^54 
crease  the  storage  capacity  for  excessive  precipitations.  This  will 
not  only  save  the  surface  from  being  washcH  and  gullied,  but  it  will 
also  increase  the  store  of  moisture  held  by  the  soil,  which  is  of  very 
great  value  in  the  timt;  of  drought. 
It  is  important  also  for  this,  as  for  other  reasons^  that  the  soil  be 
covered  with  vegetation  as  much  as  possible  throughout  the  year, 
as  the  roots  and  organic  matter  serve  to  bind  the  grains  of  the  soil 
together^  in  some  j)arts  of  Holland  tlu*  drifting  sands  of  the  coast, 
which  shift  the  position  with  every  storm,  often  cover  uj)  valuable 
farming  lands.  Vast  areas  of  these  sands  have  been  reclaimed  and 
made  productive  by  being  covered  with  vegetation,  while  the  roots 
and  remains  of  organic  matter  hold  the  soil  grains  in  place  and 
prevent  them  from  drifting  and  covering  up  more  valuable  lands. 
Any  crop  which  requires  very  clean  culture,  as  for  example,  cotton, 
is  exhausting  to  the  land  for  the  reason  that  constant  exposure  of 
the  surface  to  the  sun  and  storms  uses  up  the  organic  matter, 
makes  the  soil  less  porous,  and  the  soil  particles  themselves  are 
more  easily  washed  away;  so  that  clean  cultivation  is  in  its 
effects  very  favourable  to  excessive  erosion. 
Another  very  effective  method,  when  |)roperly  carried  out,  to 
prevent  the  washing  of  lan.ds  is  to  underdrain  the  soil  with  tile  or 
other  drains.  'These  drains  carry  off  quite  rapidly  an  excess  of 
moisture,  so  that  much  more  of  the  rainfall  is  absorbed  by  the  soil 
and  carried  off  through  the  drains  and  less  washes  over  the  surface 
of  the  land.  Not  onlv  this,  but  a vvell-underdiained  field  is  usuallv 
dryer  and  more  porous,  and  has  a greater  capacity  for  absorbing 
excessive  rainfall  and  thus  preventing  surface  washing.  A field 
thoroughly  underdrained  with  tile  drains  will  carry  off  the  water  of 
any  ordinary  rainfall  without  any  surface  erosion.  This  method  is 
very  effective,  but  is  likewise  very  expensive,  and  cannot  be  used 
economically  in  extensive  farming  solely  for  this  purpose  of  pro- 
tecting the  land  from  washing. 
While  the  land  may  thus  be  made  more  porous  and  more  absor- 
bent of  water  through  the  increase  of  the  amount  of  organic  matter 
or  of  humus  through  the  use  of  lime  and  other  fertilizing  material, 
through  the  deepening  of  the  soil  by  gradually  increasing  the  depth 
of  cultivation  by  so  cropping  it  that  it  shall  be  covered  with  vegeta- 
tion as  much  of  the  year  as  possible,  and  by  underdraining  the 
land-still,  these  methods  may  not  be  sufficient  to  so  change  the 
chemical  and  physical  texture  of  the  soil  as  to  enable  it  to  absorb 
the  rain  as  it  falls  and  to  prevent  an  excess  of  water  washing  and 
eroding  the  surface  where  the  contour  of  the  land  is  such  as  to  pro- 
mote erosion  from  the  surface  flow  of  the  excess  of  water. 
It  will  be  necessary  in  this  case  to  pro\  ide  for  a more  uniform 
distribution  of  the  flow  over  the  surface,  and  to  prevent  any  accu- 
mulation of  water  which  would  have  the  effect  of  a torrential  stream. 
This  is  secured  in  a great  measure  by  laying  off  the  rows  accoi’ding 
to  the  contour  of  the  surface,  so  tliat  each  row  will  have  a very 
slight  iiu'line  of  not  more  than  from  i to  7 iiu'hes  in  1 00  feet,  and 
