THE SOIL IN RELATION TO PLANT GROWTH 311 



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) the water, however, moves only slowly, and it 

 is not uncommon to find a fairly sharp line of demarcation 

 between moist and dry soil, and for land to crack badly 

 within a few feet of a ditch full of water ; (4) the soil readily 

 absorbs certain soluble salts and organic substances. In 

 addition the special clay properties are shown : plasticity and 

 adhesiveness when wet, and a tendency to form very hard 

 clods when dry. All these properties are much modified by 

 calcium carbonate and intensified by alkalis ; the chalky 

 boulder clay is usually fertile : on the other hand, 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. 

 Originally covered with oak forest and hazel undergrowth 

 they were early reclaimed for agriculture purposes by draining, 

 applications of lime,^ 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 enclosure, great 

 areas came into cultivation so that now only little unreclaimed 

 clay remains, 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 ccespitosa, 

 " bent " grass {^Agrostis vulgaris), yellow rattle {Rhinanthus 



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



