256 QUANTITATIVE AGRICULTURAL ANALYSIS
Calcium functions particularly in stimulating root develop-"
ment and it is thought to be connected in some way with the
development of cell wall material. Some crops, such as alfalfa,
clover, and tobacco, require large amounts of calcium for good
growth and development.
For the determination, calcium is precipitated from, the
filtrate from iron and aluminium as calcium oxalate. The
calcium may then be determined gravimetrically, as oxide, or
volumetrically by titration with standard potassium, perman-
ganate. These determinations are discussed on pages 63 to
69, Part I.
Magnesium.—This is a plant food element which plays an
important part in seed production as magnesium, like phosphorus,
moves to the seed to a great extent. In this respect it is unlike
potassium and calcium, which remain largely in the stem and
leaf. Magnesium appears also to function in oil and chlorophyll
production.
Magnesium is determined in the filtrate from calcium oxalate
by precipitating as magnesium ammonium phosphate in a solu-
tion previously made basic with ammonium hydroxide. The
precipitate is ignited and weighed as magnesium pyrophosphate.
The principles underlying this determination have been discussed
in connection with the analysis of phosphate, page 87, Part I.
When magnesium is being determined, a soluble phosphate is
used as the reagent.
Determination of Total Silica.—Weigh accurately about 1 gm of soil into
a platinum crucible, burn off the organic matter and when cool mix with
approximately 10 gm of sodium carbonate. Place the cover on the crucible
slightly to one side so that the contents may be observed. Heat gently at
first, using a small burner. Gradually raise the temperature to that of the
full flame and heat until gas evolution is only slight. Place the crucible over
a blast lamp and heat for at least 15 minutes after the evolution of carbon
dioxide has ceased. While it is still hot, rotate the crucible by manipulating
the triangle, so that the fused mass will spread over the sides as it solidifies.
When it has cooled, place the crucible on its side in a casserole and cover
with hot distilled water.
Heat until the fused mass has disintegrated, cover and gradually add 15 ec
of concentrated hydrochloric acid from a pipette through the lip of the cas-
serole. Place on a steam bath and, after all effervescence has ceased, remove
the crucible and cover, rinsing well. Use a stirring rod for this purpose.
By inserting this in the mouth of the crucible the latter can be raised out of