NATURE OF PLANTS 47 



sider the nature of soils and the relation of the substances ab- 

 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 are loosely put together and are associated with a vary- 

 ing volume of water and air. From this source are derived the 

 necessary mineral or crude foods, i. e., 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. In addition there is to be considered the 

 free water which runs oft 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 feet in sand 

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

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

 heating the soil to 100° C. The mineral particles not only retain 

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

 others 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 and potassium compounds 

 are fixed and rendered insoluble by lime and magnesia. Were 

 it not for this fact these important substances would be washed 

 out of the soils by the first rain. 



21. Relation of the Root Hairs to the Soil Particles. — We are 

 now prepared to understand the significance of the structures 

 noted in the root and their relation to the physical properties of 

 the soil. In the first place we see that these microscopic root 

 hairs can readily penetrate any cavity and they are so delicate 

 and soft that they become moulded about any soil particle in 

 their line of growth (Fig. 30, B). So firmly are they attached 



