1920] 
DUGGAR—INSOLUBLE SALTS IN BALANCED SOLUTIONS 323 
ture 15 (CaHPO,, MgSO,, ferric citrate, and KNO,) is best, 
followed by one of the controls, solution B, and this in turn is 
very closely followed by Nos. 18, 8, 20, 19, and 11. Cultures 11 
and 15 confirm the previous experience that the ferric citrate 
effects a high degree of balance in cases where the magnesium 
salt is more soluble than the caleium salt used; and neither 
the soluble ferric phosphate nor the FePO, can replace it in 
this respect where wheat is the test plant (compare with the 
cultures above mentioned Nos. 9, 10, 13, and 14). 
With due consideration of the causes already mentioned the 
value of the soluble ferric phosphate in the culture media is con- 
firmed, and the importance of ferric citrate established in certain 
combinations. No experiments thus far made have thrown any 
special light on the nature of the benefit derived in these cases 
from the soluble ferric phosphate or the iron citrate. In both 
cases, however, within the range of reaction involved in the 
studies here reported, suspension films consisting in part at least 
of ferrie hydroxide are thrown down. The writer is now en- 
deavoring to determine if floating particles of this type, or a 
substance in the colloidal state may possibly be of importance 
in the absorption and distribution of the ions. I have previ- 
ously mentioned this possibility (Duggar, '20, p. 42) while re- 
ferring especially to certain experiments of Bonazzi and of Allen 
on the culture of microórganisms. 
SUMMARY 
The value of certain relatively insoluble salts as sources of 
the necessary ions for the growth of seed plants has been tested 
in a variety of combinations covering by no means, however, 
the entire range of possibility. 
It is argued that in certain types of work many advantages 
may accrue from the use of combinations of insoluble salts, 
because of (1) the tendency to maintain a constant concentra- 
tion of the various ions furnished, and also because (2) no re- 
newal of the solution (except as to the addition of NO;) is re- 
quired from day to day. 
As sources of Ca, Mg, Fe, PO,, SO,, many insoluble salts 
have been tested, but no salt of this type is procurable as a 
practical source of NO,, so that in most experiments this ion 
is furnished by KNO.. 
