ECONOMIZING PHOSPHATES.

New Zealand Journal of Agriculture, Volume XVIII, Issue 2, 20 February 1919, Page 79

ECONOMIZING PHOSPHATES.

THE USE OF LIME.

By

B. C. ASTON,

F.I.C., Chemist to the Department.

Now that the price of fertilizers has soared so high it seems almost out of place to attempt to discuss their respective merits. Moreover, some kinds of fertilizers, such as potash salts, are altogether unobtainable, and others are so scarce that with certain limitations the choice of the farmer as to the kind he will use is extremely limited and guided by what he can purchase in a very uncertain market. Hence, although lime cannot replace phosphate, which will always be required for certain purposes —notably, for the stimulation of the turnip —lime in one or other of its forms is such a great and general ameliorator for all soils, and deposits of limestone are so generally distributed throughout New Zealand, that it seems only natural that we should turn to lime to maintain and increase the productiveness of those lands which are threatened by the fertilizer shortage. The word “ lime ” is commonly used in two senses, having either a general or special application. In the general sense it. is used to denote many compounds of lime, such as carbonate, sulphate, oxide, and hydrate. All of these are white compounds of earthy texture when in the powdered or in the amorphous (non-crystalline) form, and their external character and feeble solubility in water are such that lend themselves readily to the general designation “ lime.” This has the sanction of usage, and there is also the fact that they all contain calcium oxide, combined, however, with acids or water to form totally different substances from lime properly speaking. If we want to speak very accurately and to limit the word “ lime " to one body of definite chemical composition we must apply it only to the oxide of calcium, also popularly called “ quicklime,” “ shell-lime,” “ roche-lime ” (or “ rock-lime ”), and “ cob-lime," and retain the word “ limestone " for the commercial substance which contains the lime in the uncalcined state and therefore present as carbonate of lime. It is preferable to use the term “ ground limestone ” when speaking of this substance. To call it “ carbonate of lime ” is somewhat hyperbolical. Although it contains carbonate of lime or, more properly speaking, calcium carbonate, many , samples on the market contain as little as 60 to 70 per cent., the balance being clay or other siliceous matter In any case it is desirable that chemical terms may be restricted to substances which are fairly pure chemical products, otherwise if the farmer learns to call such stuff as a 60-per-cent. ground marl carbonate of lime he may also learn to pay the price of carbonate of limein the same way as he would if he asked for sodium chloride when he only wanted common or rock salt.

Lime or calcium oxide is the oxide of the metal calcium, in the same way as rust is the oxide of the metal iron when the latter is combined with the oxygen of the air. Calcium is one of the most abundant metals contained in the crust of the earth. Clarke estimates

the average percentage of iron present in the known terrestrial matter as 4-43, but calcium is present to the extent of 3-44 per cent. Hence it might be thought that there is no necessity to worry about adding more lime, but calcium is largely present as very refractory insoluble silicates, and where it is present as the available carbonate it has generally been washed out of the surface soil owing to its ready solubility in soil-water containing carbon dioxide, better known as carbonic-acid gas. It can thus easily be seen that a country with such a copious rainfall as New Zealand must, generally speaking, need liming if it is accepted that the soil must contain lime in the.state of carbonate. Even if the soil rests on limestone rock the leaching has been sufficient when continued over hundreds of years either to remove all the lime carbonate from the top soil or to convert it into insoluble compounds comparatively unavailable as plant-food. '

Attention may now be briefly directed to phosphates. . The theory of their fate in the soil, which is supported by much direct and indirect evidence, is that when a phosphate becomes soluble in the soil-water and it must become soluble in order to be absorbed by the root-hairs — it becomes finally precipitated by the other metallic constituents of the soil, which are either calcium, iron, or aluminium In the case of calcium it precipitates a phosphate which is easily redissolved, but in the case of iron or aluminium the compounds formed are unavailable or soluble with difficulty in the soil-water. If, however, there . is , an excess of calcium carbonate in the soil the phosphate will combine with this in preference to iron or aluminium, and this is another reason for keeping a supply of carbonate of lime in the soil. It will now be obvious that to obtain the most economical result from phosphate it must either be soluble in water or else in the case of phosphate insoluble in water it must be ground extremely fine. -This,is what is found in actual practice. In America it has become quite a well-established practice to' apply insoluble phosphates . as long as they have been ground to an almost impalpable powder, and this practice has been largely adopted in New Zealand.

It has always appeared to the writer that fineness of grinding in respect to limestone was not such an important matter as the fine grinding of phosphates, and this idea has been set forth in the Journal from time to time. It seems fairly obvious that there is not the same necessity for finely grinding a substance which becomes more soluble in the soil-water as there is for grinding one which becomes more insoluble under the same conditions. Accordingly we find a well-known American agricultural chemist advocating the use of a ground limestone which passes a | in. sieve and contains all the finer particles which have been produced in reducing the stone to that state of subdivision.

There are in New Zealand so many deposits of soft marly rock, or easily fractured hard limestone which on quarrying easily breaks down into small fragments, that the writer has in the past advocated using these in the same way as marling or chalking is conducted in England that is, by distributing the rough quarried stone on the land, leaving to the weathering influences of the atmosphere and the soil, and to a less extent the tillage operations of the farm, the task of reducing these rough particles to the state in which they may be made available for their work in the soil. Successful experiments have been carried out in furtherance of this idea on the poor gum-lands near Albany, Auckland,

where a soft marl containing 70-80 per cent, carbonate of lime, dug from a roadside cutting at a cost of a few pence per ton royalty in addition to the cost of cartage and labour, was used with striking results. (See January, 1916, Journal, p. 10.) The matter of ground limestone was discussed in an article in the Journal for April, 1914 (see also a note in the issue of November, 1914, p. 328), where it was suggested that the screenings from limestone quarries should be used on the land. On visiting the Mauriceville lime-works the writer found a large dump of rejections from the quarry. Similar material is also to be obtained at Te Kuiti, and no doubt at other quarries. The labour in handling and carting such rough moist material is at present the only bar to its use.'jThe pasture top-dressing

experiments conducted at the Wallaceville Laboratory Farm, as recorded in last month’s Journal, supply a convincing record as to the value of limestone used in the rough state.

CONCLUSIONS.

The outstanding deficiency in New Zealand soils, which has been proved by agricultural practice and also by chemical analysis, is that of phosphates —nitrogen and potash being usually present in fair amount, and addition of these fertilizer constituents to ordinary crops not having usually proved remunerative.

Enormous quantities of phosphates have been imported and used, especially on the North Island lands, during the past decade or so. The practice still survives as strongly as ever, but now neither the quality

nor the quantity of the phosphates are so easily or so cheaply obtained, on account of the war. These phosphates have only in very small part been taken off the land in the shape of crops or stock, and they have not been leached out of the soil, as that is impossible. There is only one explanation of the disappearance ' of the effect of the phosphate after a few months which renders it necessary to apply a fresh dose, and that is undoubtedly that the phosphate is changing in its character from an available to a less available form. Is there any way in which this change can be prevented and the full effect of the whole phosphate as applied by the farmer' obtained in crop or stock ? In the writer’s article in the Journal for April, 1914, he stated, Probably the most important function of. carbonate of lime on New Zealand soils (especially those deficient in phosphates), which are 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 the 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 superphosphate are applied, lime prevents the formation of the unavailable phosphates.” The 'great American soil chemist, Hilgard (see Journal just quoted),, pointed out long ago that where carbonate of lime was in excess phosphates might be present in very small amount, and yet more satisfactory returns be obtained from the land than when the converse was the case.

In New Zealand there is a great dearth of phosphate deposits, but an abundance of limestonesoft and hard—also of water-power. The inauguration of a comprehensive water-power scheme for the Dominion by cheapening the cost of grinding limestone will enable larger quantities to be used and thereby render less phosphates, necessary. The utilization of hydro-electric power for grinding limestone was suggested by the writer some years ago (see Journal for November, 1915, p. 393). . • It also seems plausible to predict that with the advent of cheap hydro-electric ’ power in districts where the roads are good—and the roads may be improved by similar methodselectric traction will play a part in cheapening limestone to the farmer; that by the employment of electrically driven lorries provided with suitable distributing apparatus the ground limestone may be delivered from the works to the fields in one operation, thus eliminating the present great cost of handling, bagging, and railage..

* In a heavy clay soil difficult to work probably quicklime would be more economical to use than ground limestone in the first instance.

Elgrass.— Very favourable reports . having been received . as to the immense amount of fodder , produced by this plant (also named “Napier’s Fodder-grass the- Department has obtained a supply of roots from the Grafton Experimental Station, New South Wales, through the courtesy of the Director of Agriculture of that State. The grass will be tested at Ruakura and at the Albany experimental plots.