f 
252 
(3) -  reforestation,  or  the  planting  of  trees,  which  act 
mechanically  to  pre'vent  washing. 
(4) .  Ry  grass  and  similar  vegetation,  which  bind  the  soil  grains 
and  prevent  their  washing  away. 
'Fhe  erosion  of  a soil  is  caused  by  the  wearing  of  the  rain  and 
snow  waters  which  cannot  penetrate  into  the  soil  fast  enough  to 
he  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  gully  in  a short  space  of  time. 
d'he  extent  of  washing  to  which  the  soil  is  exposed  depends  upon 
the  quantity  of  rainfall  in  a given  time,  the  slope  or  contour  of  the 
surface,  the  texture  of  the  sod,  ti’.e  vegetative  covering  of  the  sur- 
face. and  the  kind  and  condition  of  cultivation.  A sod  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 clay  soil,  on  the  other  hand,  into  which  the 
water  cannot  percolate  with  anything  like  the  rapidity  of  the  pre- 
cipitation will  be  washed  and  gullied  by  the  torrent  of  water  which 
must  flow  over  the  surface. 
Chemical  Relatio.ns  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 
ero'^ion  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  j)Ossible.  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. 
'I'he  most  important  thing  in  the  recovery  of  waste  fields  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 
recovery  of  another  acre,  even  if  this  be  deeply  furrowed  with 
gullies.  Where  enough  organic  matter  can  be  used  as  a surface 
dressing,  this  layer  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 
