CHARACTERISTICS OF DIFFERENT SOILS 213 



living on water-logged peaty land or on salt marshes. By 

 reducing transpiration they reduce the intake of water, which is 

 necessary when the soil water contains injurious substances, as 

 it does in the peat bogs and the salt marshes. 



Keeping these general principles in mind, we may now review 

 briefly the characteristics of some of the main soil types and 

 the vegetation associated with them. 



CLAY SOILS. There exist wide differences in structure between 

 the various soils that are commonly called clays. A soil which 

 is in the main composed of very fine sand without any admix- 

 ture of coarser particles, bound together by only 10 to 15 per 

 cent, of what may be termed true clay, will be so sticky and 

 wet that it will be regarded as a clay ; yet under cultivation it 

 will behave in many respects differently from the heavy soils 

 which on examination show 30 to 50 per cent, of true clay. 

 The most pronounced clay soils occur in the Midlands and south 

 and east of England ; in these districts the Kimmeridge, Oxford, 

 London and Weald clay formations give rise to extremely heavy 

 soils ; whereas in regions of higher rainfall the soils become so 

 much more washed free of their finest particles that their texture 

 is distinctly lighter. As clay soils are distinguished both by their 

 power of retaining water and their imperviousness to percolation, 

 it follows that they are generally wet throughout the winter 

 months, when the rainfall is greater than the evaporation. As 

 the spring advances such soils are slow to warm up because of 

 the water with which they are laden ; for not only does water 

 require more heat than soil to raise its temperature by a given 

 amount, but also it is always withdrawing heat from the land as 

 it evaporates. To evaporate a pound of water requires more than 

 thirty times the heat necessary to warm it up from freezing-point to 

 summer temperature, and even this latter amount is five times as 

 great as would be wanted to heat a pound of soil to the same 

 extent. In consequence, clay soils are cold and tend also to cool 

 the air in contact with them, and the less the water is able to 

 get away from them the cooler they remain ; drainage therefore 

 warms a clay soil by freeing it of water more rapidly and keeping 

 down the level of permanent w r ater, thus reducing the capillary 



