THE FOREST. 17 



generally very beneficial in retarding evaporation from the 

 surface soil, in retaining the snow in the spring, and in kill- 

 ing out grass and weeds. 



Forest Floor is a term used to indicate the mulch on the 

 ground in forests. This is made up of the fallen twigs and 

 leaves which remain on the ground, where they slowly 

 decay and form a cover of rich mould or humus. This 

 protective covering serves a most useful purpose; it per- 

 mits the rain and snow waters to penetrate the soil without 

 at the same time making it too compact, thus keeping the 

 soil granular so that the air can enter, and in the best con- 

 dition for conducting water, while at the same time it pre- 

 vents washing away of the land and too rapid or excessive 

 evaporation from the surface; the humus is also an active 

 agent in aiding decomposition of the mineral substances in 

 the soil. 



Subsoil. Tree-growth is less dependent on the condition 

 of the surface soil and more dependent on the subsoil than 

 is the growth of agricultural crops. For instance, in the 

 case of drifting sand overlying a moist subsoil, it has been 

 found that where pains are" taken to get the young trees 

 started they will often do well although such land is poorly 

 adapted to agricultural crops. There are many acres of 

 land in our Northern States that have such conditions, and 

 they should seldom be entirely cleared of trees. 



Washing of Soils. The soils most likely to wash badly 

 are those that are fine-grained without much adhesive 

 power, such as fine sand and some kinds of clays. When, 

 however, such soils have a forest growth on them, they are 

 protected from washing by the forest floor, tree roots, and 

 the humus in the soil. Soil which contains large quantities 

 of humus does not wash much, since the particles of or- 

 ganic matter bind it together; thus we find that newly 

 cleared timber land which contains large amounts of humus 

 may not wash much for a number of years after the clear- 



