146

POTASH—ITS NITRATE.

of iron (412-414) : a cheap acid like the sulphuric is,generally speaking, the most convenient. Gas liquorresembles putrid urine, inasmuch as it is very rich inammonia; but it differs from it in not containing thealkaline and earthy salts which that manure does. )

456. Potash, as has several times already been stated, 1is very commonly found in plants, and is consequently jan important constituent of manure. The principal value :of wood-ashes appears to depend on the potash which ;they contain. One other compound of this alkali requires ]notice in this brief sketch, namely, the nitrate. Theresult of experience has shown that, on some lands,nitrate of potash forms a most valuable manure, whilst onothers, it has produced little or no effect; this has led

to great differences of opinion as to its value.

457. Many soils contain a small quantity of nitrateof potash or of soda. The way in which these salts areformed will be easily understood when we remember,that whenever substances containing nitrogen decay in theneighbourhood of alkaline salts, a portion of nitric acid isformed (101). Under these circumstances, the ammoniawhich would otherwise be produced, is oxidised,and in place of ammonia, nitric acid and water areformed; the acid combines with the alkali, and nitrate ofpotash or soda results. These salts, together with nitrateof lime, are frequently formed in mixtures of decayingorganic manures; they are formed in the same way in thesoil itself.

458. The nature of the influence which nitrates exert