CHAPTER VIII. 



THE SOIL IN RELATION TO PLANT GROWTH. 



WE are now in a position to summarise the effect of the vari- 

 ous soil conditions on the growth of plants. The soil serves 

 several functions ; it affords a more or less continuous supply 

 of food and water, it regulates the temperature and provides 

 anchorage for the roots without interfering too much with the 

 air supply. The extent to which any of these things is done 

 depends on the nature of the particles and, therefore, varies 

 from soil to soil. As the requirements of plants also vary in 

 degree, the different soil types have come to possess their own 

 flora made up of those plants that tolerate the conditions 

 better than other possible competitors, and on cultivation they 

 show different agricultural characteristics. And since the 

 natural flora is profoundly affected by competition, a small 

 factor operating for a sufficient length of time may produce 

 extraordinary differences in vegetation, as is well seen on the 

 Rothamsted grass plots. 



At the same time the plant is not rigidly unalterable but 

 possesses considerable power of adaptation to its surroundings : 

 in rooting depth, size of leaves, glaucous characters, etc. 



In discussing the relationship between plant growth and 

 the soil it is necessary to take into account not only the 

 intrinsic properties of the soil due to the nature of its con- 

 stituent parts, but the extrinsic properties impressed by topo- 

 graphical and climatic factors. A certain indefiniteness thus 

 becomes unavoidable, because none of the latter can be ex- 

 pressed in exact measurements. This point is well illustrated 

 by the water supply. The amount of water in the soil at 



301 



