﻿PLANT 
  INDUSTRIES 
  325 
  

  

  the 
  average 
  yield 
  : 
  and 
  in 
  the 
  third 
  period 
  of 
  twenty 
  years 
  

   the 
  average 
  yield 
  was 
  36.4 
  bushels 
  per 
  acre. 
  In 
  the 
  second 
  

   twenty 
  -year 
  period 
  one 
  year 
  of 
  general 
  wheat 
  failure 
  materially 
  

   reduced 
  the 
  average 
  for 
  that 
  period. 
  

  

  The 
  benefits 
  that 
  are 
  derived 
  in 
  plant 
  industries 
  from 
  rota- 
  

   tion 
  of 
  crops 
  are 
  universally 
  recognized 
  by 
  scientific 
  students 
  

   of 
  the 
  matter, 
  but 
  the 
  explanation 
  of 
  the 
  results 
  secured 
  fur- 
  

   nishes 
  material 
  for 
  active 
  discussion. 
  Within 
  the 
  last 
  decade 
  

   it 
  has 
  been 
  shown 
  1 
  that 
  at 
  least 
  some 
  cereals 
  excrete 
  and 
  leave 
  

   in 
  the 
  soil 
  substances 
  that 
  are 
  injurious 
  to 
  the 
  kinds 
  of 
  plants 
  

   which 
  produced 
  them. 
  These 
  plant 
  poisons 
  may 
  be 
  the 
  cause 
  

   of 
  loss 
  of 
  fertility 
  of 
  soil, 
  or 
  loss 
  of 
  fertility 
  may 
  be 
  due 
  to 
  the 
  

   fact 
  that 
  the 
  soil 
  has 
  been 
  exhausted 
  of 
  the 
  available 
  materials 
  

   needed 
  in 
  the 
  growth 
  of 
  plants. 
  Soil 
  replenishment 
  through 
  

   use 
  of 
  fertilizers 
  may 
  possibly 
  bring 
  in 
  a 
  new 
  supply 
  of 
  avail- 
  

   able 
  materials, 
  may 
  neutralize 
  or 
  counteract 
  the 
  injurious 
  excre- 
  

   tions 
  within 
  the 
  soil, 
  or 
  may 
  merely 
  change 
  the 
  physical 
  nature 
  

   of 
  the 
  soil 
  so 
  as 
  to 
  make 
  it 
  more 
  favorable 
  to 
  plants. 
  What- 
  

   ever 
  the 
  scientific 
  explanation, 
  the 
  great 
  value 
  of 
  natural 
  fer- 
  

   tilizers 
  and 
  of 
  rotation 
  of 
  crops 
  in 
  replenishing 
  soils 
  is 
  evident 
  

   to 
  any 
  practical 
  student 
  of 
  soils. 
  

  

  306. 
  Loss 
  of 
  soils. 
  In 
  some 
  places 
  the 
  chief 
  danger 
  is 
  not 
  

   primarily 
  that 
  of 
  loss 
  of 
  fertility, 
  but 
  loss 
  of 
  the 
  soil 
  itself. 
  It 
  

   may 
  be 
  washed 
  or 
  blown 
  away 
  or 
  removed 
  by 
  fire. 
  If 
  a 
  pane 
  

   of 
  glass 
  that 
  has 
  been 
  moistened 
  with 
  oil 
  is 
  exposed 
  for 
  a 
  time 
  

   to 
  the 
  wind 
  on 
  a 
  dry 
  day, 
  and 
  then 
  examined 
  with 
  a 
  strong 
  

   magnifying 
  glass, 
  it 
  will 
  furnish 
  a 
  good 
  demonstration 
  of 
  

   the 
  dust-carrying 
  power 
  of 
  moving 
  air. 
  Windowpanes 
  in 
  the 
  

   houses 
  near 
  the 
  end 
  of 
  Cape 
  Cod 
  finally 
  become 
  translucent, 
  

   like 
  ground 
  glass, 
  from 
  the 
  action 
  of 
  sand 
  driven 
  by 
  the 
  wind. 
  

   When 
  cultivated 
  fields 
  become 
  dry, 
  the 
  wind 
  may 
  carry 
  away 
  

   large 
  quantities 
  of 
  soil. 
  

  

  1 
  0. 
  Schreiner, 
  and 
  H. 
  S. 
  Reed, 
  " 
  Some 
  Factors 
  influencing 
  Soil 
  Fertil- 
  

   ity," 
  Bulletin 
  40, 
  Bureau 
  of 
  Soils, 
  U.S. 
  Dept. 
  Agr., 
  1907; 
  also 
  "The 
  Pro- 
  

   duction 
  of 
  Deleterious 
  Excretions 
  by 
  Roots," 
  Bulletin 
  34, 
  Torrey 
  Botanical 
  

   Club, 
  1907. 
  

  

  