Aug. 1, 1925 
Colloidal Silica and Efficiency of Phosphates 
257 
in the ash of the whole plant (38). If growth is not abnormally 
diminished by the omission of most of this silica, it is hard to conceive 
of growth being promoted by a relatively small increase in silica over 
the average content. 
It should be noted that Lemmermann and Wiessmann obtained 
significant increases only with pure silica gel, which is adsorptively 
active and contains no ions for base exchange. Kieselguhr, kaolin, 
various finely powdered silicas, and permutite, which contains 
exchangeable calcium, gave no increase. In view of the hypothesis 
they proposed, it would have been highly advisable to have determined 
which forms of silica increased the silica contents of the plants. 
Experimental results reported in this paper confirm certain results 
of Lemmermann and Wiessmann, but contradict their hypothesis 
of the beneficial action of silica gel. Data given in Table I and II 
indicate that an increased assimilation of silica by the plant does not 
enable the plant to get along with less phosphoric acid. 
Addition of silica gel to pots receiving 0.06 gram P 2 0 5 from acid 
phosphate increased the growth of plants by 21 per cent, the quantity 
of P 2 0 5 taken up by 30 per cent, and the quantity of silica assim¬ 
ilated by 219 per cent. If we attribute the slight increase in growth 
to the slight increase in the quantity of phosphoric acid assimilated 
the 219 per cent increase in silica taken up had no effect on growth. 
Plants grown with 0.24 gram P 2 0 5 from rock phosphate made 14 
per cent less growth than plants grown with 0.06 gram P 2 0 5 from 
acid phosphate, although they took up 28 per cent more phosphoric 
acid and 295 per cent more silica. Here the increased assimilation 
of silica would seem to have diminished rather than enhanced the 
effectiveness of the phosphoric acid in the plant. Plants grown 
with 0.24 gram P 2 0 5 from rock phosphate plus silica gel made 4 per 
cent more growth than plants grown with 0.12 gram P 2 0 5 from acid 
phosphate and assimilated 20 per cent more phosphoric acid and 
444 per cent more silica. In this case the slight increase in growth 
is more than accounted for by the phosphoric acid assimilated and 
no beneficial action of the 444 per cent excess assimilation of silica 
is apparent. 6 
While it seems very apparent that the addition of silica gel to the 
quartz sand and salts of our pot experiment increased the assimila- 
bility of the phosphoric acid of rock phosphate, a complete explanation 
of the action of the gel can hardly be offered on the basis of the work 
done. The laboratory tests with solutions indicate that the silica 
gel increased the assimilability of rock phosphate by increasing the 
quantity of phosphoric acid in solution. Possibly the gel increased 
the decomposition of the rock phosphate by adsorbing calcium 
hydroxide—one of the end products of the decomposition. It is 
also possible that the gel acted by adsorbing OH ions. 7 There 
was some evidence of this in tests made of the hydrogen-ion concen¬ 
trations of the solutions in the different flasks. However, the water 
extracts of the various pots after the plants had been grown in them 
did not bear this out. 
6 It might be held that failure of the enhanced assimilation of silica to increase growth in this case was 
because growth was no longer limited by the phosphorus supply. However, the steep form of the curve 
obtained on plotting growth increase against 0.03, 0.06, and 0.12 gram P 2 O 5 from acid phosphate would 
indicate that this was not the case. 
7 The experiments of Gordon and Starkey (10,85) indicate that silica gel does not absorb an appreciable 
amount of Ca from neutral salts, especially from an acid solution, although alumina and iron gels may. 
According to Glixelli (9), silica gel absorbs OH ions and the hydroxides of Na, K, Ca, and Ba. 
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