54 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



CHAPTER XIV. DEPTH OF THE SOIL. THE UPPER LAYERS 

 OF THE SOIL AND THE SUBSOIL 



Depth of soil, that is, the thickness of the layers of incoherent soil 

 above the solid rock is obviously of great import to plants. Great 

 distinctions in the vegetation denote a shallow soil where rock lies at a 

 very slight depth, whilst deep soil is indicated when this is not the case. 

 Depth of soil affects the temperature, supply of water, amount of nutri- 

 ment, growth of the roots, and so forth. On shallow soil vegetation 

 is more adapted to dryness and is more dependent upon climatic changes 

 than it is in deep soil ; shallow soil produces no such vigorous vegetation 

 as does similar but deep soil, and the vegetation suffers more easily at 

 seasons of drought. A transition from one formation to another may 

 be caused by depth of the soil alone ; for instance, Rikli,^ dealing with 

 Corsica, writes : ' When the soil, poor in humus, becomes still more 

 shallow and consequently drier, the open maquis and garigues gradually 

 give way to typical fell-heath.' 



In soil a distinction is made between the upper layers of soil (' the 

 soil,' in the narrower sense), and the subsoil. In the former must be 

 included the completely weathered uppermost portion of the soil, which, 

 as a rule more or less intermixed with humus, is subject to the activity 

 of plants and animals, is more influenced by light, heat, and air, and is 

 richer in nutriment, partly because of the absorbent faculty of the soil. 

 By absorbent faculty we mean the character of the soil, and particularly 

 of fine soil, in virtue of which it retains, partly by chemical attraction 

 and partly by surface tension (physical attraction) certain nutritive 

 substances, which are soluble in water, and are filtered by it in such a 

 way that they cannot be washed out by rain-water, or only with great 

 difficulty. These nutritive substances are precisely the ones that are 

 least abundant but most important : phosphoric acid, potash, ammonia ; 

 on the contrary, nitric acid, and for the main part, lime are easily washed 

 out by rain-water. Soil has a noteworthy power, that of regulating 

 the nature of the aqueous solution in itself. This solution is usually 

 very dilute, and its concentration varies according to circumstances. 

 Different kinds of soil have different absorbent faculties. Certain soils, 

 clay for instance, can even abstract nutriment from the atmosphere, in 

 that they can absorb ammonia. 



The relationship between the upper layers of soil and the subsoil 

 are very important. The depth of the upper layers of soil, the amount 

 of water in them, and their other characters, all play a part ; broadly 

 speaking, it seems that the relationship to plant-hfe is the more favourable 

 the more opposed are the characters of subsoil and soil as regards power 

 of raising water and as regards the amount of water contained. Deherain 

 established the following series : 



Light soil with a permeable subsoil is entirely dependent on climate. 

 If this be dry the soil may be extremely sterile ; in a number of places 

 in France there are on such soil coniferous forests, which transpire but 

 little. If the atmospheric precipitations be abundant or the soil be 

 irrigated, it can support tall vegetation. 



* Rikli, 1903. 



