HUMUS 171 



It should be noted that a soil covered with grass 1 involves 

 a greater loss of moisture than if covered by trees ; and more 

 moisture is lost by transpiration from a soil covering of either 

 grass or trees than would be lost by evaporation from clean 

 open ground such as a ploughed field. But if this open area 

 be covered with a layer of humus, the loss of moisture will be 

 still further diminished. For this reason self-sown seedlings 

 or young nursery plants thrive much better where there is a 

 layer of humus than where it is absent, if other conditions be 

 equal. Then again, humus exerts a mechanical condition on 

 the soil which is beneficial to young seedlings. It renders 

 the surface soil less hard and more easy of penetration ; and 

 on clay soils it is especially valuable, as it prevents them from 

 " setting." 



In the previous chapter it was stated that a woodland 

 area will contain a maximum amount of moisture when the 

 canopy is as thin as is compatible with the retention of 

 humus. Hence, since by underplanting the canopy becomes 

 more dense, the total amount of moisture in the soil will be 

 lessened. But the exhaustion will take place from the subsoil, 

 and will hardly be noticeable in the surface soil. 



It is, however, very probable that this lessening of the 

 available supply of moisture for the overwood will cause them 

 to become less tapering, as the moisture and soil nutrients 

 they absorb will only be sufficient for the formation of broad 

 rings in the upper part of the boles, instead of, perhaps, an 

 equally broad ring all the way down the stem. 



From this it is evident that on soils too dry for underplant- 

 ing any attempt to that effect would be likely to cause the 

 overwood to become stag-headed, as well as being a failure as 

 far as the underwood itself is concerned. 



Underplanting will never of itself stimulate the increment 

 of the overwood ; but it will prevent the falling off in incre- 

 ment which would take place if the ground became covered 

 with grass and weeds. 



A consideration of the foregoing leads to the conclusion 



1 Certain woodland grasses do not transpire moisture to the same 

 extent as ordinary grasses, and therefore are not so harmful. 



