50 PROPERTIES OF SOILS 



plant. So we find very simple plants, as the lichens and certain 

 mosses, occupying such primitive soils. They are able to live 

 upon such soils because they are small and slow growing and 

 therefore demand meager amounts of the crude foods. Note 

 the character of the first vegetation that, appears on the slopes 

 of mountains below the solid rock. These simple plants, how- 

 ever, are very important factors in effecting further decomposition 

 of the minerals in the soil. Through the chemical activity of 

 their absorbing organs and especially through the decay of the 

 dead parts of these, plants new compounds are formed so that 

 more elaborate plants, as certain ferns, are now able to live upon 

 the soil. These latter plants in turn carry on the work until 

 the rock material is sufficiently changed to make a good soil. 

 Do not imagine that the work of soil formation stops here. You 

 are to think of every growing plant performing the same work 

 as noted in the lichens and mosses. 



The soil is one of the most remarkable things in this world of 

 ours and we must consider two of its more important features. 

 The first of its important characteristics is its power to retain 

 moisture. Water exists in the soil in three forms, — as hygro- 

 scopic, as £aj3illary and as gravitational water. The first form 

 is the water that condenses from the atmosphere as extremely 

 delicate films about the air-dry soil particles. This water is not 

 available to the plant because it is held so firmly by the soil 

 particles. It comprises only a small per cent, of the total water 

 in the soil. The capillary water also exists as films about the 

 soil particles but it is not retained so tenaciously and it is this 

 water that furnishes the chief supply to land plants. Capillary 

 water may amount to lo per cent, of the dry weight of coarse 

 sand up to 25 per cent, in clay soils. Gravitational water is not 

 retained by the soil particles and it is therefore free to run down 

 through the pores and spaces in the soil under the control of 

 gravity. This water in the upper portions of the soil is not of 

 value to the majority of plants because it fills the pore spaces in 

 the soil and so deprives the roots of the air which is necessary 

 to their existence. When gravitational water exists a few feet 

 below the surface it often becomes of great value as a source of 

 supply for the capillary water. 



