Plants in Relation to Man 475 



Cultivation of the soil kills weeds, conserves the moisture, improves 

 the soil's texture, and increases the circulation of soil air. 



Soil air, which may constitute half the volume of the soil, is essen- 

 tial to the growth of most kinds of plants. It is increased by the 

 action of animals, the decay of roots, the action of frost, and by cul- 

 tivation. 



The soil is not dead but full of minute living things, such as pro- 

 tozoa and bacteria. 



Nitrifying bacteria build the nitrogen of the soil air into nitrates, 

 which the plants can assimilate. An important group uses the legumes 

 as their hosts. 



Other bacteria break down the useful nitrogen compounds and 

 render the soil less fertile. 



The temperature of the soil depends on its exposure to the sun, 

 its color, character of surface, moisture content, and aeration. 



REVIEW QUESTIONS 



1. How does the old fable illustrate the value of the soil? 2. Give 

 a summary of important observations made from the digging of a 

 hole in the ground. 3. What is muck or peat? 4. How can humus 

 be removed from soil? 5. What is the average depth of good soil? 

 6. Classify soils on the basis of size of rock particles. 7. (a) What is 

 sandy loam? (6) Clay loam? 8. Classify soils in respect to their 

 origin. 9. Mention regions of alluvial soils. 10. What had glaciers 

 to do with soil formation in the northern states? 11. (a) How was 

 lacustrine soil formed? Give examples. (6) To what are they best 

 adapted ? 



12. Distinguish between arid and humid soils. 13. Describe how 

 the percentage of soil water can be determined. 14. (a) Which has 

 the greater capacity for water, sand or clay? Why? (6) How do 

 the soil particles hold water? 15. About how much water will soil 

 hold, expressed as a percentage of its dry weight? 16. How can the 

 moisture capacity of soil be changed? 17. What action is going on 

 between the soil and the water in it? 18. Compare well water and 

 soil water as to amount of dissolved material. 19. (a) What sub- 

 stances are dissolved in soil water ? (6) Of what importance are they 

 to plants? 



20. Mention the process by which the soil water enters plants. 

 21. Besides the food it brings, of what use is water in the life of a 

 plant? 22. Describe an experiment in which the process of osmosis 

 is illustrated. 23. How may we show that substances secreted by 



