100 



HUTCHINSON'S POPULAR BOTANY 



crops will soon be im- 

 poverished, and at last 

 become permanently 

 unproductive for plants 

 of that description. 



A fielcl that is sown 

 with Wheat for a suc- 

 cession of years will at 

 length lose all its flinty 

 matter, and will then 

 be useless, not only as 

 a wheat-producing soil, 

 but also for the grow- 

 ing of all cereal grasses 

 and silica-containing 

 plants.* On the other 

 hand, the very same field 

 may abound in nutrient 

 substances perfectly 

 adapted to vegetation of 

 another kind. Farmers 

 nowadays are well ac- 

 quainted with these 

 facts, and by a carefully 

 selected succession of 

 different crops a rota- 

 tion of crops, as it is 

 called and a scientific 

 system of manuring, they provide against the otherwise inevitable exhaus- 

 tion of the soil. The well-known Norfolk or four-course rotation is a case 

 in point (figs. 125-128). This consists of root-crops, Barley (Hordeum), Clover 

 (Trifolium}, and Wheat (Triticum), which are dealt with in the following 

 manner: "The farm is broken up into four portions. The first undergoes 

 thorough tillage and is planted with Root-crops, which need especially potash 

 and lime, and having short roots, take their food near the surface, or are 

 surface feeders. Division 2 has Barley, which takes up very little lime and 

 potash, but much silica, and is also a surface feeder. Clover, in division 3, 

 takes much the same food as the root-crops, but is a subsoil feeder that 

 is, sends its roots deeply into the ground. The Wheat, in division 4, is 

 also a subsoil feeder, but, like Barley, takes up much silica." Next year, and 

 every succeeding year, the position of the crops is changed ; and thus, at 



* Perhaps this statement needs a note. It has been shown that silica is not absolutely 

 necessary for the growth of cereals; other important constituents would be exhausted long 

 before the silica e.g. nitrogenous matter, or phosphates. 



Photo by] [S. L. Bastin. 



FIG. 134. CAPE SUNDEW (Drosera capensis). 



A. native of South Africa, which has green, leaf-like footstalks, indicating that 

 it draws more nourishment from the air than the other species. 



