HUMUS FORMATION. 67 



EXEECISE LIV. 



HUMUS FORMATION. 



1. Weigh off ten 100 gm. portions of sieved soil, in tum- 

 blers. 



2. Arrange thus: 



1 & 2 — nothing. 



3 & 4 — 5 grams wheat straw (ground). 



5 & 6 — 5 grams alfalfa hay (ground). 



7 & 8 — 5 grams wheat straw (ground) + 1 g™- CaCOs. 



9 & 10 — 5 grams alfalfa hay (ground) + 1 gm. CaCOa. 



3. Stir in these materials thoroughly by means of a 

 sterile spatula. 



■i. Add 12 ce. of sterile water to permit of the presence 

 of organic matter containing 70% water. 



5. Determine the moisture content of the soil employed 

 and add sufficient sterile water to bring the water content up 

 to the optimum. 



6. Cover with Petri dish covers, weigh, and record the 

 weight on the tumblers. 



7. Incubate Z-A months at 20° C. 



8. Weigh every 10-14 days and add enough sterile water 

 to keep the moisture conditions constant. 



9. At the end of the incubation period determine the 

 humus present in each tumbler by the usual method. 



10. Determine the humus present in the soil used as the 

 medium. 



11. The difference between the amount of humus at the 

 beginning and at the end of the experiment gives the 

 quantity formed from the organic matter supplied. 



