24  The  Production  of  Plant  Food  in  the  Soil. 
a summer  fallow  in  temperate  climates  when  the  summer  has 
been  very  dry. 
Low  temperatures  also  seem  to  be  beneficial.  The  soil  in 
parts  of  Canada  and  the  United  States  is  frozen  hard  through- 
FlG.  5. — Tobacco.  Soils  heated  to  various  temperatures  for  a short  time.  From  left  to 
right  the  temperatures  were  86°.  140°,  176°,  212°,  257°.  and  302°  F.  (S.  U Pickering,  Journal 
of  Agricultural  Science, I V ol.  3,  1910,  page  277.)  c 
Fig.  6. — Festuca  pratensis.  Soils  heated  as  in  Fig.-5.  (S.  U.  Pickering,  ibid.) 
out  the  winter  ; when  spring  comes  there  is  an  extraordinary 
rush  of  plant  growth.  Conn  has  recently  shown  that  the 
newly  thawed  soils  are  unusually  rich  in  bacteria.  The  low 
temperature  appears  to  kill  many  of  the  injurious  organisms 
