TEMPERATURE 



307 



The following table gives the average soil temperature at varying 

 depths for ten years : 



Average Soil Temperature, 1905-19 J 4 10-Year Average in Rlucgrass Sod 11 



It will be noted that the highest average temperature to a 

 depth of nine inches is readied in 'July, wbile for greater depths the 

 highest is reached in August. This is due to the slow conductivity 

 of the soil. 



!>. Tillage. In general, tillage has two effects upon soils as 

 regards temperature. It increases evaporation at first, but when the 

 surface becomes dry this layer acts as a mulch, preventing the moist- 

 ure from coming to the surface when the heat is used in evaporating 

 it. Tillage loosens the soil, making it a poor conductor of heat. 

 This concentrates the heat in the surface two or three inches of 

 soil, and gives better conditions for germination early in the spring. 

 Later in the season, when the untilled soil has become somewhat 

 dry, the conditions are reversed, and the tilled soil is cooler than the 

 untilled. 



QUESTIONS 



1. Why is a knowledge of the functions of heat and its control important? 

 '2. Illustrate the amount of heat nt-eived from the sun. 



3. Why do high altitudes receive more heat from the sun than low ones? 



Why are high altitudes colder? 



4. After the manure hecomes thoroughly decomposed and mixed with the 



soil, what ell'ect will it have on temperature? 



5. Why should water vapor raise the temperature of soil material more 



than liquid water? 

 (5. What effect does color have on radiation of heat? 



7. Why is conduction of heat downward into soil so slow? 



8. Why is slow germination of seeds undesirable? 



!). Is the temperature of the soil usually at the optimum, as shown in the 

 table on page 2fl8, when the seeds are planted? 



10. What part does osmosis play in germination? 



11. How does temperature affect it? 



12. What effect does color of soil have on a cloudy day? 



13. How does the specific heat of soils compare with other substances? 



(See tables, page 290.) 



