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(o). By reforestation, or the planting of trees, which act 
mechanically to prevent washing. 
(4). By grass and similar vegetation, which bind the soil grains 
and prevent their washing away. 
The erosion of a soil is caused by the wearing of the rain and 
snow waters which cannot penetrate into the soil fast enough to 
be carried away by underdrainage, and which, by reason of the 
slope or contour of the land, run off over the surface carrying along 
particles of sand and clay. When this water accumulates in a de- 
pression in the fields the force of the torrent may be sufficient to 
cut out a great gullv in a short space of time. 
The extent of washing to which the soil is exposed depends upon 
the quantity of rainfall in a given time, the slope o: - contour of the 
surface, the texture of the soil, tin- vegetative covering of the sur- 
face. and the kind and condition of cultivation. A soil composed 
chiefly of moderately coarse grains of sand, and having good under- 
drainage, will absorb the heaviest rainfall without much danger of 
surface erosion. A day soil, on the other hand, into which the 
water cannot percolate with any tiling like the rapidity of the pre- 
cipitation will be washed and gullied by the torrent of water which 
mus f flow over the surface. 
Chemical Relations of the soil to surface washing. 
It has been repeatedly shown by experiments and by the ex- 
perience of farmers that a soil, as a rule absorbs water more readily 
as the content of organic matter and humus increases. Surface 
erosion can, therefore, be largely prevented by such a system of 
cultivation and cropping as will introduce as large a quantity of 
organic matter into the soil as possible. A very old method of 
recovering washed and gullied lands is to place straw in the furrows 
while ploughing, the straw not only acting mechanically to hold the 
soil in place and prevent surface erosion, but also in a very efficient 
way to increase the quantity of humus, thus making the soil hold 
large quantities of water which otherwise would have passed off 
over the surface. In this simple way fields which have been badly 
washed and gullied and entirely abandoned may be recovered and 
made highly productive. 
The most important tiling in the recovery of waste Helds is the 
incorporation of organic matter of some kind in the soil, pea vines, 
stubble, briers, or leaves from the forest may be used as a source 
of the organic matter. The straw from one acre of land which has 
been recovered, as mentioned above, will be sufficient to start the 
recoverv of another acre, even if this be deeply furrowed with 
gullies/ Where enough organic matter can be used as a surface 
dressing, this Inver helps greatly to retain water and to make the 
under lying soil more absorbent. 
As soon as a sufficient supply of humus has been accumulated 
and the lands are brought up to an adequate condition of fertility, 
clover or grass should be seeded, if the land is at all suited to these 
