THE NUTRITION OF THE EOOT 39 



§ 7. In what form does the root find the nutritive substances 

 in the soil? 



Every soil, representing as it does the product of disintegra- 

 tion of various rocks, consists of a mass of fragments of various 

 kinds, endowed with certain physical powers of attraction. A 

 mechanical analysis of the soil enables us to separate any soil 

 under cultivation into its coarser constituents, the skeleton or 

 framework, and finer earth, which can be washed out of it. 

 According to the size of the fragments making up the frame- 

 work, we can distinguish the coarsest constituents as coarse 

 gravel; then follow gravel of medium coarseness, and fine 

 gravel or coarse sand. The fine sand would belong to the 

 earthy portion, which shows the characteristic physical proper- 

 ties that are of importance for the growth of plants, such as 

 the power of absorbing and conducting heat, capillary attrac- 

 tion, and the power to attract and retain gases and liquids. 

 It is the varying proportions in which the various constituents 

 of the tramework are mixed with the earthy portion of the soil 

 which gives to every soil its characteristic structure. The 

 structure of the soil is important to the plant in the first place, 

 because it regulates the access of air to the roots, which require 

 air just as much as the upper portiops of the plant. Some 

 plants, however, require a large amount of air circulating 

 among their roots, while others are content with a much 

 smaller amount of air, because they need only a very small 

 quantity of the really active constituent of the air, namely, the 

 oxygen. A loose soil, in which the several particles lie lo,osely 

 packed side by side, will not only stinmlate the activity of the 

 roots more than a binding soil, in which the particles are closely 

 fixed together, but it will also, favour the processes of decay 

 and decomposition. The most favourable structure of a soil 

 for cultivation is one which is not too loose to retain the water 

 which is necessary for growth, and , yet is not sp close as to 

 have its interstices filled with water under normal climatic 

 conditions, which would prevent the access of the necessary 

 amount of air to the roots. 



Of the nutritive substances, the one of chief importance for 



