SOILS 259
and again decant the clear solution on the filter. Dissolve the precipitate
in the beaker with a few drops of hydrochloric acid, add 15 cc of water
and reprecipitate by adding ammonium hydroxide and ammonium oxalate
solution as before. Allow to stand for an hour and filter through the
same paper. Wash the beaker and precipitate with hot water until free
from chlorides. Save the filtrate and washings from both precipitations
for the detsrmination of magnesium.
Determine the calcium either gravimetrically or volumetrically. 111 I
(a) Gravimetric Method.—Place the paper and calcium oxalate in a weighed I! i I
crucible, heat carefully until dry and then ignite in the covered crucible for ' j »-'
30 minutes over a blast lamp or a M6ker burner. Weigh as calcium oxide j'j
and calculate the per cent of this in the sample. I
(b) Volumetric Method.—Dissolve the calcium oxalate and titrate with j' :
potassium permanganate, following the details outlined on page 69, Part I. j
Calculate the per cent of calcium oxide in the soil sample. . j'
Determination of Magnesium.—Acidify the filtrate from calcium with |4
hydrochloric acid and evaporate until ammonium chloride or oxalate begins j *
to crystallize. Add 10 cc of water and stir until the salts are in solution. |']
To the filtrate add a drop of methyl red and sufficient ammonium hydroxide j /
to make the solution barely basic. Now add from a pipette, slowly and with
stirring, 20 cc of a 10-per cent solution of disodium orthophosphate. Let
stand for 20 minutes or until crystallization begins, then stir and add a
quantity of concentrated ammonium hydroxide about equal in volume to
one-ninth of the total. Cover the beaker and let stand for three hours or
over night. Filter on paper, making no effort to remove adhering precipitate
from the beaker. Wash two or three times with dilute ammonium hydroxide
and discard the filtrate and washings. Dissolve the precipitate on the filter j J
with hydrochloric acid and allow the solution to run into the beaker contain-
ing some of the precipitate. Wash down the paper thoroughly with hot
water, dilute to about 75 cc and precipitate the magnesium as before. Filter
the precipitate in an ignited and weighed alundum crucible and wash until
free from chlorides with a 2-per cent solution of ammonium hydroxide, test-
ing the washings finally with silver nitrate solution made acid with nitric
acid. Cover the crucible and heat gently over a burner until dry and finally | ',
heat for 20 minutes, using a blast lamp. Cool in the desiccator and weigh. | * *
From the weight of magnesium pyrophosphate calculate the per cent of I
magnesium in the sample.
Manganese.—Manganese is present to some extent in alluvial
clay soils but it is more abundant in volcanic clays. In small
amounts, approximating not more than abou.t 50 Ib. of manganese
per acre of soil, 6% in. deep (0.0025 per cent), it seems to have \i4
a stimulating effect on plant growth. Many plant compounds \ j
contain manganese but its biological function is not well !#
understood.