304 



SOIL PHYSICS AND MANAGEMENT 



Time Required and the \'umhcr of Plants that Came Up in the Soils of Different 

 Colors. One hundred Seeds Were Planted in Each ll 



With black the absorption is almost complete. The soils of what- 

 ever color tend to cool to the temperature of the surrounding 

 atmosphere during the night or in cloudy weather. The table on 

 page 302 shows that the lowest temperatures of the dark-colored 

 sands were not as low as the light-colored ones. Color has little 

 influence in very wet soils since evaporation is a greater factor in 

 lowering temperature than color is in raising it. 



6. Latitude or Angle of the Sun's Rays. All flat areas of 

 the earth's surface have the same number of hours of possible sun- 

 shine annually without regard to location on the earth. The effect 



Vertical 



Fia. 135. Showing the comparative areas covered by the sun's rays when vertical, 30, 60, 

 and 80 degrees from the vertical. Compare AB, AC, AD, and AE. 



of the rays in warming the soil depends upon the angle at which 

 they strike (Fig. 135). If a sunbeam striking the earth's surface 

 perpendicularly covers an area of 1, when this same beam strikes 

 at an angle of 30 degrees from the vertical, it will cover an area of 

 1.175; at 60 degrees it will cover an area of 2, and at RO degrees 

 an area of f>. The heat will be spread over a larger area the greater 

 the distance from the vertical, and the effect on temperature would 

 be inversely as the angle. The atmosphere absorbs some heat. The 



