THE COMPOSITION OF THE SOIL 133 



5. It swells when wetted^ 



6. Although the group is essentially transitional it has 

 a certain degree of permanency and only slowly disappears 

 from the soil. It disappears more rapidly from chalky and 

 sandy soils than from loams and clays. 



These properties greatly enhance the fertility of the soil, 

 and in most schemes of husbandry definite arrangements are 

 made to keep up or even increase the supply of organic matter, 

 while in forests the removal of leaves and other decomposable 

 material has led to such bad effects that in all state forests of 

 France, Belgium, Germany, etc., the practice is absolutely 

 forbidden. 



The group of substances possessing these properties is 

 generally called " humus," and so long as the word is used in 

 a collective sense as a convenient label it may be retained. 

 But the practice has been responsible for a good deal of loose 

 thinking, because it obscures the fact that the group is an in- 

 definite and complex mixture, and gives instead the impression 

 that it is a single definite substance. 



From these half-dozen general properties we may infer that 

 humus is a brown, slowly oxidisable colloid. Careful ex- 

 amination of a number of soils in their vegetation relationships 

 has shown that there must be several distinct types of humus 

 but the laboratory methods are not yet as sensitive as the 

 growing plant and fail to explain some of the differences ob- 

 served by ecologists. We have to look to field observations 

 for the facts on which to base a scheme of classification. 



Peats. — Most of the recorded investigations are on peats 



^ Peat shows this phenomenon in a marked degree ; indeed, after heavy rain- 

 fall inadequately-drained peat bogs may swell so much as to overflow into valleys 

 with disastrous results. After drainage, however, drying and shrinkage set in, 

 followed by a slow but steady erosion as air penetrates into the newly-formed 

 spaces and starts the oxidation processes. When Whittlesey Mere was drained 

 in 185 1 a pillar was driven through the peat into the underlying gault, and the top 

 of the pillar was made flush with the surface of the soil. So great has been the 

 subsequent shrinkage that over 10 feet of the pillar is now out of the ground 

 and the process has not yet reached its limit, for a perceptible shrinkage took 

 place during the dry season of 191 1. 



