HUMUS AS A GEOGRAPHICAL AGEXCY. 261 



radiation. One might then naturally expect to find it warming 

 rapidly. But this is not the case, for the specific heat of this 

 soil is greater than that of sand or clay. Moreover, an intense 

 evaporation from the usually moist surface, prevents any con- 

 siderable rise in temperature near the surface. The heat proceeds 

 slowly downwards on account of the poor calorific conductivity 

 of the material. All these properties explain how, with normal 

 temperature variations, humus is on the average colder than sand 

 in summer and warmer in winter. 



Classification op Humus. — According to the external cir- 

 cumstances which control the production of humus, it may 

 be classified into more or less well-defined groups, 



Eremacausis products. — Humuses formed in favourable con- 

 ditions of ail-, moisture, and temperature have this in common, 

 that they disappear more or less quickly, forming carbon dioxide, 

 water, and ammonia, leaving a solid residue of the mineral 

 constituents contained in organic matter, and that they give 

 an alkaline or neutral reaction. This soil is generally termed 

 mould (Mull or Terreau). 



Agricultural mould is made from harvest remains and organic 

 material brought to the land. It decomposes quickly and does 

 not gather. 



Forest mould includes the humus of the ground (Waldmtdl) 

 and the humus of the covering {StreumuU) or leaf-mould. The 

 former comes from the roots of trees that die in the earth every 

 year. The latter is spread over the soil as a thin covering, 

 which decomposes rapidly. It is overlaid by a shallow, unstable 

 layer of detritus. This humus is composed of an intimate 

 mixture of organic constituents with the mineral elements of 

 the soil. 



Slime-moidd (Schlamm mull) is formed in well-oxygenated 

 water. Should bad conditions be ofiered, the reaction grows 

 less active and turns to putrefaction. This forms rohhumus 

 or raw humus, which gathers in thick, compact layers. 



Stepj)e raw humus is mainly due to the physical nature of 

 the soil, which prevents free access of air. It is formed in various 

 climates, in very fine grained, little permeable soil, which 

 interferes with the water-supply to the deep roots, and deter- 

 mines the exclusion of the foi'est and the predominating of the 

 steppes, prairies, and similar herbaceous formations. 



