256 
Journal of Agricultural Research 
Vol. XXXI, No. 3 
This test confirms the preceding one in showing that when rock 
phosphate is exposed to a mixed salt solution the addition of a fairly 
large quantity of silica gel markedly increases the quantity of phos- 
? )horic acid in solution. The omission of the small quantity or iron 
rom the mixed salts apparently increases the quantity of phosphoric 
acid in solution. Doubtless the iron precipitates some of the phos¬ 
phoric acid brought into solution by the action of the colloidal silica. 
DISCUSSION OF RESULTS 
The yields of plants in the pot experiment, where the phosphorus 
supply was the limiting factor, the quantities of phosphoric acid in 
the plants, and the laboratory determinations of the solubility or 
decomposability of the phosphates, all indicate that silica gel under 
the simple conditions or these experiments markedly increased the 
assimilability of the phosphoric acid in rock phosphate and influenced 
relatively little the assimilability of the phosphoric acid in acid 
phosphate. If the plant experiment had been conducted in a different 
medium or with a different basic fertilization, it is quite possible that 
the silica gel would have had a more or less marked effect on these two 
phosphates. 
Lemmermann and Wiessmann (20) conducted many pot experi¬ 
ments in which a silica gel and other finely divided forms or silica were 
added to quartz sand and to soils. The phosphorus supply was 
obviously the chief factor limiting growth in these experiments and 
in all cases the silica gel increased the yield appreciably. As a 
result of their experiments Lemmermann and Wiessmann concluded 
that silica acts to a certain degree as a substitute for phosphoric acid 
(it is “ phosphorsauresparend ; Q and that the action is within the 
plant, not on the soil. WThile the results of these experiments are not 
questioned, it is believed that the conclusion drawn is hardly justified, 
especially in view of the fact that the plants were not analyzed for 
either silica or phosphoric acid. The results obtained by these 
investigators could be explained either by assuming that silica acted 
as a substitute for phosphoric acid in the plant or that silica gel 
rendered the phosphorus m the medium more available. 
The fact that plants which normally contain a high percentage of 
silica have, in certain experiments, been grown practically free irom 
silica shows that silicon is not an essential nutrient such as nitrogen, 
phosphorus, or potassium. It is evident that almost all the sflica 
assimilated by plants can be omitted without abnormally affecting 
growth/ Oats, the crop grown by Lemmermann and Wiessmann in 
most of their experiments, usually contain about 50 per cent of silica 
6 Sachs (81, p. 288) and Knop (18, p. 801 ) grew maize, and Kreuzhage and Wolff (19) grew oats, in nutrient 
solutions lacking silica, the plants containing only a fraction of the normal content of silica. Jodin (16) 
grew maize for four generations in this way. Rice, which normally contains about 10 per cent of silica in the 
dry matter (5) contained only 0.2 per cent of silica grown in a silica-free nutrient solution (6, p. 608 ). How¬ 
ever, owing to experimental difficulties, it has not been shown whether good plants can be grown absolutely 
devoid of silica. 
Several investigators have obtained increased growth by adding silica in some form to nutrient solutions 
or to the soil (19,29,11,12). None of these experiments established that the increased growth was due 
to an increased assimilation of silica. The nutrient solution with and without silica used by Rautenberg 
and Kuhn were of different compositions. Apparently none of the plants in GrSgoire’s experiments made 
a vigorous growth, and an increase was also obtained with alumina. The data considered by Hall and 
Morison were obtained from a field plot that had been treated with sodium silicate, an alkaline substance. 
The work of Livingston (22) and Jennings (16) indicate that growth may be altered in nutrient solutions 
by nonnutrient substances. While it has not been proved that plant growth is promoted by assimilation 
of silica, it is also yet to be proved that assimilation of silica does not promote growth. 
