STUDIES IN NEW ZEALAND SOILS.

New Zealand Journal of Agriculture, Volume XXVI, Issue 6, 20 June 1923, Page 329

STUDIES IN NEW ZEALAND SOILS.

THE MICA-SCHIST SILTS.

B. C. ASTON,

F.I.C., F.N.Z.Inst., Chemist to the Department

WHEN the traveller through Centra] Otago alights from his coach his first action perhaps, is to shake himself free

from the , millions of spangles of mica-schist which have settled on his clothing during . the journey. He sees the mote in the sunbeam here glisten and flash with. a new beauty of its own. These particles differ altogether from ' the dust of other journeys, being tabular or lamellar in form instead of irregular. This entirely novel experience impresses the observant visitor with the great - difference between the type of soil over which he .is travelling and that more weathered and mixed type near the east coast.

In the plains and river-terraces of this area there have been sorted out, from the harder portions of the metamorphic schists which comprise the country rock, the' softer portions, and these have been deposited in ’ the old lake-basins and river-terraces, as one would expect from the shape of the particles, in the - form of a tightly packed soil. These soils respond to cultivation and irrigation in a marvellous manner.

The characters which this micaceous rock confers on the resultant soil are quite as important chemically as physically ; for it would seem from the limited experience which one may derive from the few soils

analysed that wherever such soils are found in the South Island they exhibit the same excellent chemical characters. When the micaschist is carried away from the arid portion of Otago by rivers such as the Taieri and the Clutha, and deposited nearer the coast, forming plains and river-terraces under more favourable conditions as regards rainfall, the diluted soils are extremely fertile. Where the micaschist soil is found in wet districts the soils are likewise extremely fertile, after the land has been stirred and the physical state due to the packed condition improved by liming and green or stable manuring. The fertility of mica-schist soils in New Zealand has been remarked by many, but the cause has been attributed to either the lime or the potash they contain. Thus Captain Hutton, F.R.S., in 1875 (“ Report on the Geology and Goldfields of Otago,” page 95), stated that “ the soils of Otago taken as a whole are decidedly above the average in quality, and this appears to be owing to the great extent of mica-schist exposed at the surface, the decomposition of which has supplied more or less directly almost all the soil in the province. That this schist contains a considerable amount of lime is proved by the incrustation in nearly all the caves in it ; and the good quality of the soil derived is well seen in the Dunstan district, which is remarkably fertile when irrigated.”

The truth appears to be . that the fertility of the mica-schist soils of Otago and Westland is due not to the lime or to the potash or to the total phosphoric acid they contain, but to the comparatively large amount of available phosphoric acid present, a point of great theoretical and practical importance. Available phosphoric acid has a very great influence on the growth of the young plant, stimulating the growth of the roots in a remarkable way, a property which is very valuable when utilized in raising plants whose root-systems it is desirable to develop, such as those of the turnip, potato, and mangold. Available phosphates also hasten the ripening of a plant, and for this reason are valuable as a dressing for all cereals. The fact that available phosphate is abundant in Otago soils but deficient in most New Zealand soils, especially those of the North Island and northern parts of the South Island, suggests that some day Central Otago, with its splendid summer climate and irrigation possibilities, may become the granary of New Zealand.

The proportion of readily available phosphoric acid present in mica-schist soils is about two or three times and sometimes even as much as four times the quantity that one finds in an ordinary fertile soil. Thus, assuming that an ordinary soil yields from 0-015 to 0-02 per cent, of phosphoric acid when it is agitated with a i-per-cent. solution of citric acid for twenty hours (B. Dyer’s method, Hall’s modification), a micaceous soil may contain 0-04 to o-o8 per cent. This amount would be equal to a dressing of 8 cwt. to 16 cwt. of phosphoric acid per acre, which it would take from 2| to 5 tons of ordinary superphosphate to supply. This will give some idea of the potentiality which lies hidden in the glittering dust of these micaceous lands.

It must not be supposed that this type of soil is exceptionally rich in phosphate only when it occurs in arid districts. It is as rich when it occurs on the very wet western coasts of the South

Island. Neither must it be thought that the percentage of total phosphoric acid which occurs in the parent rock is exceptionally high, for it is not. ' There is either something in the lamellar method of weathering which enables the phosphate to become available, or, alternatively, the metamorphism pressure and heating — which the rock has been subjected has made the phosphate available, acting perhaps as the furnace acts in smelting the ironore . in the Bessemer process, when the slag of the ore becomes converted into a fertilizer in which 80 per cent, of the phosphate is available. It would seem, therefore, that while the proportion of available to total phosphoric acid in ordinary soil is about io per cent., in a mica-schist soil it would amount to from 25 to 30 per cent, of the total present.

The experience of the mica-schist type of rock in other countries is rather contradictory. Primrose McConnell (England) in “ Agricultural Geology (1902), states that mica-schist crumbles down with comparative ease and gives a soft friable deep layer of rich soil. On the Breadalbane chain of hills (Ben Lawers) this is exemplified by the intensive flora of alpine and Scandinavian character, rich and well developed in contrast to the poor appearance on the granite of Braemar and Ben Nevis. The pines and larches are magnificent on the decomposed mica-schists of the Highlands generally, but are pool' stunted sticks' on their cold clays of granite. On the Kingsbridge estuary, in Devon, there is a flourishing vegetation on this rock, including orange and lemon growing in the open air where sheltered from the south and south-west winds. In Ireland the rich green pasture and good “ heath ”, land on the same formation near Knocklayd and Ballycastle, County Antrim,, may be noted.

Hilgard, the great American soil chemist, in “ Soils ” (1906), states that mica-schist, being a mixture of quartz and mica only, not only weathers. slowly, but also supplies but little of any importance to plants in soils formed from it. Such soils would be mostly absolutely barren but for the frequent occurrence in this rock of accessory minerals that yield some substance to the soil. Yet it remains true that as gneiss and mica-schist are among rocks in which mineral veins most commonly occur, the proverbial barrenness of mining districts is frequently traceable to these rocks. The difference between American. and British opinion regarding the value of mica-schist as a soil-former is remarkable, but probably has some simple explanation, such as the amount of phosphate which is present. There are, moreover, several distinct kinds of mica, which vary greatly in the rate at which they break down or weather in the soil. It is not to be inferred from this that the phosphate is necessarily included in the mica ; but wherever mica occurs in South Island soils it appears to be an indication of the presence of . available phosphate in good quantity.

H. W. Wiley, in “ Principles and Practice of Agricultural Analysis ” (1906), points out that the schists include an extremely variable class of rocks of which quartz is the prevailing constituent, and which as rocks are deficient in potash and other important ingredients , of plant-food.

. Rastall, in “Agricultural Geology” (1916), suggests that such rocks yield soils of all kinds, their character being dependent largely on the climatic conditions. Where these are favourable the soils yielded by the rocks are often very fertile. He attributed the comparative barrenness of these formations in Britain, which occur among the ’ Highlands of Scotland, to the climate and elevation of those parts. In many tropical regions where conditions favourable to the weathering of such rocks exist great fertility results.

In' the accompanying Table 1 are given some analyses of micaschist soils made in this Department's chemical laboratory from time to time. They comprise details of the composition, of ‘ soils

(Nos. 1, 2, 3, and 8 from Dominion Laboratory reports.)

from the wet Westland and the arid central district of Otago. Included also are the results yielded by a remarkably fertile soil from Maruia Plains, near Murchison, in the south Nelson district. Dr. Henderson, of the Geological Survey, who collected this sample of soil, informs me that it is derived from a mica-schist rock. • A composite made up of samples of soil from Roxburgh (two), Beaumont, Naseby, Sutton, Manuherikia, Maniototo, St. Bathans, and Frankton, all in Central Otago, is given under No. P/278. The analysis of this composite sample bears out the opinion expressed by Professor Park in Bulletin No. 2 of the Geological Survey, that “ the soil possesses a latent richness that the casual observer would hardly suspect. By the application of water the wilderness is transformed into fruitful gardens and prosperous farms. The conservation and distribution of water for irrigation purposes will in time convert the plains and valleys of Central Otago into one of the most prosperous agricultural districts in New Zealand.”

Some analyses of unweathered mica-schist rocks are given in Table 2, from the Geological Survey Bulletin No. 6, and accompanying them are two samples, “ C ” and “ D,” of American mica-schist rocks from Clarke's “Data of Geo-Chemistry.”. Analyses of other schist rocks from Central Otago unfortunately lack the requisite, information and are therefore not given.

These mica-schist soils often show. a deficiency in nitrogen, and are all deficient in organic matter. In the wet districts the available potash is often deficient, although present in very great quantity in an unavailable state. ' The treatment of such soils with quick or caustic lime has been suggested by the writer as a remedy, in the bulletin “The Wire-basket Method of. Testing Soils” (1907). The available phosphoric acid is, as has been stated, present in great excess, but the total is sometimes present in quite moderate amounts. The happy position of Otago in the matter of available phosphate in the soil, compared with other parts of New Zealand, is summarized in the New Zealand Official Year-book for 1913, page 549, and in this Department’s Bulletin No. 48, “ Phosphates,” by the writer. Only 3 per cent, of the soils from Otago analysed were found to be deficient in available phosphate, whereas the percentage deficient in Wellington was 33 and in Auckland 65. . ■

New Rabbit ■ District. — The Waitotara Rabbit District has been constituted for the purposes of Part II of the Rabbit Nuisance Act, 1908.

Rabbit- Control and, Trapping.—Regulations under the Rabbit Nuisance Amendment Act, 1920, relating to the destruction of rabbits in the Mangapiko and Kawa West Rabbit Districts, have been gazetted. These provide that every owner of land who is served with a notice to destroy rabbits in terms of the Rabbit Nuisance Act, 1908, must do so either by laying poison or by the fumigation or filling-in of burrows. Where it is desired to substitute other means of destroying the pest, application for permission to do so must first be made to the Rabbit Board, whose decision is final. Further, within seven days of . service of the notice, all trapping on the land must cease for a period of six months. The penalty for a breach of the regulations is a fine not exceeding Zio. The full text of similar regulations governing the Hurunui Rabbit District was published in the Journal for November last (page 315).

' . Note.-—In H 572 potash by hydrochloric acid is 0-24 per cent, and potash by hydrofluoric acid 3-09 per cent.

Results, except *, are percentages on soil dried at 100° C. Laboratory No. Locality. Volatile Matter.. - Pl <D bJD O Total Nitrogen. i-per-cent. Citric-acid Extract, Dyer’s Method, Hall’s Modification (Available Plant-food). Hydrochloric-acid Extract (Total Plant-food). 6 *05 <°o • O • * 0 H. i-per-cent. Citric-acid Extract, Dyer’s Method, Hall’s Modification (Available Plant-food). Hydrochloric-acid Extract (Total Plant-food). J d 0 <0§O .9 is * 2 JO a a -2 •S <« Magnesia, CO X ■ <D O Pl MgO. “O 3 cq P-i Phosphoric Acid, P2O5. J 2 jo d O O s £ x: ■ SOrt O -t-> oq oW £* o9> o!2 Ph 0 Phosphoric Acid, P2O5. .is of <3 0 d M oi s ojQ -4-> <M O fid M 0 0 « o!S L 295 B 944 B 1037 J 11 J ' 77 P 278 B 848 E 1233 H 570 H 572 J 306 J 307 Earnscleugh, Clyde Cromwell Kurow .. • Alexandra Maniototo Composite, Central Otago Stirling . . Maruia Plains, Murchison' Kokatahi Hokitika Koiterangi 2-48 0-64 1-70 32-00 4-52 1-92 o-74 2-o-54 .4-60 o-34 15'95 2-4-62 4-70' 1-48 4-02 i-45 0-140 0-080 0-250 0-056 0-189 0-254 O-IOO 0-152 0-182 0-084 0-150 0-056 0-197 0-055 0-014 O-OII 0-018 0-008 0-031 0-037 0-015 0-014 0-013 0-006 0-016 0-012 0-052 0-069 0-058 0-064 0-041 0-031 0-072 0-050 0-056 0-080 0-056 0-087 0*35 0-41 0-90 1-23 o-74 1-02 0-38 I-OI 0-85 •1-02 o-8'o 1-26 0-20 0-16 0-16 o-66 O-IO 0-58 o-io 0-24 0-27 0-09 0-19 0-20

TABLE 1. —CHEMICAL ANALYSES OF MICACEOUS SOILS.

Constituents. (1.) (2.) (3-) (8.) C. D. Per Cent. Per Cent. Per Cent. Per Cent. Per Cent. Per Cent. Silica (Si 0 2 ). .. 68-09 70-18 71-93 70-18 70-76 64-71 Alumina (Al O 3) .. .. .. . 14’34 14-15 13’39 16-26 14-83 16-43 Ferric oxide (FeOs) ’.. ' .. 0-16 0-28 ’ 0-12 ' 0-46 1-46 1-83-Ferrous oxide (FeO) 4-39- ’ 3-53 . 3-31 2-84 3-09 ■ 3-84 Manganous oxide (MnO) 0-26 . 0-32 0-36 0-27 Trace. Lime (CaO) ■ .. ... 1-48 . 1’2-1 2-45 2-87 0-36 0-08 Magnesia (MgO) .. .. .. x-57 I’OI 0-91 2-58 ■ i-99 2-97 Potash (KO) 3'23 2-48 2-20 - 0-78 3-50 5-63 Soda (Na 2 O) 2-90 4’95 ■ 4-09 o-43 0-47 O-II Titanium oxide (TiO 2 ) o-66 0-46 o-49 0-70 o-33 0-72 Carbonic anhydride (CO 2 ) o-6o 0-08 Loss on ignition (excluding CO 2 2-09 1-09 ■ o-8o 1-84 2-79 3-ro Phosphoric anhydride (P 2 O5) .. . 0-28 0-17 0-18 0-32 . 0-26 0-02 Total .. 99-45 100-43 100-23 99-61 99-84 99'44

TABLE 2. —ANALYSES OF MICA-SCHIST ROCKS IN THE KOKAT WATERSHED, WESTLAND. (1.) Schist from Jumbletop watershed of Toaroha 'River. (2.) Quartz-mica-schist from Toaroha Gorge. (3.) Mica-quartz-schist from Mikonui River. (8.) Schistose rock from Mikonui River. "C” and “D,” American analyses quoted by Clarke in “Data of Geo-Chemistry” (1908).