CHAP. XIV DEPTH OF SOIL 55 



Light soil with an impermeaUc subsoil. In a moderately moist climate 

 such soils are of very variable value, according as they are sloping so that 

 water readily flows away, or are horizontal ; the former soils often sustain 

 a rich vegetation, but the latter are very marshy and useless for cultivation. 



Heavy soil with a permeable subsoil is as a rule fertile, as the excess 

 of water percolates into the subsoil. 



Heavy soil with an impermeable subsoil supports marsh-vegetation, 

 and requires draining before it can be cultivated. 



As the constitution of the subsoil often changes from place to place 

 with extreme suddenness, we see the character of the vegetation under- 

 going entire change frequently at very short distances. The slope of the 

 ground may essentially modify the significance of the subsoil, and in 

 general it greatly affects the quality of the soil. 



CHAPTER XV. NUTRIMENT IN SOIL 



The plant obtains its nutritive substances partly from air, and partly 

 from the substratum. It is therefore clear that differences in the 

 substratum (edaphic differences) must play a leading part in plant- 

 economy. Water will be discussed in Sections III and IV ; in this chapter 

 we shall deal with soil. 



Soil in co-operation with the specific activity of the roots, which 

 must be regarded as differing in different species, prepares nutriment, 

 which contains three kinds of constituents : 



1. Solid mineral particles ; 



2. Salts dissolved in water ; 



3. Humus substances. 



Soil, as was mentioned in Chapter XIV, collects nutriment in its upper 

 layers by means of absorption. 



Essential nutritive substances in soil. Some substances are indispensable 

 for the completion of the whole normal development of the plant. In the 

 Higher plants hitherto investigated, the elements required are invariably 

 only ten : oxygen, hydrogen, carbon, nitrogen, phosphorus, sulphur, 

 iron, potassium, calcium, magnesium. If one of these in chemical form 

 available to the plant be lacking, then the plant enters into a pathological 

 condition, or entirely refuses to grow. Beyond this, all plants absorb 

 various other substances that are of unknown utihty, and yet cannot 

 be regarded as devoid of significance ; for instance, when present they 

 may so act that certain essential substances are used in smaller quantities 

 than would be the case if they were absent. 



Amount of nutritive substances in soil. Not only the nature of nutritive 

 substances, but also their amount, is decisive. If a substance be present 

 in a quantity less than a certain minimum the plant will not thrive ; 

 but species vary greatly in their demands ; different species take in 

 different amounts (one of the reasons for the farmer's adoption of a 

 rotation of crops). The practical man distinguishes between poor and 

 rich soil. 



The amount of soluble salt in soil depends upon 



I. The minerals capable of being weathered present in the soil. 



