become warm late in the spring 

 and are said to be late ; dry soils become warm early in the 

 spring and are said to be early. These terms refer to the time 

 when soils can be plowed and planted to crops. 



Other factors affecting soil temperature are the slope of the 

 land and the color of the soil. Lands sloping to the south are 

 warmer than those sloping to the north, because of the more 

 favorable exposure to the sun. Other things being equal, dark 

 soils absorb more heat than light soils. 



The methods for modifying soil temperature are mainly those 

 of controlling the water supply. The draining of cold, wet lands 

 warms them. Increasing the content of organic matter in warm, 

 dry, sandy soils, thus causing them to retain more water, tends 

 to keep them cooler. Tillage warms the soil at the surface by 

 entrapping the heat in the loose blanket of surface soil, which 

 is a poorer conductor of heat downward than is the unloosened 

 soil. The loosened seed bed prepared for spring seeding is 

 therefore warmed and the germination of the seed hastened. 



75. Life in the soil. Few people realize that the soil teems 

 with living organisms. They are microscopic in size, but they 

 are of great importance to soil fertility. The soil is an excellent 



1 A much greater number of heat units are required to raise the tempera- 

 ture of a pound of water one degree than are required to raise the temperature 

 of an equal weight of soil one degree. 



