THE FOREST AS A SOIL-IMPROVER 55 
land becomes worn out, it should be planted again in 
forest, and fresh land cleared. In this way, by a 
process of rotation similar to the rotation of crops 
in agriculture, the productivity of the soil may be 
constantly increased. The forest will return to the 
surface the inorganic materials which have eked away 
in the process of cultivation. In forestry, “ rota- 
tion’ usually means the length of time from the 
planting of the forest until it is cut. In agriculture, 
the term rotation implies alternation; that is, for ex- 
ample, corn one year, oats the following spring, then 
wheat with grass and then corn again. 
The fertility of a soil depends very largely upon 
its ability to hold moisture, and upon the presence of 
sufficient oxygen in the soil. The interstices or 
spaces between its granules must not be too large nor 
too small. When the interstices are too small the 
soil becomes hard, and the movement of water is 
impeded. This condition is ameliorated by the pres- 
ence of organic matter, and by the holes which are 
formed by decaying roots. When the soil is too 
coarse and open, humus serves to fill up the inter- 
stices. The organic matter in the soil acts as a wick, 
which, by capillarity, carries water downward when 
it exists in abundance on the surface, and upward 
when it exists in great abundance below. By the 
action of organic matter the particles of soil are 
