=224. Introduction.=—The potash present in unfertilized soils has been
derived from the decay of rocks containing potash minerals. Among
these potash producers feldspars are perhaps the most important. For
a discussion of the nature of their decomposition and the causes
producing it the first part of volume first may be consulted. Potash is
quite as extensively distributed as phosphoric acid and no true soils
are without it in some proportion. Its presence is necessary to plant
growth and it forms, in combination with organic and mineral acids, an
essential part of the vegetable organism, existing in exceptionally
rich quantities in the seeds. It is possible that potash salts, such
as the chlorid, sulfate, and phosphate may be assimilated as such,
but, as with other compounds, we must not deny to the plant the
remarkable faculty of being able to decompose its most stable salts
and to form from the fragments thus produced entirely new compounds.
This is certainly true of the potash compounds existing in plants in
combination with organic acids. The potash which is assimilated by
plants exists in the soil chiefly in a mineral state, and that added as
fertilizer is chiefly in the same condition. That part of the potash in
a soil arising directly from the decomposition of vegetable matters may
exist partly in organic combination, but this portion, in comparison
with the total quantity absorbed by the plant, is insignificant.
It is then safe to assume that at least a considerable part of the
potash absorbed by the plant is decomposed from its original form
of combination by the vegetable biochemical forces, and is finally
incorporated in the plant tissues in forms determined by the same
powerful forces of vegetable metabolism.
The analyst is not often called upon to investigate the forms in
which the potash exists in plants, when engaged in investigation of
fertilizers. It is chiefly found in combination with organic and
phosphoric acids, and on ignition will appear as phosphate or carbonate
in the ash.
=225. Forms in which Potash is Found in Fertilizers.=—The chief
natural sources of potash used in fertilizer fabrication are: First,
organic compounds, such as desiccated mineral matters, tobacco
waste, cottonseed hulls, etc.; second, the ash derived from burning
terrestrial plants of all kinds; third, the natural mineral deposits,
such as Stassfurt salts.
All of these forms of potash may be found in mixed fertilizers. While
the final methods of analyses are the same in all cases the preliminary
treatment is very different, being adapted to the nature of the sample.
For analytical purposes, it is highly important that the potash be
brought into a soluble mineral form, and that any organic matters which
the sample contains be destroyed. If the sample be already of a mineral
nature, it may still be mixed with other organic matter and then it
requires treatment as above, for it is not safe always to rely solely
on the solubility of the potash mineral, and the solution, moreover, in
such cases, is likely to contain organic matter. In some States, only
that portion of the potash soluble in water is allowed to be considered
in official fertilizer work. In these cases it is evident that the
organic matter present should not be destroyed in the original sample,
but only in the aqueous solution. Since, however, the potash occluded
in organic matter becomes constantly available as the process of decay
goes on, it is not just to exclude it from the available supply. It may
not be so immediately available as when in a soluble mineral state,
but it is not long before it becomes valuable. Experience has shown,
moreover, that phosphorus, nitrogen, and potash are all more valuable
finally when applied to the soil in an organic form. This fact is a
corroboration of the theory already advanced that all mineral compound
bodies are probably decomposed before they enter as component parts
into the tissues of the vegetable organism.
It is highly probable, therefore, that the potash existing in organic
compounds, finely divided and easily decomposed, is of equal, if not
greater value to plant life than that already in a soluble mineral
state. The organic matter, when present, is destroyed, either by
ignition at a low temperature, or by moist combustion with an oxidizing
agent before the potash is precipitated.