316  CHLORIDE  OF  POTASSIUM  FROM  CARNALLITE. 
by  further  solution  in  hot  water  into  sulphate  of  potassa,  which 
crystallizes  out,  and  sulphate  of  magnesia  which  remains  in 
solution.  This  decomposition  does  not  take  place  so  readily  as 
that  of  carnallite  ;  at  the  same  time,  by  two  crystallizations,  a 
salt  containing  80  per  cent,  of  sulphate  of  potassa  can  be  ob- 
tained, which  is  rich  enough  to  be  roasted  with  carbonate  of  lime 
and  charcoal. 
This  process  may  possibly  be  preferable,  even  at  Stassfurth, 
to  the  treating  of  the  chloride  of  potassium  with  sulphuric  acid, 
since  the  chloride  of  potassium  occurring  there  contains  from  15 
to  20  per  cent,  chloride  of  sodium  and  other  salts,  and  therefore 
would  afford  a  potash  containing  much  soda,  while  by  the  French 
process  the  chloride  of  sodium  is  separated.  The  manufacture 
of  potashes  has  not  yet  begun  at  Stassfurth,  but  Vorster  and 
Griineberg  prepare  a  sulphate  of  potash  of  different  degrees  of 
purity  for  farmers.  They  keep  their  process  secret,  but,  how- 
ever, make  use  of  sulphate  of  magnesia,  which  the  kieserite 
(MgO,S03+HO)  abundantly  found  there  affords. 
It  is  well  known  that  plants  need  potash,  which  is,  indeed, 
found  in  their  ashes.  Although  it  appears  that  most  soils  are 
richly  charged  with  it,  yet  but  little  of  it  exists  in  such  a  form 
as  to  be  readily  assimilated  by  the  plant.  It  exists  generally  in 
the  soil  as  feldspar  (silicate  of  alumina  and  potash),  which  is  with 
difficulty  attacked  by  the  strongest  mineral  acids.  By  the  ac- 
tion of  the  weather  the  potassa  is  of  course  extracted  from  it, 
but  by  a  slow  process.  Formerly,  when  the  land  was  left  to  lie 
fallow,  this  answered  the  purpose  ;  but  with  the  present  increased 
population  and  more  exhausting  cultivation  this  will  do  no  longer. 
Hitherto  attention  has  been  mainly  given  to  the  addition  of  ni- 
trogen and  phosphoric  acid,  but  it  will  soon  be  necessary  to  re- 
store to  the  soil  the  potassa  also  which  has  been  removed  from  it. 
A  soil  which  has  grown  clover  for  some  years  soon  yields  a 
poor  harvest.  The  ashes  of  clover,  then,  show  but  about  one- 
tenth  of  their  former  proportion  of  potash,  and  a  soil  which  had 
contained  1  3*4  parts  potassa  in  10000,  then,  has  but  3  parts. 
In  the  cultivation  of  the  sugar  beet  it  has  been  often  observed 
that,  notwithstanding  the  liberal  addition  of  phosphoric  acid,  the 
amount  of  sugar  in  the  beet  suddenly  falls  off,  because  the  soil 
