3IO 



SOIL CONDITIONS AND PLAAT GROWTH 



Bones also give good results, but basic slag appears to be 

 without effect. Summer fallow has been observed to have a 

 bad effect on fen soils. 



In practice the>clay fen soils after their periodical claying 

 receive nothing but superphosphate : they are extraordinarily 

 fertile, commonly yielding 8 tons of potatoes, 90 bushels 

 of oats and 56 bushels of wheat per acre. The sandy fen 

 soils are less fertile because they cannot be clayed except at 

 great expense : the chief need again is for phosphate, but 

 potash also is wanted. 



The wonderful black earths of Russia, Canada, etc., are 

 probably akin to the fen soils. The black soils of the Canadian 

 prairies have been described by Shutt (265): under wheat 

 cultivation they require no fertiliser ; the similar Tchernozem 

 of Russia and Hungary also carry practically nothing but 

 wheat and receive little or no manure (85, i^^b). 



The " muck land " of the United States appears to be of 

 the same type : it is underlain by shell deposits containing 

 lime, and no structure can be detected in the organic matter : 

 it differs from peat, however, in that it responds to lime and 

 potassic fertilisers. 



Clay Soils. — Clay soils are characterised by the presence of 

 20 to 50 per cent, of "clay" and similar quantities of silt and 

 fine silt ; in consequence of this excess of fine particles the size 

 of the pores is so diminished that neither air nor water can 

 move freely. Clay soils, therefore, readily become water- 

 logged and in time of drought may not sufficiently quickly 

 supply the plant with water ; in our climate, however, they are 

 usually moist or wet. The high content of colloidal matter 

 impresses marked colloidal properties: (i) the soil shrinks on 



