40 THE FERTILITY OF THE SOIL [CH. 



6 to 15 per cent, of clay, 40 to 60 per cent of the 

 silts and 20 to 50 per cent, of coarse and fine sand. 



Loams contain more plant food than sands and in 

 general have a better water supply ; they therefore 

 yield heavier crops. But the crops are often not as 

 early as those grown on sands. 



The next group, the clays, cannot be sharply 

 marked ofi* from the loams but can only be described 

 generally as sticky soils, persistently wet in winter 

 and spring, and drying to hard clods in dry weather. 

 They contain more actual clay^ and less sand than 

 the loams, but none appears to contain more than 

 50 per cent, of clay, very few contain as much as 

 40 per cent., and most "clays" contain only about 

 25 per cent, or less. In consequence of their sticki- 

 ness clays do not allow very free root development. 

 The root range being thus restricted, plants do not 

 draw on anything like the same volume of soil for 

 food and water as in the case of loams and sands. 

 Hence the clays are less fertile than these soils in 

 spite of the fact that they often actually contain 

 more food and water. Of course if the plant is in 



1 Unfortunately soil chemists use the word "clay" in two distinct 

 senses: (1) the soil or mineral as a whole, (2) the fine material less 

 than -002 mm. in diameter (-005 mm. in the United States). In past 

 years another meaning was given which does not appear to have been 

 very definite ; this survives in the statement handed solemnly down 

 through eighty years of text books that a clay soil contains "75 to 

 95 per cent, of clay." Ceramic chemists adopt a different definition. 



