254 QUANTITATIVE AGRICULTURAL ANALYSIS
There are also formed soluble chlorides of iron, aluminium,
calcium and such other metals as were present in the soil.
The silica is separated almost completely from the other
compounds by evaporation to dryness and heating to about 120°
to decompose the silicic acid:
H2SiO3 -» H20 + SiO2. (3)
The residue is taken up with water and hydrochloric acid and
the insoluble silica is separated by filtration. However, this
separation is incomplete as there is a tendency to form a hydrosol
of silicic acid. The error thus produced may be avoided by
filtering off the silica formed on first evaporation and repeating
the dehydration of soluble silica by a second evaporation. The
silica finally obtained is not pure but the amount of impurities
may be determined by treating the ignited and weighed pre-
cipitate with hydrofluoric acid, thus converting the silica into
silicon tetrafluoride. The latter is volatilized by heating, leaving
oxides of iron and aluminium as a residue.
Silica.—The function of silicon in plant growth is not well
understood. There is a considerable amount of this element in
some plants (notably oat and rye straw) and it may serve some
useful purpose, not yet understood.
Aluminium.—Compounds of aluminium are present in normal
soils in rather large quantities.. The per cent of aluminium in
sandy loam is about 1.5, in clay loam, 4.5, and in residual soils
formed from gneiss or limestone about 13. Residual soils
usually contain much more aluminium and iron than do glacial
soils. Salts containing aluminium are present in some acid
soils in sufficient amount to exert a toxic influence on certain
plants; barley and corn are particularly sensitive t'o it. This
effect is probably due to the existence of colloidal basic aluminium
salts which are capable of being absorbed by the plant. The
toxicity may be corrected to a considerable extent by an applica-
tion of calcium silicate, acid phosphate, or limestone to the soil,
thus causing the aluminium to form a less soluble compound.
Iron.—The iron content of soils is quite variable. In soils
only slightly tinted, from 1.5 to 4 per cent of iron, calculated
as ferric oxide, is found. Ferruginous loams contain from 3.5
to 7 per cent and the red lands from 7 to 14 per cent.