THE CONSTITUTION OF THE SOIL 



67 



The variations in water content follow very closely the variation in 

 the amount of organic matter present. So marked are these physical 

 effects that if i S or 20 per cent, of organic matter is present in a soil 

 the operation of other factors ceases to count for much, and the dis- 

 tinctions between sands, loams, and clays are obliterated. Thus, much 

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

 coipposition 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' on the Texture of Soils. 



5. It swells when wetted. ^ 



' Measured by the loss on ignition. 



' 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 vras drained in 1851 a pillar was driven through the 

 p eat into the underlying gault, and the top of the pillar was made flush with the surface of 



