124 SOIL CONDITIONS AND PLANT GROWTH 



grow well and many other crops can be raised. The black soils of the 

 Canadian prairies have been described by Shutt (264) : 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. 



Clay Soils. — Clay soils are characterised by the presence of 20-50 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 be- 

 come waterlogged and in time of drought may not sufficiently quickly 

 supply the plant with water ; in our climate, however, tl\py are usually 

 moist or wet. The high content of colloidal matter impresses marked 

 colloidal properties: (i) the soil shrinks on drying and forms large 

 gaping cracks which may be several inches wide and more than a foot 

 deep ; (2) it absorbs much water, a good deal still being held even when 

 the soil appears to be dry ; (3) it readily absorbs soluble salts, or parts 

 of them, and organic substances. In addition the special clay proper- 

 ties are shown : plasticity and adhesiveness when wet, and a tendency to 

 form very hard clods when dry. All these properties are much modi- 

 fied by calcium carbonate and intensified by alkalies ; liquid manure 

 (which contains ammonium carbonate) and nitrate of soda (which gives 

 rise to sodium carbonate in the soil) are both to be avoided. 



Clay soils have had rather a chequered agricultural history. Origin- 

 ally covered with oak forest and hazel undergrowth they were early re- 

 claimed for agricultural purposes by draining, applications of lime,i and, 

 later, of ground bones. Wheat and beans were the great clay crops, 

 and in the early part of the last century, under the combined influence 

 of high prices, large drainage schemes and artificial stimulus to enclo- 

 sure, great areas came into cultivation so that now only little unreclaimed 

 clay j-emains, excepting where the forest was preserved for hunting. 

 Crops grew well but ripened late ; a wet harvest was a terrible calamity. 

 Bare fallowing was always necessary once in four years and any of 

 the intervening years might, if wet, be lost by the difficulty of getting 

 on the land to sow the crop. When the price of wheat fell in 

 the 'eighties many of these soils went out of cultivation and became 

 covered with a mixed growth of grass and weed, which was grazed by 

 stock and gradually deteriorated as the old drains choked up and the 

 land became more and more waterlogged. Aira aespitosa, "bent" 

 grass {Agrostis vulgaris), yellow rattle {Rhinanthus Crista-galli), and 

 in dryer places the quaking grass {Briza media) and ox-eyed daisy 



' E.g. see Gervase Markham, Inrichment of the Weald of Kent, 1683. 



