' FP

252 QUANTITATIVE AGRICULTURAL ANALYSIS
and immediately available forms and those not so available.1 The
determination is no longer official.
Determination of Humus.—Five-gram samples of air-dried soil, ground
to pass a 60-mesh sieve, are placed in 500-cc wide-mouthed bottles and
washed repeatedly by shaking with a 1-per cent solution of hydrochloric acid
until calcium and magnesium are no longer extracted, as shown by testing a
small quantity of filtered solution with ammonium hydroxide and ammo-
nium oxalate. The first washings need not be tested. The wash-
ing can be hurried by manipulating the bottle in a shaking machine for
15 minutes (Fig. 51). After calcium and magnesium have been removed,
filter the solution and wash the soil free from acid by decantation. Return
the filter and its contents to the bottle and add 250 cc of 4-per cent ammo-
nium hydroxide. Shake in the machine for three hours, or every 30 minutes
by hand for six hours, then place the bottle in a horizontal position for twelve
hours.
Again shake the bottle well and pour the contents upon a 24-cm filter
paper in a funnel. Cover the funnel with a watch glass. The filtrate may
be very turbid for an hour or more. In this case, refilter. When the filtrate
comes through clear, save 100 cc or more of it in a clean flask. Pipette
50 cc of the clear filtrate into an 8-cm evaporating dish. Evaporate to dry-
ness on a steam bath, dry in the oven for an hour at 100°, cool in a desiccator
and weigh. Burn the carbonaceous matter to an ash in the muffle furnace,
cool, weigh and calculate the difference between the two weights as per cent
of humus.
Extraction of Material Soluble in Strong Acid.—As in the case
of organic matter, the inorganic constituents of the soil are
combined in forms which differ widely in degree of availability.
Calcium may be present either as limestone (calcium carbonate)
which is easily soluble in acids, or as one or more of a variety of
silicates, such as anorthite (calcium aluminium silicate) which is
nearly insoluble. A similar variation exists with potassium,
which may be present as a soluble carbonate or as orthoclase, a
silicate of potassium and aluminium which is highly insoluble.
Extraction of the soil with hydrochloric acid provides an approxi-
mate distinction between materials of small availability and the
more available ones. The acid extract may be evaporated to
dryness and the extract simply weighed, or the residue may be
subjected to a partial or complete analysis as outlined for the
original soil.
The amount of material dissolved by the acid varies with the
concentration of the latter, the fineness of division of the soil
1 Soil Science, 3, 515 (1917).