SOILS 243

or its

until a slight precipitate of hydroxides..........- - • ' .

02, beginning with "Add ammonium hydroxide uu^11 a sll«ht
persists............."

aJid Sodium.—Potassium is essential to plant |,

a,nd it is present in most soils in sufficient ^mounts to |i

plant needs, but only partly in an available form, it j j

very gradually changed to soluble potassium carbonate by Ij

of carbonic acid upon orthoclase, which is nearly insoluble g\

xxs not readily available to plants. Sandy soil often con- »|

-Oiss than 0.1 per cent of acid-soluble potass!"U^*1* sandy j|J

f rom O.I to 0.3 per cent, loams from 0.3 to 0.45 per cent jj

clays 0.45 to 0.8 per cent. |

functions notably in the photosynthesis and I

of starch within the plant. Lack of starch- formation I

ovement is one cause of shriveled and sterile grain. An- |

effect of a lack of potassium is to make the plan-t less resis- I

> disease. This may be said of a plant suffering from any jj

^ood deficiency but it seems to be especially ~fcrxie in the |l

potassium.

urn. is not of great importance in plant nutrition. It is
d with delaying potassium starvation but i"fc will not
y [prevent this condition.

rrtethod generally used for decomposition of insoluble
Is, preliminary to the determination of potassium and
L, is the J. Lawrence Smith method. It is based upon
sion. of calcium chloride (formed from calcium, carbonate
xmonium chloride) upon complex silicates at temperatures
n 800° and 900°. Sodium and potassium chlorides,
. a,s silicate of calcium, are formed. The reaction taking 1

b>ei>ween orthoclase, calcium carbonate and ammonium
e maay be represented as follows:

l*O8 + 6CaCO3 + 2NH4C1->2KC1 + Al2O3-h

6CaSi03 + H20 + 2N"HC3 -f 6CO2.

platinum crucible (Fig. 56) is preferable for -fche decom- f! \ •
OL but an iron or nickel1 crucible of 50-cc capacity may be 11 (
S\zch base metal crucibles deteriorate rapidly when used nil
.
, J. Ind. Eng. Chem., 11, 1139 (1919)..