THE CONSTITUTION OF THE SOIL 



69 



of the famous Red River prairie soil of Manitoba is identical in mineral 

 composition with certain poor infertile wealden soils, but the presence 

 of 26 per cent, of organic matter completely masks the harmful effect 

 of the clay and fine silt. A similar pair of soils, owing their difference 

 in agricultural properties to their different organic matter content, 

 have been analysed by C. T. Gimingham (105) : 



TABLE XXX. EFFECT OF ORGANIC MATTER 1 ON THE TEXTURE OF SOILS. 



5. It swells when wetted 2 



6. Although the group is essentially transitional it has a certain 

 degree of permanency and only slowly disappears from the soil. 



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 indefinite 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, but unfortunately we cannot get 

 much further. Careful examination of a number of soils in their 

 vegetation relationships has shown that there must be several dis- 

 tinct types of humus, but the laboratory methods are not yet as 



1 Measured by the loss on ignition. 



2 Peat shows this phenomenon in a marked degree, indeed after heavy rainfall inade- 

 quately-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 1851 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 1911. 



