140 



Humus: Nature and Formation 



ducing special forms of humus, such as composts, forest htter, or 

 peat. Addition of sufficient inorganic fertiHzer will also result in 

 an increase in the amount of plant residues, which will yield greater 

 amounts of humus to replace, partly at least, the loss resulting from 

 cultivation. 



Shutt found that, on cultivating a virgin prairie soil, there was a 

 loss of over 100 pounds of nitrogen per acre annually for 22 years; 



7.0 



5.0 



3.0 o 



1.0 



16 



20 



4 8 12 



Temperature, °C 



Fig. 62. Schematic representation of the widening C:N ratio of soil humus with 

 decreasing temperature ( from Jenny ) . 



only one-third of the loss was accounted for by the crop grown. 

 About 25 per cent of the total organic matter of the soil was lost as 

 a result of cultivation during this time. Lipman and Blair also 

 found an annual loss, for 20 years, of 70-100 pounds of nitrogen 

 from a heavily manured soil. This loss could not be accounted for 

 in the crop, but could be accounted for largely by the nitrogen in 

 the drainage water. Some losses may also occur in the gaseous forms 

 of nitrogen. 



In dry-land farming, the most important soil problem, aside from 

 that of water, is maintenance of organic matter. Russell calculated 

 the loss of organic matter under dry-land farming to be 6.5 per cent 

 during the first 3-7 years under cultivation, 12.4 per cent in 8-15 

 years, 26.8 per cent in 17-30 years, and 28.0 per cent in 45-60 years. 

 The depletion of soil organic matter leads to erosion by wind and 



