FORMATION OF HUMUS IN SOIL (OPTIONAL) 79 



6. Allow the experiment to run for twelve days. 



7. Arrange the results in a table. 



TABLE 28. The Influence of Organic Substances on the Evolution of Carbon 

 Dioxid from Soil 



Exercise 7 

 Formation of Humus in Soil (Optional) 



1. Prepare a uniform sample of soil (air-dry) by passing 

 through a sieve. 



2. Weigh out 5oo-gram portions and treat as follows: 



(a) Control. 



(b) Add 2 per cent, finely ground wheat straw. 



(c) Add 2 per cent, finely ground alfalfa. 



(d) Add 2 per cent, finely ground dry manure. 



(e) Sterile, no additional treatment. 



(/) Add 2 per cent, finely ground wheat straw; sterilize. 



(g) Add 2 per cent, finely ground alfalfa; sterilize. 



(h) Add 2 per cent, finely ground dry manure; sterilize. 



3. Pint or quart Mason jars can be used as receptacles 

 for the various soils. 



