THE SOIL 321 



worn. Everything is suggestive of an old river bed, and 

 such alluvium has been carried there by water. In many 

 places large areas are covered with similarly mixed materials , 

 but more angular, and whose rocks are of different kinds. 

 These have been deposited by ice-sheets and glaciers, and 

 are known as glacial drift (Fig. 213). The soil in these 

 areas is not derived from the underlying rock, but from 

 material which has been carried from a distance. Such 

 soils are therefore called transported soils, and are often 

 very complex in character and liable to vary much even in 

 short distances. 



Effect on growth of different soils. In all cases we find that 

 the roots of plants occur mainly in the dark soil. Why is 

 this ? Do they find more available food there than in the 

 subsoil ? Test this by sowing a few seeds in pots, one 

 filled with dark surface soil, a second with sand, a third 

 with subsoil, and a fourth with clay, and compare the results. 



Fig. 207 is from a photograph of such an experiment and 

 shows how differently they have fared, though all the other 

 conditions are the same. Those in the surface soil are 

 sturdy and healthy, in sand they have not grown so well, 

 while those grown in subsoil are very poor and starved, and 

 have evidently been unable to extract from it a suitable 

 amount of food ; the seeds in the wet clay failed to ger- 

 minate. 



Composition of the soil. As is shown by water-culture 

 experiments, the compounds which form plant food must be 

 soluble, and contain at least the elements oxygen, hydrogen, 

 nitrogen, sulphur, phosphorus, potassium, magnesium, cal- 

 cium, and iron. But we have seen that a plant obtains 

 carbon from the carbon dioxide of the air in sunlight, and 

 the other elements are absorbed by the roots in the form of 

 compounds such as are used in a culture-solution. Other 

 elements also occur, e. g. sodium, silicon, and chlorine, but 

 some of these are not essential to all green plants. A soil, 



1296 X 



