262 QUANTITATIVE AGRICULTURAL ANALYSIS
Determination of Manganese: Persulphate Method.—The standard per-
manganate solution prepared as for the bismuthate method is used in this
case.
One gram of soil is fused and treated as directed above for the bismuthate
method. Do not add sodium bismuthate but after the residue from, evapora-
tion has been dissolved in nitric acid, add 15 cc of a 0.2-per cent silver nitrate
solution, following immediately by 1 gm of ammonium persulphate. Heat
by placing the beaker or casserole in hot water until the pink color is fully
developed. Cool and rinse into a tube of a color comparator. Place in
another tube enough of the standard permanganate solution (measured
accurately) to make a somewhat greater intensity of color, when viewed from
above, dilute to the mark and mix. Place both tubes in the comparator
(Fig. 52, page 237) and adjust to equality of color. Calculate the per cent
of manganese in the sample.
Sulphur.—The sulphur content of most soils is usually less
than that of phosphorus, while considerable sulphur is needed
by certain plants to produce proteins and flavoring oils. It has
been shown that onions, mustard, and cabbage usually respond
favorably to the addition of either elementary sulphur or sul-
phates to the soil. The function of sulphur in the plant metab-
olism is not well understood.
The determination of sulphur in soil is preceded by fusion with
sodium carbonate in the presence of a small amount of an oxidiz-
ing agent, the latter in order to convert protein sulphur to the
form of sulphates. The sulphate thus formed, together with
sulphates originally present as such, is later precipitated and
weighed as barium sulphate. The heating should be done with
an alcohol burner or in an electrically heated muffle furnace
instead of with a gas flame because of danger of absorption of
sulphur dioxide from the burning gas (which always contains
hydrogen sulphide) by the sodium carbonate.
Determination of Sulphur.—Mix 2 gm of 100-mesh soil with 7 gm of
anhydrous sodium carbonate (free from sulphates) and 0.5 gm of potassium
nitrate in a platinum crucible. Place the covered crucible in an electrically
heated muffle and heat to dull redness until well fused, after which remove
the crucible and tip it in such a manner as to cause the contents to solidify
on the sides. While it is still hot place the crucible in 75 cc of cold water in
a 200-cc beaker (use care). Cover and heat the beaker and contents to
boiling. Stir until all lumps of the fused mass have been disintegrated, then
filter into a 400-cc beaker and wash the residue until the volume is about
200 cc. Reduce any sodium manganate present by boiling with a few drops