CHEMICAL CONSTITUENTS OF THE SOIL 165 



is also possible that the lime, by combining with the peat resin, 

 causes the peat and muck to oxidize more rapidly. 



Certain investigators claim to have separated ulmic acid, crenic 

 acid, and other acids from peat. 



Importance of Humus. The functions of the organic matter 

 in the soil may be summed up briefly as follows : 



1. It contains the store of nitrogen of the soil. 



2. It furnishes nutriment for bacteria and other forms of life, 

 which aid in changing plant food so that the plant can take it up. 



3. It produces carbon dioxide and other acids in its decay, 

 which increase the solvent action of the soil water on plant food. 



4. It increases the retentiveness of sandy soils for water, and 

 binds the fine particles of clay into compound particles, so that 

 the clay has better tilth. 



5. Like lime, an abundance of humus renders a soil productive 

 even though only small quantities of plant food are present. 



Classification of Soils with Respect to Humus. Knop 1 makes 

 the following divisions, using the term humus to mean the entire 

 quantity of organic matter present : 



Per cent. 



Poor in humus o.o to 2.5 



Fair 2.5 to 5.0 



Good 5.0 to 10.0 



Rich 10.0 to 15.0 



Excess 15.0 and over 



The humus in the soil decreases from the surface. The follow- 

 ing analyses by Kosticheff of the black soil of Russia illustrates 

 this: 



Depth Percentage 



inches of humus 



i to 6 5.4 



6 to 12 4.8 



12 to 18 3.6 



18 to 24 2.6 



24 to 3c 2.6 



301036 1.9 



3 6 to42 1.3 



1 Quoted by Wollny, Zersetzung d Orgnanischen Stoffe, p. 192. 



