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Volume 56, 1926
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Soils of the Rotorua County.


In the last volume of the Transactions (vol. 55, pp. 720–23) was given some account of a deficiency disease in ruminants which was proved to be caused by a lack of iron in the pasture-plants grown in the affected country, arising from the unusual soil-conditions present.

These conditions are the result of the soils having been derived from showers of material ejected from the neighbouring volcanoes or earth-vents. Where the material deposited has consisted of coarse rhyolitic pumice the soils have been of the lightest and most porous kind. These soils have not had time to become weathered and compacted by natural agencies sufficiently to enable the staple fodder plants (cocksfoot and clovers), when grown in the ordinary way on a bush burn as a surface-sown pasture, to be of sufficient nutritive value to support ruminant stock unless special treatment of the soil or animal is given. The special treatment of the soil includes (1) stumping and ploughing, thereby compacting the soil so that a given volume weighs more after ploughing than before, and thereby retaining water better; (2) applying fertilizers containing phosphates and iron; (3) a third treatment should undoubtedly be the ploughing-in of green crops (green-manuring), but experiments in this have not yet been conducted.

The special treatment of the animal consists in (1) feeding on top-dressed pasture; (2) giving extra winter feed, such as turnips and hay; (3) giving medicinal treatment when the beast shows signs of becoming anaemic; (4) supplementing the natural food of the animal, and especially that of calves and yearlings, with concentrated foods such as molasses, bran, &c. It will be argued that a good deal of what is here called special treatment is what every farmer would aim at doing on a well-ordered farm; but it must be remembered that the country is covered with forest, and that something like twenty years must elapse after a burn before the stumps can profitably be got out and the plough got in. It is this first phase of bush farming that is so fatal to the farmers on pumice lands when winter feeding is not practicable, and precautionary measures must be sought in special top-dressing of the soil, and remedial treatment in the administration of medicine supplying iron.

Where the showers have consisted of finer material, other factors—altitude, aspect, climate—being, as far as can be determined, equal, the result is a greater retention of the soil-water and the growth of a different type of forest with a distinctly different undergrowth.

When the showers have been so recent as 1886 (Tarawera Eruption), and consist of very much finer material, locally known as “mud,” there results a soil having a larger proportion of fine particles and also of calcareous matter soluble in hydrochloric acid without effervescence.

It is noteworthy that the deficiency disease in ruminants only occurs in the animals depastured on the coarsest soil and not on those of finer

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texture. A preliminary examination of the weight of a given volume of moist soil when the weights are all taken about the same time, so that each soil has had the same rainfall, seems to indicate that the incidence of iron-hunger varies inversely as the weight of the soil.

No. 1

type: Fine gravelly sand (Kaharoa). Rimu-tawa forest. 68.75 lb. per cubic foot. Very bush-sick.

No. 2

type: Sandy silt (Mamaku Farm). Rimu-tawa forest. 75.47 lb. per cubic foot. Not so sick.

No. 3

type: Sandy loam (Oturoa). Beech forest. 78.35 lb. per cubic foot. Not bush-sick after top-dressing.

No. 4

type: Coarse sand (Rotoiti). Rimu-tawa-mangeao forest. 79.40 lb. per cubic foot. Healthy to cattle; not bush-sick after top-dressing.

No. 5

type: Calcareous sandy loam (Te Ngae). Tutu and fern scrub. 85.81 lb. per cubic foot. Perfectly healthy country.

Weighings made of equal volumes of soil taken from ploughed and unploughed pasture-land side by side demonstrated that the land which had been ploughed was in every case appreciably heavier, showing that ploughing was an efficient method of compacting the soil. The above types of soils are arranged in the descending order in which bush sickness or iron starvation occurs on them. In No. 1 the trouble is so acute that the local practice is to change the stock twice a year. No. 2 is an average sick-country soil upon which the practice would be to change the animals once a year. No. 3 is the country of the Oturoa Road District on the beech forest side, which is so free from the sickness that no precautions are taken; No. 4 country is sick for sheep but not for cattle. In both of these cases (Nos. 3 and 4), after top-dressing, no difficulty is experienced. No. 5 is the well-known Te Ngae district, upon which no sickness has ever been observed. Sheep (the most susceptible animal to the sickness) rear their young without difficulty, fat lambs being one of the staple products. There is, then, a graduated series of soils given in the above list com mencing with “very sick” lands and leading gradually to those which are perfectly “healthy.”

Another fact which emerges from a study of the large area comprised in the northern sheet of the Rotorua County map is the uniformity of these air-deposited soils over wide areas. Some 330 square miles of land have been mapped in this soil-survey, and very little variation is found in each area where running water has not had an opportunity to alter the soil by sorting the particles into river-terraces. This is only what one would expect. Districts where matter had been ejected vertically and spread by air-currents over some hundreds of square miles of country in which the physiography is such that running water cannot sort out the fine from the coarse particles would naturally alter equally over wide areas and therefore present little variation. The sandy silts therefore occupy a large area around Mamaku and extend into Matamata County, so that it is impossible at present to guess at the extent of this type. A large area of this soil extends down the Patatere plateau into the Rotorua crateriform depression, where the higher lake-terraces have been overlain by the Rotorua shower. From a soil aspect the only differences between these west and south-west Rotorua terrace soils and the Mamaku soils is that the Mamaku soils are some 1,760 ft., whereas the Rotorua terraces are only from 1,000 ft. to 1,400 ft. above the sea, thus having the benefit of lower altitude and hence of a milder climate, while the natural vegetation of the Rotorua

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terraces has never been anything more difficult to “break in” to pasture than fern and manuka scrub. Hence at the very outset the farmer on terrace land had the opportunity of ploughing and compacting the soil, of manuring it with phosphates, and of growing winter feed, three very good weapons in fighting “iron-hunger.”

Pumice soils, by which is meant a soil chiefly and obviously consisting of pumice, vary greatly in their quality, which is governed chiefly by their texture. This ranges from those soils which are so coarse or so elevated that they must permanently remain as desert, heath, or forest, to those which are so fine that they may be profitably utilized in the grazing of sheep and cattle. “Pumice” is a word which is supposed by etymologists to be related or derived from the same root which gives “spume” (spuma—foam). Pumice is, indeed, a solidified foam or volcanic froth, and highly absorbent. One finds pumice in all stages of decay, of comminution, and of compaction. It had probably been well leached before being redistributed to positions in which it is now found. The final stage of decay as pumice is where the fine lumps have become so acted upon by natural agencies that the resultant mass is plastic with very little feeling of grittiness. The final stage of comminution is when the pumice is reduced to a fine sand or silt; the final stage of compaction is found in river-terraces in which the weathered and powdered material is laid down into a wonderfully fertile alluvial soil. Soils of all these grades may be found, and what may be termed good pumice lands is where the improvement has gone on to such an extent that farming may profitably be carried on without resorting to exceptional methods. These studies are for the present limited to the soils derived from the Rotorua pumice-shower. There are many areas of pumice lands derived from other outbursts, possibly of different age, origin, and composition, which may therefore not be comparable with that of the Rotorua shower: these must be studied separately in their turn. A warning should be given against the practice of speaking of all pumice soils as a class, without limitation or definition. To make general statements disparaging to pumice soils is to court disproof by hundreds of successful farmers on what may justly be called pumice lands. On the other hand, it is dangerous to extol pumice lands indiscriminately as fields for settlement, as instances may readily be given of large areas which are not suitable for settlement by the methods in common use. Seeing that there is this great diversity of soils under the term “pumice,” it will be conceded that some scheme of classification must precede an intensive study of the subject, and the initial step in this direction is exactly what a soil-survey of the Rotorua County hopes to accomplish.

In the area under discussion—the northern sheet of the Rotorua County map—the wind-borne pumice, unsorted by river or lake action, had covered a large area of rhyolitic country rock. As there is no sign of weathering or of pre-existing vegetable life at the junction of the pumice and the country rock, one suspects that the pumice fell on the rhyolite, either when recently ejected, and perhaps still hot, or while it was submerged in shallow water. One should premise the description of the Rotorua soil survey by explaining that the method of soil-classification is based on the examination of the proportion of particles present belonging to differently sized groups, each group containing particles which vary in size only within definite limits. The groups have names attached which, although in common use and therefore used loosely, in this connection are used in a technical sense and are intended to denote material of a definite

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size. Thus the groups of sizes are variously designated “fine gravel,” “coarse sand,” “fine sand,” “silt,” “fine silt,” and “clay.” The fine gravel is the coarsest and the clay is the finest fraction. The method of separating these groups of sizes in each sample is based on the British official method, but special machinery has been devised in the Agricultural Chemical Laboratory to deal with large numbers of samples (see Grimmett, N.Z. Journal of Science and Technology, 1926, vol. 8, No. 2, p. 118).

The chief types of soil in the area already dealt with are,—


Sandy silt, of approximately 113 square miles.


Sandy loam, 12 square miles.


Fine gravelly sand, 37 square miles.


Coarse sand, 120 square miles.


Calcareous sandy loam, 12 square miles.

Further work will no doubt greatly extend the area of Nos. 4 and 5 types.


Sandy Silts.—These cover a very large extent of country west and south-west of Lake Rotorua. At the altitude of the Patetere plateau the natural vegetation is rimu-tawa (Dacrydium-Beilschmiedia) forest, with the exception of a patch of country described under “Sandy loam.” Within the Rotorua Lake basin area, where the land rises gradually from lake-level (915 ft.) to 1,500 ft., the plant-covering is fern (Pteridium) and tutu (Coriaria ruscifolia), with occasional manuka (Leptospermum scoparium). The area from the west shore of the lake to the boundary of Matamata County is, with the exception of the beech-clad country of the Oturoa Road and Mangorewa-Kaharoa Block of forest reserve, essentially a soil of very definite mechanical composition, technically a sandy silt.


Sandy loam of the Oturoa Road and Mangorewa-Kaharoa Block: This is a distinct departure from the sandy silt series, and carries a forest the dominant trees of which are two species of beech (Nothofagus fusca and N. Menziesii). The underscrub is also characteristically different.


Fine gravelly sand occupies an area stretching from near the north shore of Lake Rotorua back to the Mangorewa River. This is the coarsest soil met with. It is very sheltered land, at a low elevation, and, although the land is affected with bush sickness worst of any, the proximity to healthy country round the edge of the lake, and also the milder climate, make for more hopeful results in farming. On the western edge of this gravelly sand is a wedge-shaped strip of intermediate material, ranging between a fine gravelly sand and a coarse sandy silt. The vegetation of this No. 3 area is forest of tawa-rimu type merging into that described under (4).


Coarse Sand.—To the north and east of the gravelly sand is a very large area of coarse sand which requires very detailed investigation. It is here that a detailed survey may be able to locate patches of healthy country, which will much hasten settlement. This area has been much altered by rivers, and the vegetation is the tawa-rimu type modified by the inclusion of some smaller trees requiring a milder climate than that of Mamaku, such as mangeao (Litsaea calicaris), kohekohe (Dysoxylum spectabile).


Calcareous Sandy Loam.—This is the area stretching from the eastern shore of Rotorua Lake to the shores of Tarawera Lake. It is free from bush sickness, and is very good sheep-country. The natural vegetation has been much altered by recently ejected material from the Tarawera Eruption, but is largely fern (Pteridium esculentum), with patches of rimutawa forest similar to that of type (3).

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Map of Northern Part of Rotorua County Showing Different Types of Soil

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Typical mechanical analyses of the above are as follows:—

Mechanical Analyses.
(Results are percentages on air-dried soil.)

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Laboratory No. Description of Soil. Loss on Air-drying. Analysis of “Fine Earth” passing 2 mm.Sleve. Stones and Gravel.
Loss on Air-drying. Fine Gravel. Coarse Sand. Fine Sand. Silt. Fine Silt. Clay. Moisture and Loss on Ignition.
T/525 Sandy silt 38.9 1.7 26.1 27.3 18.9 7.3 1.7{ 2.6
T/499 Sandy loam 38.7 0.8 16.3 20.9 19.8 8.7 5.5{ 9.5
T/564 Fine gravelly sand 36.4 23.2 46.6 9.9 8.7 3.5 0.9{ 1.4
T/606 Coarse sand 35.2 9.4 47.9 15.8 9.5 6.4 1.1{ 1.5
R/885 Calcareous sandy loam 31.3 4.8 21.8 28.1 21.2 9.4 5.1{ 1.4

T/525 from Section 12, Block 14, Rotorua, Mamaku-Rotorua Road; T/499 from Rotorua and Horohoro Blocks, Oturoa Road; T/564 from Dudley Road, Te Pu (fern slopes); T/606 from Whataroa Road, Ngawaro; R/885 from Wairoa Road (cultivated, clover).

This account does not pretend to exhaust the descriptions of soil-types occurring in this county, but aims at describing the predominant soils which are recognized in the course of a reconnaissance survey.

The work at the Mamaku Demonstration Farm has proved beyond doubt that the citrate of ammonium and iron is a cure for the deficiency disease now known as “iron-hunger,” when the animal is decidedly affected. No food such as molasses, bran, or other concentrated foodstuff has been successful in curing such a beast. Molasses has been found to be of great assistance in the rearing of healthy calves. Experiments at the State Farm have demonstrated that it is possible to raise cattle on the farm and keep them free from deficiency disease. Top-dressing the pasture with phosphates has increased the iron content of the pasture when compared with that grown on unmanured parts. The best dressing for grassland is found to be a mixture of equal parts of superphosphate and basic slag, but all kinds of phosphate give a good and immediate response.

Ill-treatment of stock will, of course, predispose them to suffer from iron starvation more than when they are properly treated. Thus, as is pointed out by the writer (“Mineral Elements in the Feeding of Stock,” N.Z. Jour. Agriculture, vol. 31, p. 351, Dec., 1925, and vol. 32, p. 78), a good supply of pure water is essential to success with cattle. A deficiency of drinking-water will interfere with the animal's digestive and other functions, and so weaken it that the beast becomes more readily affected by any other adverse conditions. Exposure to cold is also a similar weakening factor, proving the necessity for rugging cows and giving other shelter in cold or stormy weather.

An examination of the country involving a traverse along the main roads, and much careful laboratory-work, show that the soils may be clearly separated into distinct types which are uniform over wide areas. If the

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physical composition only of some of these types is considered in the light of the experience of other countries it will be found that they would be considered too coarse for profitable occupation. Thus the composition of the fine gravelly sand and coarse sands is such that they approach that class of soils in England stigmatized by Hall (1911, Jour. of Agricultural Science, vol. 4, p. 209) as too coarse for cultivation, and therefore abandoned to waste lands and commons. In America the Miami (New York) stony sand, gravelly sand, and gravel are not of much agricultural value under present conditions. The three factors which render Hall's dictum inapplicable to the Rotorua type are (1) the greater rainfall of Rotorua; (2) the porosity of the particles of pumice; (3) the rich chemical composition of the particles and the capacity of weathering down and becoming compacted with cultivation converts them, under the influence of plantlife and the copious rainfall (almost double that of England or New York), into fertile soils.

In conclusion, the author considers that the results of the investigation encourage the hope that the pumice lands of the Rotorua County may in the future prove of great agricultural value to the Dominion.

The following papers in the New Zealand Journal of Agriculture should be consulted for the full details of the investigation: “A Reconnaissance Survey of Pumice Soils, Rotorua County”—1, Mamaku, vol. 29, p. 333, Nov., 1924; 2, Rotorua Basin, p. 369, Dec., 1924; 3, Kaharoa and Te Pu, vol. 30, p. 1, Jan., 1925: “Iron Hunger in Ruminant Stock,” vol. 30, p. 175, March, 1925. Also, in the New Zealand Journal of Science and Technology, vol. 7, p. 216, Nov., 1924, a paper on “Iron Starvation on the Patetere Plateau.”