VOLCANIC SOILS OF THE CENTRAL NORTH ISLAND TERRITORY.

New Zealand Journal of Agriculture, Volume 43, Issue 3, 21 September 1931, Page 201

VOLCANIC SOILS OF THE CENTRAL NORTH ISLAND TERRITORY.

A lecture entitled “The Volcanic Soils of the Central North Island Territory and their Agricultural Utilization,” was delivered by Mr. T. Rigg before the Wellington Philosophical Society on 26th August, 1931The following summary of the lecture is supplied by the Department of Scientific and Industrial Research: —■

Although it has been known for many years that large deposits of volcanic ejectamenta cover enormous tracts of country in the RotoruaTaupo zone, it is only during the last year that a full realization has been arrived at of the very widespread nature of volcanic deposits throughout the whole of the central North Island territory. From the work that has been done in the course of the soil reconnaissance survey during the last twelve months it has been ascertained that volcanic deposits cover the whole country from Kawhia Harbour and Mount Egmont on the west coast, to Taurangaand Whakatane on the Bay of Plenty, and Napier on the east coast. In particular districts the ash coating has been removed by erosion or by slumping, and on the river-flats extensive resortment of ash from different sources has taken place.

It is now clear that three great centres of volcanic activity were concerned in this widespread distribution of ash. These centres were located in the Rotorua zone, the Taupo zone, and Mount Egmont. In the case of volcanic activity in the Rotorua and Taupo zones, it. is evident that several vents were responsible for the distribution of volcanic material. In the Taupo zone the more important vents were situated in or near Lake Taupo, but Mounts Tongariro and Ngauruhoe also have contributed to the covering of ash over particular areas of country. Volcanic activity has been taking place over a very long period, and it is thought that even in Miocene times eruptions on a vast scale took place. At such widely separated points as Napier and Te Awamutu deposits of volcanic material 15 ft. to 50 ft. in thickness can be seen. An examination of these deposits shows that a number of separate showers of ash were deposited. In all probability, great periods of time frequentlyelapsed between the deposition of the various showers that are visible in these sections. An examination of the deposits near any great centre of activity shows that a number of eruptions were of restricted distribution and are confined to the country in the vicinity of the vent. Of the volcanic deposits which are responsible for the soils of the central territories, the most extensive deposit is one that has come from a vast eruption near Lake Taupo. Material from this' eruption, which apparently took place in comparatively recent times, covers the whole country from Te Kuiti and. Taumarunui in the west, and extends almost as far as Napier in

the east. Waiouru in the south, and Rotorua in the north, mark the limits of well-defined deposition in the. north and south.- Eruptions from Mount Egmont are responsible for the soil-types covering a great part of Taranaki, extending almost as far as Ohakune in the east, and probably as far as Ohura and the Mokau River in the north. Between the Mokau River and Te Awamutu the principal soil-covering is derived from ash from another source. This ash appears to be much older than that from the eruptions of Mount Egmont and from Taupo. The same ash in the district between Te Awamutu and Putaruru contributes largely to the soils of this district, but there appears to be a superficial coating of ash from an unknown source which materially influences the characteristic properties of these soils.

Ash from vents in the Rotorua centre covers large tracts of country between Rotorua and Tauranga and Whakatane. The eruption of Mount Tarawera in recent times covered, in its turn, earlier deposits from the Rotorua centre lying between Mount Tarawera and Opotiki. Ash from the Ngauruhoe eruption ■ covers a somewhat restricted area in the vicinity of the mountain, and overlies the Taupo ash. Volcanic ash from Mount Tongariro is almost entirely covered by ash from the last Taupo eruption, but a considerable area of soil between Waiouru and Taihape has been identified as ash from the Tongariro eruption.

Examination of the different volcanic deposits has revealed striking differences in the chemical and textural properties of the soils derived from them. Chemical analyses show that ash from the Egmont, Tongariro, and Ngauruhoe eruptions is andesitic in origin, and is well supplied with lime and other bases. The phosphoric content of ash from these eruptions is notably high. The chemical analyses suggest that the soils derived from the weathering of these ash deposits are of high potential fertility. The rhyolitic deposits of Taupo and the Rotorua centre, compared with soils derived from the recent Egmont and Ngauruhoe deposits are low in lime and phosphoric acid. The ash coating in the Te Kuiti. district probably is andesitic in origin, but great leaching of lime and other bases has taken place.

Although textural changes in the soil derived from any particular ash deposit occur in traversing the country from the eruptive vent to the limit of deposition, it is surprising how uniform the texture is in particular localities from any one volcanic deposit. The texture, however, of soils derived from different deposits, varies widely from loam to gravelly sands.

The great chemical, and textural. changes which are associated with the volcanic deposits very clearly point to the desirability of very detailed laboratory examinations in order to determine more accurately the distinctive properties of the volcanic soils. This is a matter of great importance to the development of agriculture in the volcanic territory, as farming experience already has shown that difficulties are frequently encountered in connection with the grazing of stock on established pastures.

A particular type of stock ailment affecting sheep in the country west of Te Kuiti in recent years has occasioned much concern to farmers in this district. Investigations carried out by Mr. B. C. Aston under the auspices of the Department of Scientific and Industrial Research have shown that the pastures contain low amounts of lime and phosphate. Treatment with mixtures of lime and superphosphate, accompanied by close grazing of the pastures, has resulted in great improvement in the health of stock on certain farms, and also has greatly improved the carrying-capacity of the land.

The incidence of stock-ailment* in the country to the west of Te Kuiti must not be confused with bush sickness, which has presented such great difficulties to the extension of agriculture in certain areas covered by Taupo and Rotorua rhyolitic deposits. Bush sickness in its acute form has been noticeable in particular localities from the moment when the land was broken in. Generally, bush sickness has diminished in severity under ordinary

farm-management. The reverse appears to be the case in connection with the “ dopiness ” affecting sheep in the districts west of Te Kuiti.

The long and careful work of Mr. B. C. Aston for the. Department of Agriculture has shown that typical bush sickness on volcanic soils is associated with soils of a somewhat coarse texture. As a result of pasture and soil analyses a deficiency of iron in the feed of stock has been suggested as the cause of bush sickness. It has been demonstrated that administration of iron drenches and the provision of iron licks effects a cure or greatly reduces the incidence of the ailment. :

During the conduct of the present soil survey the observations that have been made show that bush sickness is very definitely associated with particular volcanic deposits, and is not common to all the soil-types derived from ash-deposits. The ryholitic ash from the Rotorua centre, covering the country north of this centre to the Bay of Plenty, and the Taupo deposit of widespread distribution, are very definitely associated with bush sickness. The ailment occurs at such widely separated points as Mamaku, Ngaroma, and Kopaki, which are all located on the Taupo deposit.

Investigations conducted by the Cawthron Institute, at Glenhope, in the Nelson territory, have shown that a similar form of stock ailment occurs on certain granite soils. The administration of ferric ammonium citrate has given great improvement in the condition of sheep, clearly pointing to a close similarity to bush sickness in the volcanic soils of the North Island.

An examination of healthy and unhealthy pastures in the Glenhope district does not support the theory of low iron content of pasture being the sole cause of the stock ailment. There is good reason to suspect that the iron in soil particles digested along with grass provides a portion of the iron requirements of stock. On healthy soils the percentage of iron and its availability is much higher than on bush-sick soils. In the case of weathered soils a relatively large proportion of the available. iron is held by the clay and silt fractions. This explains why bush sickness is not found in any acute form on loamy soils. A chemical examination of bush-sick soils shows that they all have a low content of iron, and that relatively little of this iron can be extracted by weak acids. The evidence points to the incidence of bush sickness on those pastures where soil .contamination of the grass is not extensive, or where the soil particles contain relatively little iron which, can be extracted by weak acid solutions.

In connection with the development of volcanic soils, the whole area of country covered by the Taupo deposits must be regarded with some suspicion in view of the incidence of bush sickness at at least three points widely separated. The success that has been obtained by the use of iron licks in combating this ailment, however, gives great hope for the eventual development of intensive agriculture on large areas of the Taupo deposit. In selected localities in this sector the carrying-capacity of pastures sown to English grasses and clovers and top-dressed with superphosphate, has been notably high, and if the bogey of bush sickness could be definitely removed there is little question that very large areas of the Taupo deposit could be successfully farmed. •

In concluding his address, Mr. Rigg stated that although the Soil Division established by the Department of Scientific and Industrial Research had been in existence for only a little more than a year, very valuable information, had been secured. The different volcanic deposits had been identified,, the approximate limits of deposition had been ascertained, and the combined work of geologists and chemists of the Department of Scientific and Industrial Research and of the Cawthron Institute already had. resulted in new information of great value to farmers in the volcanic territory. Much further work, however, was required in order to enable the greatest use to be made of the great areas of unoccupied country which presented such great possibilities for agriculture.