216 INTRODUCTION TO GENERAL SCIENCE 



the humus. This nitric acid acts upon some of the insoluble 

 compounds of the soil, changing them into nitrates, which are 

 readily absorbed by the plants. In the decaying humus 

 carbon dioxide is liberated, which dissolves more of the min- 

 erals, forming plant food. The humus also aids the growth 

 of the nitrogen-fixing bacteria. The study of the bacterio- 

 logical influences upon agriculture will be considered under 

 Bacteria. See Sections 155 and 173. 



A further study of humus will be taken up in the considera- 

 tion of green manures. See Section 155. 



References : 



1. 1205 : 20-21. How Humus and Subsoil are Mingled. 



2. 1601:64-68. Humus Soils. 



3. 1601 : 95-96. Chemical Constitution of Humus. 



4. 1605:95-96 The Uses of Humus. 



5. Farmers' Bulletin No. 78 : 5-7. Importance of Humus. 



6. Farmers' Bulletin No. 192:30. The Cumulative Effect of 



Barnyard Manure in Producing Humus. 



7. Farmers' Bulletin No. 245 : 11. Improving the Soil by In- 



creasing the Humus, 



a. 1404:107-108. Humus. 



6. 1602 : 42-43. Humus in the Soils. 



c. 1603 : 21. Adding Humus to the Soil. 



d. 1606:52. Effects of Humus on Soil Water. 



e. 1607:76-77. Humus. 

 /. 1608:31-32. Humus. 

 g. 1610:25. Humus. 

 h. 1611:24-25. Humus. 



i. 1612 : 286-290. Getting Humus into the Sott. 

 j. 1713:148-149. The Humus Theory. 



154. ENRICHING THE SOIL 



If the soil is lacking in any of the plant food which has been 

 mentioned, that material should be added in order to obtain 



