Volume 32 1923 > Volume 32, No. 126 > The Maori gravel soil of Waimea West, Nelson, New Zealand, by T. Rigg and J. Bruce, p 85-93
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DURING the progress of a soil survey of the Nelson district, attention was drawn to certain areas of so-called Maori gravel soil. This soil was prepared by the Maoris for their crops, presumably kumaras (sweet potatoes), before the occupation of these lands by the Pakeha. Examination of the soil in the field and in the laboratory has elicited several facts which will be of interest to students of the Maori race.


The Maori gravel soil occurs in well-defined areas, on both sides of the Waimea river. The main stretches occur in Waimea West where there are 800 acres of the soil. On the eastern side of the Waimea river there is another large area of the soil, covering about 200 acres. Smaller patches occur along the banks of the Wai-iti river and Eve's creek. The total area of Maori gravel soil in this part of the Nelson district is more than 1,000 acres. The accompanying map shows the location of the soil and the extent of the various areas.


Maori gravel soil, as its name implies, contains a large percentage of fine gravel and coarse sand. This makes the soil easy to cultivate and frees it from standing water, even in the wettest weather. The soil is conspicuous by its dark colour and the presence of small stones and coarse sandy particles, in fairly constant proportions. The soil is now used by farmers for general farm crops which are invariably good and equal in yield to those obtained from any soil in the Nelson district. Considering the sandy nature of the soil this is surprising, particularly as other sandy soils in the vicinity are remarkable for their poverty. The sandy soil of Rabbit Island may be cited in this connection. In texture it bears a close resemblance to the Maori gravel soil, but it is, nevertheless, so poor, that even tea-tree (Leptospermum spp.) fails to grow on parts of the island. Another feature of interest is the fact that while Nelson soils generally respond to liming and the application of phosphatic manures, the Maori gravel soil shows little response to this treatment for ordinary farm crops.

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In order to ascertain the reason for the high fertility of Maori gravel soil, several samples were obtained from different parts of Waimea West and analysed. The following table shows the amounts of “available” plant food contained in Maori gravel soil, as well as other soils of the district.

Percentage of “available” 1 Plant Food and “Lime Requirements.” 2
Soil. Phosphoric Acid (P2O5) Potash (K2O.) Lime Requirement.
Maori gravel 1 ·042 ·019 ·11
Moutere loam ·003 ·014 ·32
Waimea loam ·011 ·017 ·18
Waimea stony loam ·007 ·010 ·26
Stoke fine gravelly loam ·018 ·024 ·20
1   The percentages given are the average from three samples.

The results of the chemical analyses show that Maori gravel soil is much richer in mineral food, available for plants, than other soils of the district. It contains about fifteen times as much available phosphoric acid as is contained in Moutere loam. The lime requirement of Maori gravel soil is low when compared with that of other soils. It is not surprising, therefore, that the soil has a remarkably high fertility. The absence of effect in the application of phosphatic manures and lime for ordinary farm crops is adequately explained by the results of the chemical analysis.


Since farmers who cultivate the Maori gravel soil use little manure for their crops, it follows that the large stores of plant food in this soil were not introduced by Europeans. The soil has, indeed, been growing good crops since settlers first took up this land some sixty years ago. As a result, large quantities of mineral plant food must have been removed by crops from the soil. The manures which have been used would compensate for a part only of that removed by the crops.

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Two explanations of the large amount of mineral plant food in Maori gravel soil are possible:—

  • 1. The soil used by the Maoris in making their kumara beds was naturally very rich in plant food.
  • 2. The Maoris introduced large amounts of mineral plant food in the preparation of the land for their crops.

In order to determine the origin of its high fertility an examination of the soil and substrata in situ was made. Pits were dug in several places, on typical areas of the soil, and notes were made of the nature and depth of the various layers which form Maori gravel land. Commencing with the surface soil, the following layers were found:—

No. 1.—Fine gravelly sand thickness 10′-16′.
No. 2.—Loam thickness 18′.
No. 3.—Fine sand thickness 10′-30′.
No. 4.—Coarse Sand and Gravel thickness several feet.

Layer No. 1 constitutes Maori gravel soil which was spread over the loam by the Maoris. The lower layers have been deposited by the Waimea river in past ages, and have not been disturbed by the Maoris except at the pits from which they excavated sand, silt and gravel for use in spreading over the loam. These pits are still plainly visible, and despite the fact that they have been partly filled in by farming operations of the present day, vary in depth from three feet to six feet. An examination of the pits show that the Maoris must have utilised the material from layers 3 and 4 for spreading over the loam. Undoubtedly some of the loam from layer 2 became admixed with the lower layers during the process of excavation, but mechanical analysis of the Maori gravel soil shows only a small percentage of fine particles such as are contained in layer 2.

One noticeable feature connected with layer 4 is the fact that it contains about 10 per cent. of larger gravels which range from 1½′ to 4′ in size. Stones of this size are rarely found in the soil made by Maoris. The larger gravels must therefore have been picked out from layer 4 either during the process of excavation or at the time of spreading on the land. Large piles of stones which have been thrown aside, are plainly visible, usually in the pits themselves, in different parts of the Maori gravel area. Another interesting point of difference between the original excavated material and the prepared soil lies in their colour. The excavated material freshly dug from layers 3 and 4 is light brown in colour, while the Maori gravel soil is so dark that in wet weather, when the colour can be observed to advantage, it appears to be nearly black. Natural soils in the vicinity do not exhibit this dark colour. They are invariably brown, somewhat similar to the freshly excavated material. While it is natural to - 88 expect a darker colour in the topsoil when compared with its subsoil or substrata, this can hardly explain the very dark appearance of Maori gravel soil. An examination of the soil revealed the fact that it contained considerable quantities of charcoal, which could be separated to some extent by the simple immersion of the soil in water. The charcoal was noticed in several areas of the Maori gravel soil, and occurred, not only in the top 9′, which may be regarded as the true top-soil, but was found to the full depth of 12′ to 16′, which constitutes the whole layer of the prepared soil. The dark appearance of Maori gravel soil results, in fact, principally from the presence of the charcoal, which apparently was introduced from the very beginning, in the preparation of the land for their crops.


Table II. shows the analytical results of samples of the above mentioned layers taken near the pit sites on Messrs. O'Connor's property, Waimea West.

Plant Food content of the Layers associated with Maori Gravel Soil.
    Soluble in Hydrochloric Acid. Soluble in 1% Soln. Citric Acid.
Layer Character Total Nitrogen. Phosphoric Acid, (P2O5) Potash (K2O) Phosphoric Acid, (P2O5) Potash (K2O)
No. 1 Top 9′ of Soil ·078 p.c. ·16 p.c. ·60 ·046 ·024
Bottom 7′ of Soil ·026 p.c. ·12 p.c. ·55 ·047 ·022
(Maori Gravel Soil) No. 2 Loam ·078 p.c. ·09 p.c. ·63 ·015 ·012
No· 3 Fine Sand ·023 p.c. ·10 p.c. ·60 ·027 ·008
No. 4 Coarse Sand and Gravel ·016 p.c. ·10 p.c. ·57 ·032 ·009

NOTE:—The analysis of further samples taken from different parts of Messrs. O'Connor's property agree closely with those given above.

The results of the analyses show that the whole layer of Maori gravel soil is much richer in available plant food than the layers of the underlying strata. There is an accumulation of .016 per cent. phosphoric acid (P2O5) and .015 per cent. potash (K2O) over the amounts contained in layers 3 and 4 which were used by the Maoris in the preparation of the soil. The accumulation of phosphates, as represented by the amounts contained in the hydrochloric acid extract, is even greater, being on an average .04 per cent. higher than that

- i
The mounds in the foreground consist chiefly of large stones discarded by the Maoris. Photo by W. C. DAVIES.
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of layers 3 and 4. One interesting feature of the chemical results is the comparatively high percentage of available phosphoric acid contained in layers 3 and 4. These layers are much richer in this plant food than the original loam topsoil which occurred in this part of the district. Though the high content of phosphoric acid in these layers is, no doubt, partly responsible for the present high fertility of Maori gravel soil, it is insufficient to account for the whole of it and some other explanation of the exceedingly high fertility must be sought.

Chemical analysis of samples taken from another area of Maori gravel soil confirm the results which have been given already, though the enrichment of the soil has not taken place to such a great extent on this second area. The enrichment, in this ease, amounts to .005 per cent. available phosphoric acid (P2O5) and .008 per cent. available potash (K2O).


The presence of charcoal throughout the whole depth of Maori gravel soil suggests that very large quantities of wood must have been burnt on the soil. It is well-known that wood ashes contain considerable amounts of both phosphates and potash salts. Analyses of tea-tree and bracken ash, made in the laboratory, show that they contain about 1.1 per cent. of phosphoric acid and from 8 per cent. to 17 per cent. of potash. The ash of these plants is alkaline, and contains carbonate of lime, which would remedy soil acidity. It is not enough to assume that the Maoris simply burnt, from time to time, the vegetation growing in situ on their soil. This would not account for the large increase in either the available phosphoric acid or that soluble in hydrochloric acid. The increase can only be explained by a long continued policy of burning wood, or more probably scrub, taken from other lands in the vicinity.


The Moutere Hills flank the whole of the land used by the Maoris. Nowhere are the hills more than one mile from their fields. Tea-tree, or Manuka (Leptospermum ericoides) flourishes on almost every part of the hills, and if cut, rapidly re-establishes itself. It seems probable that this was the source of the wood used for burning on their soil. It is difficult to give any estimate of the total quantities of wood which were burnt on the lands during the Maori occupation. The accumulations of the present day represent a portion only of the quantities of plant food which must have been introduced by the Maoris. Large quantities of potash would quickly leach out of such a loose textured soil, and the crops of both Maoris and Europeans - 90 must have removed very large amounts of both potash and phosphoric acid. On the richer portions of the Maori gravel lands, several hundred tons of vegetable matter must have been burnt on each acre.


It is well-known that the three important plant foods obtained from the soil by crops such as kumaras are phosphates, potash, and nitrogen. There is no doubt that the burning of large quantities of wood on the Maori gravel soil resulted in a plentiful supply of the two plant foods first mentioned. The supply of nitrogen for the kumaras would not be increased but reduced by the burning of wood on the land. In view of the statements, by observers of Maori practice, that no animal manure was used for their crops, it is at first difficult to understand how good crops were obtained on a soil which has such a low nitrogen content as that of the freshly excavated material from layers 3 and 4. During the early stages in the formation of the Maori gravel soil, the lack of nitrogen would not be quite so noticeable, for the roots of the kumaras would quickly penetrate to the underlying loam, which is comparatively well supplied with this plant food.

As the thickness of the Maori gravel soil increased, starvation from lack of nitrogen must have resulted, unless some means of supplying this fertiliser was employed. If fish, taken in times of plenty, had been utilised for fertilising the land, undoubtedly great benefit to the kumara crop would have resulted from the nitrogen contained in it. Peat taken from swamps and low-lying ground and incorporated into the soil, would also have benefited the soil considerably, not only by reason of its comparatively high content of nitrogen, but also by its property of retaining moisture and thus increasing the water supply of the crop. There are many swampy areas in the vicinity of the Maori gravel lands, where such peaty material could have been obtained. Whether it was used, however, is purely a matter of conjecture.

Perhaps the simplest explanation of the maintenance of the nitrogen supply of Maori gravel soil, is the supposition that periodically, the land was left uncropped. Under such conditions, the land would quickly revert to Nature's vegetation. The work of the Rothamsted Experimental Station has shown that Geescroft field, which was left uncultivated and untouched, gained nitrogen at the rate of 44 lbs. per acre per annum. This occurred, despite the fact that members of the order leguminoseae were absent from the herbage of this field. Sir Edward Russell has attributed this gain in nitrogen to the activity of a specific bacterium, Azotobacter chroococcum. - 91 Since a good kumara crop requires less than 40 lbs. of nitrogen per acre yearly, it seems possible that land periodically left uncropped would accumulate the nitrogen required for the years when kumaras were grown.


It is interesting to compare the methods adopted by the Maoris in the treatment of their kumara soil with present day experience in growing sweet potatoes. In a bulletin issued by the U.S. Department of Horticulture, the following statements are made:—

  • 1. Sweet potatoes require a light, well-drained soil, underlaid by a clay or loam subsoil.
  • 2. Excellent results can be obtained by manuring with commercial fertilisers containing nitrogen, phosphate and potash.
  • 3. Soils very rich in nitrogen tend to give excessive growth of the vines at the expense of the tubers.
  • 4. The sand overlying the clay or loam subsoil should not be too deep, otherwise the tubers are apt to be long and stringy.
  • 5. The loam underlying the sand must be sufficiently porous to allow surplus water to drain away quickly.
  • 6. Moderate applications of lime are advisable in order to secure the best results from sweet potatoes.

Viewing Maori practice in the light of American experience, it becomes apparent that modern agricultural practice agrees closely with that adopted by the Maoris.

The soil which they made, from the point of view of texture, drainage and underlying loam, met exactly the specifications of American experts for the sweet potato crop. In regard to manuring, the Maoris definitely supplied phosphates, potash and lime, from the ashes obtained by burning wood on the soil. They did not make the mistake of manuring too heavily with nitrogenous manures, although it is a matter of surmise how the nitrogen supply of their soil was maintained. The incorporation of charcoal into the soil was a material advantage, as the whole soil was blackened. This resulted in a greater absorption of heat and an earlier crop. A similar practice is adopted by the market gardeners of the Biggleswade district, England, where soot is used in large amounts, partly for the sake of the contained nitrogen, and partly for the purpose of obtaining early crops.

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A map has been prepared showing the location and extent of the Maori gravel soil in the Nelson district. There are over 1,000 acres.

Maori gravel soil consists of fine gravel, coarse and fine sands, which have been dug out from lower depths, and subsequently spread out on top of the loam which is the prevailing topsoil in this part of the district. The depth of the prepared soil varies from 10 inches to 16 inches. Large stones were removed from the material excavated. The sand used by the Maoris in the preparation of their soil is naturally rich in available phosphoric acid. A comparison of the Maori gravel soil with freshly excavated material similar to that used by them, shows that the prepared soil is richer in available phosphoric acid and potash than the original sand. This enrichment of the soil has not resulted from European farming practice, for little if any manure has been used by English settlers on these lands. The source of the enrichment was apparently wood ashes, since the soil is black, owing to the presence of much charcoal. Wood, scrub, or other vegetable matter must have been brought on to the land and there burnt. Tea-tree (Manuka) is suggested as the form of vegetable matter which was employed for this purpose. The ash of tea-tree is rich in phosphates, potash and lime.

It has not been possible to ascertain how the Maoris maintained the nitrogen supply of their soil, although several methods have been suggested by which this could be effected.

The soil prepared by the Maoris and the general treatment of the land agrees well with the specifications for soil and treatment demanded by agricultural experts for the successful growing of sweet potatoes.


  • “The Maori Race,” by E. Tregear.
  • “Food Products of Tuhoeland,”by Elsdon Best, Tr. N.Z. Inst., Vol. XXXV.
  • “On the Vegetable Food of the Ancient New Zealanders, before Cook's Visit,” by W. Colenso, Tr. N.Z. Inst., Vol. XIII.
  • “Sweet Potato Growing,” Farmers' Bulletin 999, U.S. Department of Agriculture.
  • “The Book of the Rothamsted Experiments,” by A. D. Hall.
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[It is interesting to note in connection with this valuable paper by Messrs. Rigg and Bruce, the practice of the Taranaki Maoris in the preparation of a compost for use in the cultivation of the kumara. It was the custom of these people in by-gone times, after a flood in the rivers, or a storm at sea, for the experts in agriculture (the tohunga or priests who taught this and other branches of learning and ancestral lore, in the Whare-wānanga, or Maori College) to examine most carefully the deposit of sand and silt thrown up or left by storm and flood. If, in the opinion of the tohunga these deposits were suitable for the purpose, and the omens propitious, the people were at once assembled, and what we may term a “kete brigade” (on the lines of the familiar bucket brigade) was formed, and the sand and silt gathered in ketes, or baskets, and passed from hand to hand to a spot selected, where it was mixed with vegetable matter, gathered in by other bands of workers. This consisted mainly of the succulent ground-fern called Moukū (Asplenium bulbiferum), which grew in abundance throughout the neighbouring forests, and the leaves and tender branches of certain shrubs of the coprosma family—the taupata, karamu, raurakau—and probably leaves of other trees and shrubs in a lesser degree.

This deposit when thoroughly mixed was carefully covered, and after due religious ceremonies, was set apart and left to mature in readiness for the planting season, when it was opened up and apportioned out by the tohunga to the various family plots prepared for the growing of the kumara.

The term applied to this mixture was whakaparapara, a free translation of the meaning of which is, to add or blend ingredients (into a compost) for the purpose of producing a vigorous growth.

Mr. W. J. Gray of Okato, Taranaki, to whom I am indebted for the above information, states that there is one of these whakaparapara heaps near his homestead, on the banks of the Hangatahua (Stony River), the only one known to the Maoris of that district, where it has remained untouched for over a century.—EDITOR.]

1   Soluble in 1 per cent. solution of Citric Acid.
2   As determined by Hutchinson & MacLennan's method.