156 SOIL PHYSICS AND MANAGEMENT 



Determination of Humus. 13 Ten grains of soil are treated 

 on a filter successively with a one per cent solution of hydro- 

 chloric acid until the liine is removed, as shown by testing a 

 few cubic centimeters of the filtrate with ammonium oxalate after 

 neutralizing with ammonia. Wash the soil with distilled water to 

 remove the acid. The filter and soil are placed in a bottle or stop- 

 pered cylinder and a definite amount of a four per cent solution of 

 ammonia is added. The amount added should vary from 150 to 

 500 cubic centimeters, depending upon the organic-matter content 

 of the soil. Digest with frequent shaking for 12 hours and allow 

 to stand for 12 hours. Filter the supernatant liquid and use an 

 aliquot part of the whole for evaporation. Dry at 100 C., weigh, 

 ignite and weigh again. The loss by ignition is the humus. Cal- 

 culate the amount in the entire -sample. 



QUESTIONS 



1. Define organic matter. 



2. Distinguish between humus and organic matter. 



3. What is the source of most of the organic matter? 



4. Why are upland forest soils low in organic matter? 



5. Where are chernozem soils found? 



6. What kinds of organic matter in soils ?_ 



7. What is a "run-down" farm usually? 



8. What amount of organic matter should soils contain? 



9. What effect does moisture have on organic-matter content? 



10. Are prairies increasing or decreasing in extent? 



11. How many tons per acre of organic matter in timber soils to a depth 



of 6% inches? 



12. In the subsurface? 



13. Why are soils rich in limestone usually rich in organic matter? 



14. Why do soils of northern latitudes have more organic matter? 



15. Give the changes that organic matter undergoes in the soil. 



16. Which elements increase and which decrease in proportion? 



17. What is the origin of the coal-like materials in soils? 



18. Compare the humus of arid and humid regions in nitrogen. 



19. If the nitrogen content of a surface soil is 0.287 per cent, what per cent 



of organic matter does the soil contain ? 



20. How many tons per acre? 



21. How is organic matter distributed in the soil strata? 



22. How is the money value of organic matter to be determined? What 



factors are involved? 



23. Of what value is granulation? 



24. What effect does organic matter have on retention of water? How 



many tons of water per acre will an addition of 5 per cent of peat 

 enable the surface to hold? 



25. How does organic matter prevent puddling? 



26. How does it aid in correcting it ? 



27. What effect does it have on temperature? How? 

 2B. How does it affect biological activity? 



