44 MATURE < >F S< ill. S 



sider the nature of soils and the relation of the substano 

 sorbed by the plant to the soils. 



20. The Nature of Soils.— The soil consists of minute mineral 

 particles derived from disintegrating rocks, and organic particles 

 or humus, formed from decaying plants and animals. These 

 particles arc loosely put together and are associated with a vary- 

 in-' volume of water and air. From this source are derived the 

 necessary mineral or crude foods, i. c. potassium, calcium, mag- 

 nesium, iron, nitrogen, phosphorus, sulphur, and also water. 

 These substances are distributed in the soil in a rather remark- 

 able manner. The water, for example, either fills a portion of 

 the space between the earth particles and is then known as capil- 

 lary water or it forms thin films about the earth particles, termed 

 hygroscopic water. Tn addition there is to be considered the 

 free water which runs off through the larger spaces in the soil. 

 The capillary water is more or less mobile according to the fine- 

 ness or coarseness of the soil particles and may move up or in 

 any direction towards drier soils. In this way water is drawn 

 to the dry surface of the soil from a depth of four feqt in sand 

 to six feet in clay. The hygroscopic water, however, remains 

 untouched even on the hottesl days and may only be driven off by 

 heating the soil to loo° C. The mineral particles not only retain 

 water, but some of them furnish necessary crude foods, while 

 other- are equally important because they enter into combination 

 or fix important soluble mineral substances. The nitrogen in 

 ammonia is fixed in this way by the compounds of aluminium 



in clay soils and soluble phosphoric ami potassium compounds 



xc<\ and rendered insoluble by lime and magnesia. Were 



it not for this fad these important substances WOUld be washed 



i .nt i if the soils by the firsl rain. 



21. Relation of the Root Hairs to the Soil Particles. \\ 

 now prepared to understand the significance of the structures 



note.! in the root and their relation to the physical proper! ; 



the soil. In the first plat e thai these microscopic root 



hairs can readil) penetrate an\ cavit) and tln\ are so delicate 



rind SOfl that they become moulded about an) Soil particle in 



their line of growth < Fig. .;<>. B). So firmly are the) attached 



