:!8 I,(){'.\I,ITV AND FOltKST VKCxETATION. 



luiiicTiil in;itt(.T coiilaiiit'd in llu; liiuims (ashes) ; secuiully, ii 

 lurj^o (juanlity of carbon dioxide is produced, wliicli accelerates 

 the decomposition of the mineral part of the soil. Final!}', 

 humic acid renders many important substances, such as 

 potash, magnesia, lime, etc., soluble in water, so that they 

 become available for the plant. 



There are, however, different kinds of humus : — Mild humus, 

 or forest humus is formed, if air and water act in proper 

 proportion upon fallen leaves, moss, etc. J>ii/ imntJd is 

 formed by the action of an excessive supply of air, or rather 

 absence of moisture, on certain plants, such as heather. 

 Acid humus is the result of deeomposition, if there is an excess 

 of moisture and a deficiency of air in the soil. Only mild 

 forest humus acts altogether favourably upon forest vegetation. 



C. ]Valn: 



Water is the most important component part of the soil, as 

 plant life is impossible without a certain (piantity of moisture. 

 It affects vegetation principall}" in the following manner : — 



{(I.) It assists in the decomposition of the rocks. 



(/>.) It assists in the formation of humus, and regulates 

 botli the admission of air into the soil and its temperature. 



(r.) It is an important agent in the process of nourishing 

 and shaping the plant. More; especially' it carries through 

 the roots the mineral substances from the soil into the 

 plant. 



However favourably a certain degree of moisture in the soil 

 may act, an excess of water, especially if stagnant, is always 

 injurious; it reduces the activity of the soil (by driving out 

 air), lowers the temperature, increases danger from frost, and 

 is lial)le to render the soil acid. 



The soil receives water from one or more of tlu^ following 

 sources : — 



(1.) From the atmosphere, as dew, rain, snow, hail, or as 

 vapour condensed by the hygroscopic action of the soil. 



(2.) From ground water resting in the subsoil. 



