i64 CHEMICAL COMPOSITION OF THE SOIL [chap. 



must be dressed frequently and heavily if they are to 

 be made fertile. From the chemical point of view, they 

 often show certain special deficiencies ; very generally 

 they are lacking in carbonate of lime, with the result 

 that turnips and similar cruciferous crops are found 

 to suffer from finger-and-toe or club root. Lacking 

 clay, they are also apt to be deficient in potash, and 

 crops like mangolds, potatoes, clover, and tobacco, 

 which require plenty of potash, should always receive 

 potash manures on sandy soils. All sandy soils, from 

 their warmth and friability, are apt to be very weedy ; 

 the characteristic weeds are often those associated with 

 lack of lime rather than with the dryness of the soil. 

 Good examples of this type of weed are the Spurreys 

 (^Spergula arvensis and Spergularia rubra), Sheep's Sorrel 

 {Rumex acetosella), and Corn Marigold {Chrysanthemum 

 segetum), all common on light sandy soils, but indicating 

 also great lack of lime if not actual acidity. The 

 Knawels {Scleranthus annuus 2t.v\d perennis), Knot Grass 

 (^Polygonum aviculare), and some of the wild Poppies 

 {Papaver dubiuni) and the small Bindweed {Convolvulus 

 arvensis'), are among the most troublesome weeds of 

 sandy arable land ; sometimes also a form of Bent Grass 

 {Agrostis alba) is very abundant. On the grass land 

 various tufted species, Cock's-foot {Dactylis glomerata), 

 the Oat Grasses {Arrhenatherum avenaceum, Avena 

 flavescens), and the Soft Brome {Bromus mollis), are 

 characteristic. Leguminous plants, except Bird's-foot 

 Trefoil {Lotus comiculatus), and more occasionally some 

 of the Vetches like Vicia cracca, are not abundant; 

 though various weeds — Hardhead {Centaurea Cyanus), 

 Buttercup {Ranunculus bulbosus). Rattle {Rhinanthus 

 Christa-galli) and Silver Weed {Potentilla anserina) are 

 common. On the wastes, Gorse ( Ulexeuropceus and nanus) 

 Broom {Cytisus scoparius), Heather and the Heaths, the 



