170 CLEARANCES AND UNDERPLANTING 



and establishing a crop on perfectly clean land with seedling 

 plants. 



And it may be pointed out that an additional outlay of 

 i at the beginning of a QO-year rotation will, at 4 per cent, 

 amount to 34, or at 3^ per cent, to 22, thus rendering the 

 final crop at the end of the rotation debtor to that additional 

 amount. 



Hence it will be seen how essential it is to keep the land 

 clean, if it can be effected without additional expense. 



Other benefits derived from underplanting can, for the 

 most part, be traced to the effects of the soil covering of 

 humus which results from the growth of an undercrop. Now, 

 humus prevents rapid changes in the soil temperature ; and 

 also lessens the danger from late spring frosts, both by pre- 

 venting excessive radiation of heat from the soil, and by 

 delaying active vegetation in the early spring. This delay- 

 ing of spring growth will often result in the production of 

 denser spring wood, as the weather, when active growth 

 begins, will usually be warmer. It will also have the effect 

 of reducing the proportion of spring wood to autumn (or 

 summer) wood, which, in the case of all timbers is, generally 

 speaking, a very important matter, since the smaller the 

 proportion of the spring wood, the more valuable will be the 

 timber. 



Then again, a layer of humus will tend to prolong the 

 growing season. 



Humus, by its own decomposition, affords a gradual supply 

 of plant food in an easily available state ; though if it decays 

 at a greater rate than it is formed by the annual fall of 

 leaves, soil deterioration will quickly follow. Then also, as 

 it decays, it provides a constant supply of CO 2 , which is 

 instrumental in gradually setting free hitherto insoluble 

 plant food. 



Humus also assists in the percolation of water through 

 the soil. It absorbs water like a sponge, and prevents the 

 rain water from running too quickly off the land, and retards 

 evaporation, and thus insures a more plentiful and constant 

 supply of moisture. 



