152 SOILS AND FERTILIZERS 



Sandy soils are less likely to be cold in spring than are 

 clay soils, because the former usually hold water less tena- 

 ciously. In vineyards a covering of stones on the soil 

 has been found to facilitate the warming of the soil in the 

 spring, but it is doubtful whether, in view of their other dis- 

 advantages, stones are desirable. 



Good tilth is, next to drainage, the best aid to warming soil 

 in spring, as it allows the water to pass down into the lower 

 soil and thus decreases evaporation from the surface. Har- 

 rowing in the spring produces this result, while rolling, by 

 compacting the surface, increases evaporation and cools the 

 soil. 



QUESTIONS 



1 . Describe the conditions that govern the volume of air in soils . 



2. State the two principal factors that affect the movement 

 of soil air. 



3. How does the composition of soil air differ from that of 

 atmospheric air ? 



4. What are the sources of carbon dioxide in soil air ? 



5. What are the functions of the oxygen of soil air ? 



6. What are the functions of the nitrogen of soil air ? 



7. What are the functions of the carbon dioxide of soil air? 



8. In what ways may the volume and movement of soil air 

 be controlled ? 



9. Describe the direct and the indirect effect of temperature 

 on plant growth. 



10. What are the sources of soil heat ? 



11. Describe three factors that modify soil temperature. 



12. By what means may soil temperature be controlled ? 



LABORATORY EXERCISE 



Exercise I. — Movement of soil air as influenced by texture and 

 moisture. 



Materials. — Dry sand, dry clay loam, 6" funnels, cotton, aspi- 

 rating bottles (10 liter). 



Procedure. — Place a large funnel through the cork of an aspi- 

 rating bottle, fill to the mark with water, as shown in Fig. 26. 

 Place a small piece of cotton in the bottom of the funnel and fill with 



