CHAP. XVI . ROCK AND SAND 59 



CHAPTER XVI. KINDS OF SOIL 



In accordance with the different constitution of soil the following 

 main kinds may be distinguished : 



Rock soil Clay soil 



Sand soil Humus soil 



Lime soil Saline soil 



These are connected with one another by gradual transitions and 

 countless admixtures, so that in reality there exist innumerable varieties 

 of soils of multifarious character. As, however, the kinds of soil above 

 named have extremely different properties, and therefore necessarily 

 support plant-communities very different oecologically, their charac- 

 teristics are briefly recorded here : 



1. Rock soiL In this case it is the nature of the rock that determines 

 what vegetation can develop upon it. And in this there come into play 

 distinctions in the hardness, porosity, specific heat, and thermal conducti- 

 vity. Among the most important kinds of rock are : granite, gneiss, 

 limestone, dolomite, sandstone, slate, basalt.^ 



2. Sand soiL Sand consists of loose particles of various minerals, 

 mainly of quartz, but also of felspar, hornblende, mica, even hme (for 

 example, in coral-sand), volcanic products. The nutritive value of sand 

 varies with the chemical nature of its particles ; pure quartz-sand is 

 sterile, because particles of quartz are incapable of acting as nutriment ; 

 sands containing hme, mica, felspar, and other minerals, have a greater 

 nutritive value. Humus is formed only with difficulty in dry, loose, sandy 

 soil, because in this organic bodies are easily decomposed and oxidized 

 by the admission of air. In addition, sand, particularly quartz-sand, 

 which is the most frequent kind, has only a slight absorbent faculty in 

 regard to substances nutritious to the plant. 



Sand is loose soil, because its particles have but little cohesion, and 

 this diminishes as the particles increase in size. Atmospheric precipita- 

 tions easily percolate through sand, and with a facility that is greater the 

 larger are the particles. Sand generally contains only a small amount 

 of water ; the coarser are its grains the less water does it retain approxi- 

 mately from three to thirty per cent. ; dune-sand from Bordrup in 

 Jutland, according to Tuxen, takes up twenty-seven per cent. The 

 power of sand to raise water from the subsoil is as a rule very slight ; 

 water is usually raised at most one-third of a metre. 



Sand dries as a rule very quickly, and consequently becomes rapidly 

 and intensely heated in sunlight, but it also cools very rapidly and intensely 

 at night. The surface of a shifting sand-dune becomes covered by a 

 dry layer of sand, which becomes strongly heated in sunlight, yet though 

 this layer is of but slight depth it hinders evaporation from the subjacent 

 sand, which consequently always remains moist and cool ; this considera- 

 tion is of very great importance for the proper understanding of dune- 

 vegetation. In the deserts of Arizona, according to Livingston,^ a 

 powdery superficial layer appears to act in like manner. The difference 

 * See Section VIII. * Livingston, 1906. 



