CH. Ill] Undeconii^osed Residues 37 



done by mixing good fibrous turf in the borders {e.g., 

 cucumber borders) so that the soil shall always remain 

 open and aerated in spite of the constant heavy water- 

 ing. After a time the fibrous roots disappear and then 

 the soil is much more likely to become sodden, covered 

 with green growths, and "sour," than it was while the 

 fibre lasted. In outdoor horticultural work it is equally 

 an advantage to have sufficient undecomposed or fibrous 

 material to keep the soil open, and afford what the 

 gardener' calls a proper root run. On heavy farm soils, 

 also, undecomposed material, such as stubble, straw, 

 long manure, is very helpful for the same reason. On 

 the other hand this material is a disadvantage on Hght 

 soils because these are already open enough especially 

 in dry seasons. Any fibrous or undecomposed plant 

 material or manure containing long straw or peat moss 

 is therefore added in autumn so that it may have a 

 good chance of being broken up before the summer 

 droughts come on. On many good light-land farms, 

 indeed, the use of these materials is reduced to a 

 minimum by a method that will be discussed later 

 (p. 113). 



This fibrous material contains many of the chemical 

 substances that occur in the plant: among them are 

 protein, cellulose, and waxes. The decompositions that 

 go on in the soil are not known in full detail, but it has 

 been found that the protein breaks down to form 

 ammonia and other substances, some of which, along 

 with the cellulose, give rise to the black mixture humus ; 

 carbon dioxide is also given off during the process. There 

 must be many products formed during these decom- 

 positions, but very httle is known of them with cer- 

 tainty. The waxes only disappear slowly; they tend to 



