THE NEED FOR LIME.

Hokitika Guardian, 18 August 1921, Page 4

THE NEED FOR LIME.

and how to meet it. (By B. H. Bedell; in the “Journal of the Ministry of Agriculture, Eng. land.) Before Iho general use of artificials enabled the fanner to forget for a time his ancient friend the lime kiln, he had limed not always wisely, but frequently too well, with the Result that such stores of Time were laid up in the soil that many fields to-day owe much of their fertiltity to excessive dressings of lime applied perhaps forty years ago. This happy state of affairs is, however, becoming more and more rare as the years succeed one another j and no lime is returned to the s oil to replace the inevitable losses. It has been computed that to meet the lime requirements of the arable land only in England and Wales, 3) million tons of burnt lime would be needed, and that the losses due to all causes on this 1 same land do not fall far short of 800,000 tons a year. As it is improbable that more than 350,000 toils are applied annually it is not difficult to see that wo are heading straight for national lime bankruptcy. In fact a time is approaching, and on some farms has already arrived, when no amount of artificial manure can restore los s of fertility due to soil acidity which only lime in some form can. correct. The writer feels, therefore, that no apology is needed for drawing attention to some of the considerations incidental to the production of lime to meet this urgent need. There arc two forms in which lime may he applied to the soil, neglecting unessential modifications. The first of these is burnt, lime, and the second is ground limestone (or ground chalk), and each of those can be obtained by the farmer in two ways; he can cither buy tliem, or if bis land overlies a chalk or limestone formation, he can produce them himself. At the present time there is a feeding among agriculturalists that lime producers are demanding much more profit than they are economically entitled to, or than thr increased costs of production warrant. The writer believes that this feeling is bv no means always justified bv the facts, but where it is well founded. the purchasers ("either individually or collectively a.s a co-operative society) might think well of adopting the second alternative and eliminate the producer’s profit altogether by providing their own lime. With this possibility in view, it is proposed to offer a few suggestions as to the plant required and the processes involved, first in the production of ground limestone (or chalk) and secondly in burning these materials to obtain quick lime. THE STONE. The first consideration is obviously the discovery of a bed or outcrop ol ( limestone of suitable composition upon (In' estate or near enough to it, to make haulage not too serious an item. All the upper chalk is entirely suitable for burning an I for grinding; the lower, or grey chalk, is by no means good, uni often contains so much combined silica that when burnt it forms a -irongly hydraulic lime ol doubtful use 1,, the agriculturist. Limestones oiler a mueb wider range of texture and chemical eon)position than chalk, and before am decision is arrived at witli regard to grinding or burning a limoaloiic. samples should be analysed. It may be decided at once that any stone which contains 92 per cent, of calcium carbonate (('a('t’3) is suitable either burnt or ground. Many rocks which fall short of this percentage of calcium carbonate arc also good, but a little eoiisidera!ion of their other constituents is necessary before deciding upon their use. MAGNESIAX LIMESTONE. Many deposits ol limestone, some of I hem of considerable extent, and consequently of importance, contain the ■ dement magnesium in varying proportions. Magnesium lias a close chemical relationship to calcium, and also occurs in limestone in the form of a carbonate. Magnesium carbonate (MgCOS) is met with in all proportions from mere traces ii]> to 45 per-cent., at which point it is in chemically equivalent proportion to the calcium carbonate; such limestone s containing a high proportion of magnesium carbonate are termed Dolomites.

Tliero Itiis boon inuoli controversy ns to whether, to wlmt oxtont, and in '.vlint circumstances dolomitic limestones iiv injurious to plant growth when ap- ■ liod to soils oitlior before or after burning. Trials have shown that an excess of magnesia in a soil has a toxic action on crops, hut the extent of such action depends on the type of Soil, the condition of the soil, and particularly on the amount of lime present in the ■soil with the magnesia, It may. however, be safely assumed that a stone containing up to 10 per cent, of magnesium carbonate can always be used on any soil: higher proportions of magnesium carbonate should he regarded with some suspicion where it is intended to npplv the ground stone to land already heavily charged with magnesia, (■round limestone has been particularly mentioned, for where we are dealing with burnt lime made from dolomite limestone, we are fared with a new source of danger not connected with anv toxic action of Hip magnesia, but arising from the fact that magnesian lime always tabes longer to slake and revert to carbonate' than a straight calcium line, and sometimes takes so long that the soil remains partially sterile for months after it is applied. This “ burning ” effect is particularly noticeable on light sods. When, therefore, dolomite limestone is under consideration, our conclusions will he very largely dependent upon whether the stone is to he ground or burnt; if the former, we shall probably deride to use it, making a mental note that, wo would rather it had been straight calcium limestone, but if lime is to he made from it then wo shall proceed with the utmost caution, getting a few tons burnt in a clamp or a neighbouring lime kiln as an experiment, and testing the lime before embarking on kiln building and qularry plant. Before leaving this thorny question of the magnesia content of the stone, there is one other point on which much misunderstanding exists, which ft may he useful to discuss. The calcium carbonate content of a dolomite is fre-

quently spoken of as though that alone < could have an agricultural value; this 1 is quite erroneous, for the magnesium carbonate is also capable of neutralising soil acidity, and from this point of view a stone analysing 59 per cent of calcium carbonate (C'aCo3) and 39 per | cent of magnesium carbonate (MgCO3) is not a 59 per cent limestone but a 98 per cent., neglecting its somewhat slow . er action, and the possible toxic e/l’ccL on certain soils. not only an adulterant but withdraws Another impurity frequently met with in limestone is combined silica, which can have no toxic effect, but in both the ground and burnt stone it is an equivalent amount of the calcium ! present from taking an effective part in soil neutralisation. Other impurities , i likely to be met with are iron, uncombined silica, and organic and earthy matters, none of which need be considered as they are only diluents, and usually occur in quantities too small to be of much importance. METHOD OF UTILISATION. ) Having found and analysed the stone we decided whether it will bo better to reduce it. to powder in a mill, or to burn it to quick-lime in a kiln. If the limestone contains a high percentage of calcium and the distance over which it will have to be drawn is great, and j providing there is an amply supply of j coal and wood readily available, then j burning the stone is clearly tiie right j thing,; on the other hand a dolomite is usually better applied to the soil a--1 ground stone. Between those extremes 1 there are a great number of interl mediate cases which must be decided on 1 their merits.

Tn comparing the two products it, must always lie borne in mind that one ton of quick-lime is the equivalent of a little more than If tons of ground limestone, and that consequently, in cases where a long “ draw ” is involved, there may bo ample compensation for the additional quantity of coal consumed in burning, instead of grinding (be stone. This consideration brings us to the question of the relative fuel costs involved in tlic two cases. To supply the answer in any specific case if would be necessary to know all the conditions, hut the following information will be found useful in arriving at a just comparison.

In the average small pot kiln, lime can usually he burnt with an expenditure of from 7 to lOewt of .culm per toil of lime produced In small draw kilns, where tilt" heat losses are considerably lessened by the continuous nature of the process, a fuel consumption of 5 to 7 cwt of coal per ton of lime may lie anticipated, and in large draw kilns as little as 4 ewt of coal is sometimes used. On the other hand, to grind one ton ol chalk or limestone will require about 8-10 horse-powor-hours, corresponding to th e consumption of

40-5011) of coal in an average farm steam engine or of a gallons of paraffin if an oil engine is used to drive the mill.

Although nearly double as much ground stone must he produced in ordet to b*‘ equivalent to the lime, it will be Mam that so far as fuel alone is concerned. there is great saving in cost when the stone is ground instead ol burnt. If the question is worked out on the present prices of oil and coal the above figures will show that for iuol alone, burnt lime will cost ihout H>s ■a ton, while the equivalent 12 tons ot ground stone will cost only about o s Gd for power fuel. Dunning costs and the first capital cost of the installation. however, will generally he slightly in favour of the kiln. Tin comparative cost of two equivalent plants, one to burn lime and the other to grind stone, both on a very small scale, would he in the ratio of about 1-3 in favour of the kiln. Afore skill in manipulation and greater previous experience arc needed in the case of the kiln than in that of the grinding plant. Assuming that the choice of process has hoi'ii made, wc will now consider matters connedod with limestone grinding plants of small size, such as many farmers could easily instal. SMALL (.’BINDING PLANTS.

Tlio object of all agricultural stone grinding plant is to produce ground stone or chalk in a sufficiently line state of sub-division to enable it to react readily with the constitituents of the soil and dissolve ; lienee it is important to know how small the particles of stone must he in order that their function may lie fulfilled. If the stone were all reduced to impalpable dust, the object would ho attained; that is, however, far from being a practicable proposition, on the ground ol both initial and running costs. We have, therefore. to decide, not what is absolutely the best, hut what is fine enough to give thoroughly satisfactory results in practice. Not all authorities are agreed on the point of fineness in grinding, hut there is a very general belief that a fair mean is struck between the ideal and the commercially practicable, when nearly all the ground stone will pass through a screen having ten meshes to the linear inch, and all the fine material produced in grinding is included. Tin’s product contains about 10 to lb per cent of stone which is in too coarse a state of sub-division to he immediately useful, hut t ie reason it is recommended is that the type of mill which will produce it is much cheaper, and the power required to drive the mill is so much less than -'or a similar output of finely ground stone, that there i s no doubt about the advantages attaching to the rougher method.

•Experiments have been conducted both in this country and in America in order to ascertain what is the actual size of grain below which no .appreciable improvement in fertilising value cun he detected. Although opinions did or it seems that particles which pass through a (50 x (50 screen are at any rate fine enough for all practical purposes. Tt might at first seem that ground stone passing through a 10 x 10 screen would not he fine enough to he of very much service; such a material, however, would he found in general to contain about 70 per cent small enough to pass through the 60 x 60 screen, 'flu’s is a fact not infrequently overlooked by producer., of very finely ground hut very highly priced agricultural limestone. Tn the matter of distribution there is an actual advantage attaching to the more coarsely ground produce, for it does not tend to clog and hang together like the very fine limestone dust, and for that reason

gives much less trouble in the distributor. The plant necessary to produce this comparatively coarsely ground limestone is so compact and simple that it is possible to mount it in portable form so that it can he drawn by a tractor to any point where it can be fed conveniently with broken stone. A tractor or other portable source ot power can bo used to drive it. There are many makes of machine, but the disintegrator ri tho only type of mill which can be used to fulfil these conditions In the case of the small portable machines the stone to be ground is fed into the | mouth of the mill in pieces not much I larger than 2i-iu. cubes; it then passes into a circular chamber in winch there i are two or more massive, manganese steel hammers, flexibly attached to a rapidly rotating, central shaft. The stone *is immediately broken and rebroken by percussion against both the ' swinging "hammers and the walls of tho : chamber, some part of wim-ii, generally • the lower half, is composed of stout, manganese steel bars arranged with narrow spaces between them, through which the stone dust can escape when fine enough. After leaving this screen the ~-round stone falls into a hopper whence it- can be shovelled or bagged; it is, however, better to arrange tlm portable plant so that the stone is fed into the machine almost at ground level, and delivered into a cart. This can bo readily done by providing a feed elevator to lift the finished product and

shoot it into a cart. A plant as described above would cost at the. present time about £6OO, mounted on wheels, complete with two elevators. The output 'would be about 1 V tons of ground stone per hour, and the power taken to drive the mill and jt* two elevators would not exceed about 15 horse power. It is obvious that on small estates where the initial cost of such a plant is more than the needs of one owner warrant, a cooperative scheme should be possible. Success will depend entirely upon careful selection of tho plant and attention t„ certain details, some of which will now be described.

i There are many makes of disintcgrator on the market, hut only a few are ! r eallv suitable for limestone grinding. : It is important that the swinging ham- | users should he as massive as possible, j linc i for this reason there cannot he I many of them: two are enough, any ! aiiv 'number above four is certainly too j many. Tf the machine offered has four I hammers, an opposite pair should lie j dismantled in order to determ ini whether the mill will not run easier and give the same output as before. Accessibility is an important feature, and the mill chosen must he of a pattern which can he easily opened up for examination of the grinding chamber and tackle provided to take the weight of any heavy parts not swung on a vertical axis. The screen bars must lie easily renewable', preferably in single units, not <ast up together in segments; and should he made of manganese steel like tlic swinging hammers, as no other material is tough and durable enough. ]t is a mistake to su, pose that the spacing of the screen bars is a measure of the average or largest particles which will come through; the pieces of stone in the disintegration chamber are being I driven round with a very high circumferential speed, and only when they artvery much smaller than the width of the s lots* between the bars <!<)' they find their wav through the screen. I‘or this reason it will seldom he necessary to space tin* bars nearer than 1,-in., and for grinding chalk ;t to A-in. will he suitable. The harder and more friable limestones will require the closest spacing of screens in order to give » satisfactory proportion of very fine -lust. When the mill lias become much worn •taro should he taken when renewing oid screen bars that no ridge exists between tin' old and new ones, since any cue veil ness of the periphery of the grinding chamber will cause a totally dis) roportionato quantity of large parte les to go through. The best results are obtained when the mill is run right up to the safe working maximum speed as stated by the manufacturers; loss of speed means not only smaller output, hut a less finely ground product. ,V|| varieities of limestone, it freshly quarried, i.e., not, much exposed to the weather after being broken, will he dry enough to grind without any Preparation. This is. however, far from being the case with chalk. Soft chalk will need to he dried before it is passed through the mill, or it will “ pug ” and form a paste which will effectively block up the screen and stop the machine. It is not easy to devise means for artificially drying chalk. Undoubtedly the best course to adopt, where covered storage spare is available, is to quarry the chalk and leave it stacked under cover for several months to dry before it is ground. Where there i s t.o such storage space, a drying floor is probably the simplest means, hut experience shows that it will take nearly lewt of coal to dry a. ton of chalk sufficiently to enable it to he ground. Some of the harder chalks will give much less trouble than soft chalk, such as that of the North or South Downs. Tti any given instance it is easy to ascertain whether the raw material will require to he dried, for if on taking a fair sample and drying it out completely it is found to lose more. than. 12 to II per cent of its weight, it will not satisfactorily go through one of these small portable mills without being dried beforehand.