140 SOILS AND FERTILIZERS 



poor to cultivate. It grew up mainly to grass, with a very 

 few legumes, and in the course of twenty years it had gained 

 nitrogen at the rate of twenty-five pounds to the acre an- 

 nually. With the exception of about five pounds to the acre 

 that was brought down by rain, dust and the like, the accu- 

 mulation was doubtless due to the free-living germs. 



QUESTIONS 



1. Explain the difference between the directly injurious and the 

 indirectly injurious effect of soil germs on plant growth. 



2. Are the numbers of bacteria in soils rather uniform, or do 

 they vary greatly in different soils ? 



3. Describe the relation of soil bacteria to the air supply. 



4. Their relation to moisture. 



5. Their relation to temperature. 



6. Their relation to organic matter. 



7. Their relation to soil acidity. 



8. Their relation to soil fertility. 



9. Describe the cycle through which carbon passes from plant 

 to soil and back to air again. 



10. Explain the fermentation known as ammonification. 



11. Describe what is meant by nitrification. 



12. How do soils of arid and humid regions differ in respect to 

 the depths at which nitrate formation occurs ? 



13. Why does nitrate formation not take place in early spring ? 



14. Describe three fermentations by which the nitrogen of ni- 

 trates is converted into other forms. 



15. Describe the two processes by which atmospheric nitrogen 

 is fixed in the soil by germs. 



16. Describe the cycle through which nitrogen passes from the 

 plant to soil and back to plant again. 



LABORATORY EXERCISES 



Exercise I. — Test for nitrates in soil. 



Materials. — A rich garden loam, a 500 c.c. vessel for mixing 

 the soil and water, wooden stirrer, funnel and filter paper, hydrate 

 of lime, water bath, ammonium hydrate solution, evaporating dish, 

 phenoldisulphonic acid. 



