The  Production  of  Plant  Food  in  the  Soil.  23 
and  is  finding  increased  favour  among  practical  men  ; many 
details  have,  however,  to  be  worked  out  and  many  difficulties 
overcome.  It  is  not  yet  known  how  the  heating  should  be 
done  to  kill  effectively  all  pests.  Again,  Pickering1  has 
shown  that  germination  is  retarded  in  heated  soils  because  of 
the  production  of  a toxic  substance.  Dyer  records  that  certain 
growers  who  rely  on  the  fermentation  of  the  added  manure 
to  keep  up  their  soil  temperatures  are  placed  in  this  difficulty 
when  they  heat  their  soils  : if  they  add  the  usual  amount  of 
UP-TO-DATE. 
Heated  Soil.  Untreated  Soil. 
Weight,  4 lb.  lb. 
Potatoes  grown  in  heated,  soil  and  in  untreated  soil.  University  of  Leeds  Agricultural  Department. 
Bulletin  No.  70. 
manure  the  crop  becomes  over  rank  ; if  they  add  any  less  they 
do  not  get  up  a sufficient  soil  temperature.  None  of  these 
difficulties  is  insuperable,  and  they  are  all  under  investigation. 
The  kind  of  effect  produced  is  seen  in  the  photographs  in 
Figs.  3 to  7. 
Tropical  sunshine  has  much  the  same  effect  on  the 
soil  as  heat.  “ It  has  been  the  practice  of  the  ryots  for 
centuries  past,”  write  Mr.  and  Mrs.  Howard,2  “ to  expose  the 
alluvial  soils  of  the  plains  of  India  to  the  intense  heat  and  light 
of  the  Indian  hot  weather  in  April  and  May.  The  beneficial 
effect  on  the  succeeding  crop  is  extraordinary,  and  has  all  the 
effect  of  a nitrogenous  manuring.”  It  appears  that  the  soil 
temperature  rises  to  105°  F. — 125°  F.  or  even  higher  ; the 
drying  also  is  no  doubt  very  thorough.  Considering  the  length 
of  the  exposure  to  sunshine,  either  the  heat  or  the  drying  could 
kill  the  injurious  factor  even  if  the  direct  sunlight  had  no 
effect.  Something  of  the  same  sort  has  been  observed  after 
1 Journal  of  Agricultural  Science. 
2 Nature , February,  1910,  page  456. 
