GROUND LIMESTONE.

King Country Chronicle, Volume VIII, Issue 669, 16 May 1914, Page 6

GROUND LIMESTONE.

ITS VALUE ON SOILS. Ground limestone, often sold under the name of "carbonate of lime," is any more or less finely powdered limestone which is approximately pure. Such a stone should not contain less than 80 per cent, of carbonate of lime, the remaining impurity being mostly sand and clay, or, to speak in chemical terms, iron oxides, alumina and ailica. In most districts it should be possible to obtain a stone which contains between • 90 and 100 per cent, of carbonate of lime, and, if such is the case, the cost of cartage and bagging, labour in handling and spreading, power in grinding, etc., will generally be much less than if a stone containing 10 to 20 per cent, more of impurity were emploved. Each case must, however, be dealt with on its merits; and it is quite conceivable that local conditions on a given farm may favour the use of a rock containing a high percentage of clay, rather than going further afield for a pure product. Crour.d limestone, although it may have been kilp-died, or heated by other means to expel moisture, and thus enable it to be finely ground without balling or clogging in the grinding machinery, has not been heated to the temperature necessary to expel all the carbonic acid gas, and thus differs chemically from quicklime—calcium oxide—or slacked lime —calcium hydrate. In order to make quicklime, large iumps of limestone are heated in a kiln to redness for some huurs, and both moisture and earbrmic acid are expelled. On gradually adding water, or allowing it to sbForb the moisture of the air, the lime becomes bydrated or "slake," and falls to the finest powder—fa? finer than any machinery could economically produce. Henca a given weight of quicklime is more powerful than the same weight of carbonate of lime, not only because it contains twice the amount of calcium 'or because of the changed nature of the substance, but also because it is in such a finely powdered condition that the powder may be spread with greater facility over a larger area of soil than ia possible with an equal amount of ground limestone. It will be seen that "burnt" lime is, hwoever, a much more expensive article to porduce than ground limestone, or, in addition to the extra handling in what ib called "burning" or calcining the stone, the farmer ®hjacts to spread slacked lime owing to tho difficulty in distributing it on windy days and the unpleasant caustic action of the powder on the hands, eyes, etc. Hence is has become common practice to grind the lime after burning, which introduces us to another lime product—namely, ground brunt lime, on no account to be confused with ground limestone. This ground quick lime may be sown with a drill, and there is no loss in sowing the fairly coarse powder, as the atmosphere soon slakes it to a fine flour. All the good effects resulting from the employment of quick lima or slacked lime on the soil may with greater safety be produced by ground limestone, being also alkaline in reaction and coagulating clays—that is, tending to destroy the sticky nature of soils it renders them more porous, and eaiser to work. It supplies a base so necessary in the operation called "nitrification" —the formation of nitrates from ammonium salts. If lime is not present in the soil there is a tendency for ammonia to, be lost Limeatono promotes an alkaline rtacion in the Boil, which is more favourable than a neutral acid reaction for the growth of the beneficial bacteria which transform the inert nitrogenous matter of soils into availabe nitrogenous compounds. Carbonate of lime liberates potash from the insoluble silicates and prevents potash fertilisers from passing into an unavailable condition. A good supply of lime in the soil prevents it from being invaded by such organisms as the destructive "finger and toe" of cruciferous plants. Cut probably the most important f uncti -n of carbonate of lime on New Zealand soils, espsciarly deficient in phosphates, which ai'd usually well supplied with nitrogen and potash and are not too difficult to work lies in the favourable influence which a good supply of carbonate of lime exerts both on the" phosphates naturally present in tho soil and on those which are artificially added; for in the first place, it decomposes the phosphates of iron and alumina which are with difficulty available as plant food, with the formation of calcium phosphates, which are easily available and, further, if acid phosphates such as superphosphates are applied, lime prevents the formation of the unavailable phosphates. Hilgard, the great American authority un soils, has shown in a very luminous manner the relation which may exist between phosphoric acid lime content of the soil and its fertility. He gives four instances cf soils of high productive capacity which only contained 0.03 to 0.05 per cent, of "total" phcaphcric acid, but the lime content was high, varying from 0.6 per cent, to 1.4 per cent. He then gives four examples of soils so low in productive capacity that they were wholly unprofitable unless artificial fertilisers wera applied. The phosphoric acid in these soils varied from 0.03 to 0.17 per cent., but their lime content was only from 0.07 to 0.11 per cent. I know of an instance in New Zealand where a similar state of things occurs—the phosphoric acid content being very low, tho lime content exceptir.ally high, and the land extremely fertile. Where the soil contains a fair proportion of clay liberal dressings of ground limestone will undoubtedly exert a most beneficiial influence on the availability of soil phosphates. I know of no better way of economising the store of phosphates than by utilising the great deposits of limestone which exißt throughout this Dominion.—

From an article contributed by Mr B. G. Aston. Chief Chemist of tho Department of Agriculture, to' the Department's Jorunal.