SOILS 251
excess of acid may be attained. Note the volume of acid required to dis-
charge the color. The pink color may return as the solution is allowed to
stand but this is not considered in the reading.
. At the end of 15 minutes from the time the second set of bulbs was inserted,
replace the first bulbs, recharged with barium hydroxide. The titration of
the second solution constitutes the second "blank" and the average of this
and the first is to be taken as the acid equivalent of the barium hydroxide ', I
solution. i *
While one or more blank determinations are running weigh 2 gm of 100- I \
mesh soil and transfer it to an alundum boat (about 10 cm long and as wide ' f ,
as the tube will allow) and mix the soil with aa equal weight of 20-mesh / TI J
alundum. Replace the Meyer absorption tube by another, containing , ,, r
exactly 50 cc of fifth-normal barium hydroxide solution. Place the boat in ^ ;
the combustion tube, connect and continue to pass oxygen for 20 minutes. ^ \ \
At the end of this period, disconnect the absorption tube and replace by a ' j
second, containing barium hydroxide as before. Without interfering with „ ^
the flow of oxygen, immediately withdraw the boat from the tube and insert { * ,'
another, containing a sample weight as before. Insert the stopper carrying \ ,
the oxygen tube and while combustion is proceeding with the second sample, ' / *
rinse the barium hydroxide from the first Meyer tube into a 500-cc Erlen- '
meyer flask, using 50 cc of carbon dioxide-free water, and titrate the unused i J 'f
excess with standard acid, using phenolphthalein as indicator. Calculate } 111
the per cent of total carbon and from this the per cent of organic carbon, ', \ *
deducting that present in the carbonate form. ' \
\ I \
, * f *
Soil Humus.—This is a somewhat indefinite term, used to f fi
designate an intermediate stage of decomposition of the complex ': 11
organic residues usually found in the soil. The term " humus" is '3/1f
arbitrarily used to include that part of the soil organic matter ;, J ?
which has reached a stage of decomposition in which it is soluble ? <
in 4-per cent ammonium hydroxide. Part of this decomposed ; ^ /
organic matter contains certain substances having acid prop- ,' 11
erties, which combine with basic materials to form organic
' I ' r f
salts called humates. Total humus material is the active avail- - '/ T |
able organic plant food, while the residual organic matter is V,
useful in improving the soil texture. j \
There has been considerable discussion concerning tbe real 4*
value of the humus determination. While it must be admitted I
that the term " humus " does not cover a sharply defined class of \;.
compounds and that the result of the determination is subject ||
to considerable variation unless the method is rigidly standard- |*
ized, it yet appears that some useful information is obtained, {i
in at least approximately classifying organic matter into easily ||
li