INFLUENCE OF THE QUALITY OF THE SOIL 439 



or when a number of different plants are sown closely together. Various 

 conditions decide which plants will survive, and a change in the quality or 

 quantity of the nutriment supplied or in other external factors may cause 

 a different result to be produced. Thus in a soil kept excessively moist 

 a particular type of plant may conquer, while the physical properties of the 

 soil, such as its texture, consistency, depth, &c., may exercise a decided 

 influence upon the struggle for existence, and a very trifling difference 

 may suffice to change the course of events and enable a plant to survive 

 which was less adapted to the previous conditions. 



It is possible indeed that a plant may be suppressed in a rich soil, 

 whereas in a soil deficient in one or more of the essential ash constituents it 

 may gain the upper hand, either because of its more modest requirements or 

 because by means of a more highly developed root-system it is able to 

 exhaust the soil, and especially the deeper layers, more completely. 

 Further, plants which can assimilate free nitrogen are at a distinct 

 advantage in a soil poor in combined nitrogen. Again, Glanx, Salsola, 

 and other saline plants grow perfectly well upon soil free from salt, although 

 in nature the competition with other plants restricts them to a saline habitat, 

 where their growth is perhaps somewhat more luxuriant than in an ordinary 

 soil and where most plants are unable to compete with' them. The presence 

 of a certain amount of salt seems to be an essential condition for the 

 existence of many marine algae (Sect. 73). 



Similar relationships are responsible for the specific floras of siliceous 

 and calcareous soils, and when these are in close proximity the transition 

 from the one flora to the other may be extremely abrupt, although 

 ubiquitous plants may flourish in both soils. The old supposition that these 

 differences were due to the amounts of calcium or of silicon required by 

 the plants in question is obviously incorrect, and, as a matter of fact, the 

 former is essential to all Phanerogams, and may be found in greatest 

 abundance in the ash of plants which prefer a sandy soil. Moreover, 

 almost all plants characteristic of sandy soils may be readily grown in 

 calcareous humus and vice versa, while the various other factors which 

 regulate distribution may bring it about that plants typical of calcareous 

 regions may be found growing wild on sandy soils 1 . Hence, when the 

 addition of chalk causes certain plants to be driven out from a given 

 area -, it does not follow that they are unable to grow upon the changed 

 soil. The presence of chalk does, however, appear to act injuriously upon 

 certain plants, and it is perhaps on this account that mosses which, grow 

 upon siliceous rocks may disappear when continually sprinkled with chalky 



1 Cf. Nageli, Bot. Mitth., 1866, Bel. n, p. 159; Drude, Pflanzengeographie, 1890, p, 49, 



2 Schulz-Fleeth, Der rationelle Ackerbau, 1856, p. 201. 



