SOILS f 255 1 ' | /

Iron and aluminium are precipitated together as hydroxides. *) \

If titanium is present In the soil the precipitate will contain also / |

titanium hydroxide. Phosphorus will be precipitated here as I \

basic ferric phosphate. The combined precipitate is ignited and ] *

the oxides and phosphate weighed together. Iron is then ' * / 1

determined by dissolving and titrating with a standard per- ' * •' <

manganate or dichromate solution. Phosphorus is determined , ; ]

in a separate sample and calculated to . the pentoxide, while \ I

titanium is usually ignored unless it is known to be present in ; s !*|

appreciable quantities, as it has no known biological signifi- / f |
cance. The sum of the per cents of oxides of iron and phos-
phorus, subtracted from the per cent of total residue, gives the
per cent of impure aluminium oxide.

Direct Method for Determining Aluminium. — The above |

procedure necessarily throws the combined errors of all of these , J

determinations upon aluminium. If an accurate determination i

of the latter is required, a direct determination may be made. i

In this case the precipitate of hydroxides is redissolved without « ' J
previous ignition and the iron is reduced to the ferrous condition
by sodium thiosulphate :

{

2Na2S203 + 2FeCl3 -^ Na2S4O6 + 2FeCl2 + 2NaCl.
The aluminium is then precipitated as phosphate, ferrous phos-
phate remaining in solution. \
Purification by Double Precipitation.—The precipitates of f
iron and aluminium hydroxides, of calcium oxalate and of mag-
nesium ammonium phosphate are difficult to purify by simple
washing. If accuracy is important, purification is usually
accomplished by dissolving the partly washed precipitate,
redissolving and reprecipitating. In the solution from which the
second precipitation is made the concentration of soluble salts
is only a small fraction of that in the original solution and the
amount now carried down by the precipitate and not removed !
by washing is extremely small. ! I
Calcium.—Many soils that are noted for their fertility have a
high calcium carbonate content. Examples of such are the
blue grass soils of Kentucky, the calcareous prairie soils of * fc
Illinois and Indiana and the black prairie soils of Texas and
Mississippi.