312 SOIL CONDITIONS AND PLANT GROWTH 



Crista-galli), and in drier places the quaking grass {Briza 

 media) and ox-eyed daisy {Chrysanthemum Leucanthemum) are 

 among the more obvious plants on these neglected fields ; the 

 only relics of the past are the field names and the high ridges 

 or "lands" made years ago to facilitate drainage. But 

 recently marked improvement has set in. Drainage is 

 gradually being attended to, improved implements allow of 

 better cultivation : mangolds, kohlrabi, and other fodder crops 

 suitable for heavy soils have been grown, whilst additions of 

 lime and phosphates (as basic slag) have markedly improved 

 the herbage, favouring the development of white clover {Tri- 

 folium repens) and the pasture grasses, and crowding out the 

 weeds. Fig. 30A and B shows specimens of her age from some 

 of Robertson's plots at Horndon-on-the-hill, -ssex (London 

 clay). Potassic fertilisers are not usually needed. Only in 

 the dry eastern counties has the old arable cultivation survived. 



It is commonly observed that plants growing on clay 

 soils tend to have larger leaves, and to make shorter-jointed 

 growth, than plants on sandy soils. 



Many of the ecological and agricultural observations on 

 clay need revising in view of the distinction which has recently 

 been set up between the silty clays and the true clays. The 

 former owe their heaviness to the presence of much fine silt 

 which differs in colloidal properties from clay (p. 107), and is 

 not flocculated by lime, frost, etc. Indeed, no way is known 

 for ameliorating these soils and they are generally left as 

 rather poor pasture. The true clays are often indistinguish- 

 able on casual inspection, but they behave differently on 

 cultivation and respond to lime and good treatment whenever 

 it is deemed worth while to improve them. 



Sandy soils are formed of large silica particles deficient 

 in colloidal matter, and therefore possessing little power of 

 cohesion, or of retaining water or soluble salts. Hence they 

 tend to be dry, loose, and poor in soluble substances — 

 "hungry," the practical man calls them. Their behaviour 

 towards vegetation depends very largely on their position, 

 their depth, and the nature of the subsoil, these being the 



