On  the  Power  of  Soils  to  absorb  Manure. 
33  7 
Thus,  then,  it  has  been  proved  that  for  the  bases  of  the  salts 
of  ammonia,  potash,  magnesia,  and  soda,  the  absorptive  power  of 
the  soil  is  available  either  directly  or  through  the  agency  of  lime. 
It  has  further  been  proved  that  this  property  extends  to  the  sepa- 
ration of  lime  and  its  carbonate  from  solution  ; whilst  it  has,  so 
far  as  we  can  yet  learn,  no  action  upon  the  ordinary  soluble  salts 
of  lime. 
The  compounds  which  are  thus  arrested  form  a most  important 
part  of  the  mineral  ingredients  of  manure ; but  there  is  yet  one 
mineral  element  of  vegetable  nutrition  which  is  so  very  valuable, 
that  we  should  learn  with  a certain  amount  of  dissatisfaction  its 
want  of  subjection  to  a law  which  cannot  be  regarded  in  any  other 
light  than  as  a direct  impress  of  the  wonderful  hand  of  Provi- 
dence. This  substance  is  phosphoric  acid,  which,  it  is  almost 
unnecessary  to  say  in  this  place,  is  of  vital  consequence  to  vege- 
table existence.  As  an  acid  substance  it  could  not  obviously  be 
subject  to  an  action  having  reference  only  to  bases  ; but  owing  to 
the  insoluble  condition  of  one  of  its  salts,  it  is  in  a subsidiary 
but  most  perfect  manner  retained  and  preserved  in  the  soil. 
Phosphate  of  lime  is,  like  the  carbonate,  insoluble  or  nearly  in- 
soluble in  pure  water;  therefore  when  an  alkaline  phosphate  of 
potash  or  ammonia  is  brought  in  contact  with  the  soil,  the  ab- 
sorbing action  of  the  latter  for  the  potash  or  ammonia  causes  the 
phosphoric  acid  to  combine  at  the  same  time  with  lime,  forming 
a phosphate  of  lime,  which  from  its  insolubility  is  retained. 
Phosphate  of  lime  is,  like  the  carbonate,  soluble  in  carbonic 
acid  water ; and  it  is  probable  that  this  is  the  principal  source  or 
mode  of  supply  of  phosphoric  acid  to  plants.  The  same  action 
which  enables  a soil  to  remove  carbonic  acid  from  a solution  of 
bi-carbonate  of  lime  may  also  be  expected  to  separate  its  solvent 
from  phosphate  of  lime  so  dissolved.  Such  is  actually  the  case, 
and  thus  the  phosphoric  acid  of  phosphate  of  lime  is  incidentally 
subject  to  the  same  law  that  serves  for  the  preservation  of  the 
alkaline  bases. 
Again,  phosphate  of  lime  is  soluble  in  an  excess  of  phosphoric 
acid,  which  forms  with  it  the  acid  or  bi-phosphate  of  lime,  which 
has  become  so  valuable  an  accessory  to  turnip  cultivation  under 
the  name  of  “superphosphate  of  lime.”  It  would  appear  pro- 
bable that  the  same  action  which  in  the  cases  just  quoted  sepa- 
rates carbonic  acid  from  bi-carbonate  of  lime,  and  which  is  un- 
doubtedly that  of  a base  for  an  acid,  should  come  into  play  in  the 
case  of  bi-phosphate  of  lime  ; and  it  has  been  experimentally 
proved  that  soils  have  the  power  to  arrest  entirely  the  acid  phos- 
phate of  lime.  It  is  to  be  observed,  however,  that  in  this  parti- 
cular instance  the  newly  observed  action  is  not  absolutely  neces- 
sary to  such  a result,  since  the  mere  mixture  of  carbonate  of  lime 
VOL.  xi.  z 
