66 AGRICULTURE 



and is most likely to occur, therefore, in the 

 subsoil, or in soil saturated with water. 



Important as are the biological and physio- 

 logical aspects of humus, its physical effects 

 upon the soil are no less important. It is 

 not too much to say that no soil can be quite 

 satisfactory for agricultural or horticultural 

 purposes that does not contain a considerable 

 proportion of humus. Its effects on sand 

 are to make it more binding, to add to its 

 absorptive properties for soluble plant food, 

 to render it more capable of taking up and 

 retaining water, and to prevent its becoming 

 overheated in bright sunshine, and chilled 

 during the night. When added to clay, 

 humus improves drainage, facilitates the 

 entrance of plant roots, diminishes the 

 tendency for the surface to become encrusted, 

 and in its decomposition sets free carbonic 

 acid gas, which attacks such substances as 

 potash compounds, and in this way liberates 

 mineral food for the use of plants. While, 

 however, a reasonable amount of humus, say 

 up to ten per cent., is entirely beneficial, 

 excess may produce undesirable results, as 

 is seen in a pronounced form in the case of 

 peaty land, which is often sour, and incapable 

 of growing satisfactorily the better class of 

 plants. 



