238 QUANTITATIVE AGRICULTURAL ANALYSIS
After 5 minutes dissolve and dilute to 1000 cc in a volumetric flask. This
makes a permanent color standard, 1 cc of which will contain 0.001 ing of
nitrate nitrogen.
Place two 100-gm samples of 40-mesh soil and 5 grn of calcium hydroxide
(to aid in securing a clear solution) in salt mouth or shaker bottles and add
400 cc of nitrate-free distilled water (tested as below) to each bottle. Mix
in a machine for 30 minutes and then remove the bottles and let stand over
night. Pipette 10 cc or more of the clear, supernatant solution into a 8-cm
porcelain evaporating dish and evaporate to dryness on a steam bath.
Remove from the steam bath as soon as dry, cool, add 2 cc of phenoldi-
sulphonic acid and mix well with the aid of a glass rod. After the acid
has stood in contact with the residue for 15 minutes add 5 cc of cold dis-
tilled water, stir and add enough ammonium, hydroxide (1 to 1) to produce
a permanent yellow color. The standard (a suitable measured quantity
of which has been made basic with ammonium hydroxide in the same mariner
as the unknown) is rinsed into a cylinder for a colorimeter, such as that illus-
trated in Fig. 52, and diluted to the 100-mm mark. Rinse the unknown
into another tube and dilute to the 100-mm line, provided that the color
of this solution is not over two-thirds as intense as that of the standard.
Place both tubes in the colorimeter and move the tube containing the more
intense color up or down until the intensities of color in the two are equal.
The nitrogen concentrations are inversely as the lengths of column equiva-
lent in intensity of color. Take three readings on each sample and from
these calculate the per cent of nitrate nitrogen in the sample.
Ammonia.—The amount of ammonia nitrogen in soils is
usually very small, although in certain, swamps it is present in
considerable quantities as ammonium, salts. Such plants as
rice, which grow in water, secure considerable nitrogen in the
form of ammonia or of nitrogenous organic decomposition
products- Among these compounds are amino acids, e.g.,
arginine and glycocoll.
The chemistry of a possible mode of ammonia production from
amino acids may be represented by the following equations:
RCHNH2COOH + O2-» RCOOH -f CO2 + NH3; (1)
• Amino acid Patty acid
RCHNHsCOOH + H2O -» RCHOHCOOH + NH3. (2)
Thus an arnino acid when oxidized or hydroli^ed produces
ammonia as an end product.
Determination of Ammonia.—Place 25-gm samples of soil, together
with 5 gm of sodium carbonate, in aeration flasks (Fig. 53), add three drops
of light hydrocarbon oil (to prevent frothing) and 1OO ec of boiled distilled
water to each flask. Connect the flask with a wash bottle containing 25 cc