SOIL MAKING. 



3 



no agricultural value : weathering generally takes ages rather than 

 years and is of little interest to the agriculturist or horticulturist. 

 The soil particles change, but very, very slowly. There are still 

 enormous differences between the grey soils formed in the Lias period 

 and the red soils formed in Triassic times : very much greater than 

 the differences between the particles of the top 9 inches of a natural 

 soil exposed for thousands of years to the weather, and those of the 

 lower depths which have lain protected from the weather : indeed, 

 one of the most striking features of modern soil work is the small 

 difference in mineral matter between surface and subsoil. As the 

 particles were left when they were deposited from sea or wind currents, 

 so in the main they have remained, and seem likely to remain for any 

 period of time that interests us. In the south the process of soil forma- 

 tion consists mainly in the washing away of the cement that has held 

 the rock particles together : the particles then tumble out from the 

 position in which they were consolidated millions of years ago. Adj oin- 

 ing formations still show differences not much less than might have 

 been seen in those days. In the north and west there are great areas 

 derived from igneous or very old rocks which are not agglomerated 

 in this way : soil formation here has proceeded by the chipping off 

 of fragments. This process depends very much on climatic factors ; 

 hence there is some tendency to uniformity of soil type in a given 

 district, even when different kinds of rock occur. This is shown below : 



Soil Types in N. Wales and S.E. England. 





S.E. England. 

 Soil types widely different. 



N. Wales. 

 Soil types tend to uniformity. 



London 

 Clay 



Thanet 

 Beds. 



Pre-Cambrian, Cambrian, Ordovician, 

 Silurian. 







Anglesey. 



Paleozoic. 



Sand. 



Fine gravel. 



04 





8-3 



IO-6 



51 



Coarse sand 



08 



16-9 



180 



80 



500 



Fine sand . 



65 



573 



21*2 



I 4 -I 



156 



Silt .... 



15-8 



8-2 



13-6 



141 



96 



Fine silt 



163 



39 



i8- 7 



28- 8 



91 



Clay .... 



4°'5 



60 



5-o 



6-6 



2-4 



The horticulturist and agriculturist are both more interested in 

 the size of the particles than in their composition, because it is size of 

 particles that determines whether a soil is a sand, a clay, or a loam. 

 Typical examples have the following composition : 





Sandy Soil. 



Loam. 



Clay Soil. 



Coarse and fine sand • 



65 



30 



IO 



Coarse and fine silt . 



20 



45 



50 



Clay . J 



5 



15 



30 



