CHAPTER TWEL VE

Stony Plains, Silty Downs

E ;;;;h:"~~~~d~~~~!~~~:~~t,~et:;i~:s~~~~~;,~;:~~~ ~:~~~~~:I:~ landscapes journey. At the southern end of the Canterbury Plains the frontal ranges press closer to the coastline until , at Shag Point 50 !un north of Dunedin, they run right down to the sea. Here, the Kakanui Mountains constitute such a topographic and climatic barrier across the lowlands that they are also a convenient point at which to divide the eastern lowlands. This chapter deals with the lowlands of Canterbury and North Otago as one region - the drier (500 - 750 mm) of the two regions, with rainfall more variable and usually spread over only 60 - 100 days. Chapter I 3 covers eastern Otago and Southland where the rainfall is slightly greater (600 -900 mm), much less variable and spread over a much longer period (100 - 150 days). Furthermore, the soils of the Canterbury-North Otago lowlands lie within the mild temperature zone while those south of the Kakanui Mountains are within the cool temperature zone (see Fig. 14.3).

In Canterbury, although the mountains have now been left behind on the western horizon, they stHl exert a major influence on the region's climate and land use. The north.westerly winds, which lose most of the ir moisture when pushed up over the barrier of the Southern Alps, now descend to the leeward plains with increased ve locity and drying power. The dryness of the plains is a major constraint to agricultu ral use and it is only the plentiful wate r from these same mountains - flowing in huge braided rivers or underground in the deep gravels - that allows irrigation and the successful growth of a diverse range of pastures and crops.

The landscapes of the region fall into three broad groups:

o the terraces, fans and floodplainS of the Canterbury Plains, and the lower Waitaki; o the rolling loess-covered downlands of South Canterbu ry and North Otago,

and the downlands and coastal hills of North Canterbu ry; o the hilly, volcanic landscape of Banks Peninsula.

Canterbury Plains and the Lower Waitaki The Canterbury Plains are the largest alluvial plains in New Zealand . They cover 750 000 ha, and extend for 180 km from Amberley in North Canterbury almost to Timaru in South Canterbury. At their widest point in mid-Canterbury they extend for 70 !un from an altitude of 400 m near Springfield below the frontal Torlesse Range (Plate 12.1), down to sea level in the Canterbury Bight; annual precipitatio n decreases from 1000 mm to 600 - 700 mm over this same distance.

To the motorist travelling to and from the mountains, the plains seem flat but they are, in fact, a series of huge, gently sloping (1: 17 5) fans built up by four major rivers - the Waimakariri, Rakaia, Ashburton and Rangitata. DUring successive glaciations vast quantities of greywacke detritus were produced in the mountainous hinterland, to be washed out into the river systems and deposited to depths reaching over 500 m in places. The depressions between these four main fan systems have become occupied by smaller rivers - the Kowai, Ashley, Selwyn , Hinds, Orari and Opihi - which rise in the frontal ranges, not in the glaciated mountains of the interior.

179

Plate 12.1 (opposite)

Looking north-west from between Sheffield and Springfield, showing the sharp transition between the frontal ranges and the gently sloping fan surface of the Canterbury Plains. In this locality the annual precipitation is higher (around 900 mm) than the rest of the pl ains and the Lyndhurst soils have sufficient depth of loess (40 - 50 cm) over gravels to allow cropping and the establishment of good pastures.

180 The Uvinn MamIe

Plate 12.2

View south across the lower Waitaki River below Kurow to the fans and lower slopes of the St Marys Range. With an annual precipitation of only 500 - 600 mm these are the driest coastal lowlands in New Zealand and drought is a recurring problem. The Waimakariri soils of the low terrace in the foreground are being irrigated by the border-dyke technique but the patchiness of the pasture response indicates soil differences or inadequate distribution of the water.

Fig. 12. 1

Stylised cross-section of terraces on the south bank of the Waimakariri River , Canterbury Plains. The cross· section is approximately along the 60-m contour and 650-mm annual rainfall isohyet. The vertical scale is considerably exaggerated.

The lower Waitaki Plains are smaller (80 000 hal consisting of the fans, terraces and floodplain of the Waitaki River only (Plate 12.2). The soils are similar to those of the Canterbury Plains, but drier (500 -600 mm annual rainfall).

The single most important factor determining the properties of the soils of the plains is the depth of fine material overlying the outwash gravels. Observations of crop and pasture production in relation to depth of fine material over gravels has indicated that 45 cm is an important dividing line; soils with more than 45 cm of fine material over gravels are described as 'deep' and those with less than 45 cm as varieties of stony terrace soils (see p.188).

The variation in depth of fine material over gravels is shown in Fig. 12 . I, a stylised cross-section of the four terrace and floodplain levels on the fan surface on the south side of the Waimakariri River. This simple type of terrace/soil·depth sequence is repeated with only minor variations on the other fan interfluves associated with the Rakaia, Ashburton and Rangitata Rivers.

HIGH TERRACE

sw

INTERMEDIATE TERRACE

Depth

(C~I ~Fin.s 100 ,,0.0' 200 . -J:t!ir. Gravelsandstones

LOW TERRAC E

FLOOD PLAIN

NE

The high terraces are the remnants of the huge alluvial gravel fans of the Otiran Glaciation and were probably laid down about 20000 years ago. For the next 10 000 years loess from the riverbeds gradually accumulated on this surface under the influence of the prevailing strong north-westerly winds, and the process is still occurring today along the margins of the major braided rivers (Plates 12.3 and 10.5). On the high terrace surface on the south bank of the Rakaia River there is a sequence of soils (BarrhilllHatfield/ChertseylLismore) in which the depth of this loess declines from over 2 m near the river (Barrhill soils, Plate 12.3) to less than 25 em farthest from the river (Lismore soils). The shallow, stony Lismore soils (Plate 12.4), and the Steward soils in the lower Waitaki, are by far the most prevalent soils on these high terraces.

The intermediate terraces were buHt up from river al1uvium deposited from 10 000 to 3000 years ago and carry a mosaic of both stony terrace soils and deep, finer-textured soils (Fig. 12.1). The shallow, stony Eyre soils are similar to the Lismore soils, and the deeper soils include both well-drained Templeton soils (Plate 12.5) in fin e sandy to silty alluvium, and others with impeded drainage in finer silty and clayey deposits (Wakanui and Temuka soils, Plate 12.6). The Templeton, Wakanui and Temuka soils are among the most fertHe, agriculturally important soils in Canterbury. They cover around 170000 ha, with the Templeton and Wakanui sons supporting most of the inte nsive mixed farming (wool/lambs and cash crops) on the plains. Most of the Temuka soils (and associated Wakanui soils) are in permane nt pasture as high-producing dairy farms supplyi ng milk to Christchurch, Ashburton and Timaru .

This patte rn of shallow and stony, deep, and gleyed soils is repeated on the low terraces and floodplains, where the alluvium is much younger, having been deposited in the last 3000 years. The most Widespread are the Waimakariri and Selwyn soils, recent alluvial soils which are quite variable in depth and stoniness (Fig. 12.1). The land·use patte rn again depends upon soil depth; the stony terrace soils are used for extensive grazing, since any increased production is generally not worth the cost of irrigating suc h porous soils on very undulating surfaces. The deeper soils are friable, free draining and fertile, and are suitable for intensive sheep and cattle farming, cash cropping, and horticultural production.

Stony Plains, Siley Downs 181

Plate 12.3

Loess, mantling the outwash gravels to depths of up to 4 m, is the most important factor causing soi l variability on the high terraces of the Canterbury Plains. In this view across the Rakaia River to the frontal ranges north and south of Mt Hutt, loess is being lifted out of the riverbed by the prevailing north -westerly wind and deposited on the terraces along the southern bank. Some of the coarser drift material has been oriented by the wind into low dunes (Fig. 12.1) on the surface in the foreground. The areas of deep, accumulating loess closest to the river carry Barrhill soils, which are fertile, free-draining soils well suited to cropping although they can be susceptible to drought.

182 The Livins Mantle

Plate 12.4

The Lismore soils are the best known example of the stony terrace soils of the Canterbury Plains. They occur on over 200 000 ha of the highest terraces; closely related Glasnevin and Balmoral soils occur on simi lar surfaces (50000 hal in North Canterbury, and Steward soils (13 000 hal in the lower Waitaki valley of North Otago.

Because the total volume of fine soil in the profile is so low, Lismore soils are free draining and have a !ow capacity for storing water. This factor, coupled with low rainfall. usually makes their soil moisture inadequate for crops, so most are committed to dryland farming for lamb meat and wool. The stoniness can give. rise to the severe limitation of windthrow in exotic forest plantations, but pasture and lucerne roots can penetrate to considerable depths if moisture is available. Lismore soils predominate in most of the community irrigation schemes in mid-Canterbury. With careful irrigation, pasture y ields have been doubled, sufficient for diversification to dairy farming.

Plate 12.5

Templeton soils cover about 75000 ha of the intermediate terraces of the Canterbury lowlands. They consist of deep layers of silty /sandy alluvium deposited 10 000 to 3000 years ago. They have a much higher water-holding capacity than the associated shallow and stony Eyre and Lismore soils (Plate 12.41. and pasture roots can be seen penetrating to below 1 m. These deep soils of the seasonally dry Canterbury Plains are important because of their greater ability to withstand drought, and to provide the most efficient use of water when irrigation is possible. Where the water table is intermittently high, Templeton soils grade into Wakanui soils which have darker topsoils, strong brown mottles and dark iron /manganese concretions in their subsoil. Both Templeton and Wakanui soils are important for cropping: small seeds (ryegrass, white clover, lucerne and lupin), cash crops (wheat, barley, oats, peas and potatoes) and fodder crops (turnips, rape, swedes, chou moellier ).

Plate 12.6 (right)

Temuka soils occur on around 45 000 ha of the intermediate terraces of the Canterbury Plains, especially at the toe of the fan surfaces in the Kaiapoi, Lincoln-Ellesmere, Ashburton River mouth, and Temuka districts. They are gley soils because of the influence of a high water table (at 70 cm below the soil surface in this photograph); their clayey-textured subsoils exhibit the characteristic rusty mottles and grey or bluish grey colours Associated Wakanui soi ls are more yellow-brown, coarser textured and better drained

Temuka soils are mainly used for dairying, with cash crops (peas, barley, wheat) usually only grown when

~~n~~~~; :~~t~~~s~;~~:. ~Z ~~i~: ;;,~t advantage of plenty of moisture for spring pasture growth; they dry out in summer and supplementary sprinkler irrigation is often used to maintain production.

Stony Plains, Silty Downs 183

Plate 12.7 (below)

The late Quaternary loess that mantles the downlands of South Canterbury and North Otago can be nearly 10m thick near the coast. This section, at Dashing Rocks near Timaru, shows fou r distinct loess layers overlying polygonal blocks of the Timaru Basalt Timaru soils (Plate 12.9) are formed in the upper 1 m of the top loess layer.

184 The Living Mantle

Plate 12.9

The Timaru soils cover an area of 60 000 ha of the coastal downlands of South Canterbury and North Otago below 150 m in altitude. They are dense grey soils formed in deep loess. They are only weakly weathered (mica and vermicu lite are the main clay minerals) , show little sign of clay movement down the profile and have a mottled 8g horizon (seen here at 45 - 70 cm) above a firm, compact gam mate fragipan (Cx horizon, 70 - 11 5 cm) with coarse prismatic structure. Dark manganese concretions can be seen in the 8g horizon of this profile.

Although Timaru soils are suitable for intensive stock production and a wide range of crops such as cereals (wheat and barley) , linseed, peas and seeds (grass and clover). yields are often severely limited by drought.

Although the Canterbury and lower Waitaki Plains consist of a relatively simple repeating soil landscape unit in a north-east/south·west direction - high, intermediate and low terraces and present-day floodplains (Fig. 12. 1) - it must also be remembered that there is a climatic gradient cutting across this at right angles. Annual precipitation drops from 1000 mm in the no rth-west to 600 mm in the south-east near the coast. The terrace soils sequence shown in Fig. 12 . 1 applies only to the drier end of the plains; closer to the frontal ranges the soils are brighter (yellow-brown B hOrizons) and more leached of nutrients. Like any natural continuum, this soil transition is not sharp, for above 900 mm Lismore soils grade into Ruapuna soils; Chertsey to Lyndhurst; Templeton to Mayfield; Eyre to Hororata; and so on. Generally, these soils on the more humid apex of the fans have better structures and water-holding capacities, but lower avail able nutrients, than their drier counterparts.

Rolling Downlands of Canterbury The downlands of North and South Canterbury and North Otago lie around the inland margins of the plains, or between the frontal ranges and the coastline where the ranges press closer to the sea. For the most part they are moderately rolling, ranging from sea level to 500 m altitude. Most of the down lands (especially in the Timaru and Oamaru districts) have a dry climate (600 -800 mm) and experience seasonal droughts. They are generally too high above the regional water table and local rivers for irrigation to be practical. The distinguishing soil feature of these downlands is their ubiquitous parent material - late Quaternary loess. The depth of the loess varies widely depending on the angle of the slope, aspect and exposure; it can range from about 1 m on the higher parts of the downs to 10m near the coast (Plate 12 .7).

In North Canterbury the downlands occur around the Ashley River and the Amuri Plains inland of the Lowry Peaks Range. Waipara and Mairaki soils, typical dense grey soils (see Chapter 6), occur widely where loess (and colluvium from loess) has accumulated. They are currently used for semi-intensive sheep and cattle farming but have the potential for very intensive mixed farming of stock and cash crops. To the north of the Ashley River, rolling downlands give way to low dissected coastal hills consisting of an extremely complex mixture of Tertiary sediments (marine siltstone and sandstone, calcareous mudstone, siltstone and sandstone, limestone, and glauconitic sandstone). These Cheviot, Stonyhurst, Glendhu and Leader hill soils cover around 50000 ha and, although droughty, respond well to superphosphate and oversowing with clovers. This North Canterbury hill country is an important source of meat and wool production, farm cattle and coarse-woolled sheep.

In South Canterbu ry the downlands are very extensive. The loess originating from the bare, aggrading surfaces of the Canterbury Plains during the late Quaternary was depOSited as thick layers below the eastern fl anks of the Two Thumb Range and the Hunters Hills, from Geraldine and Fairlie to Waimate, and in the Hakataramea valley. At Geraldine and Timaru this loess overlies the Timaru Basalt (Plate 12. 7), but mostly it mantles Tertiary siltstones, sandstones and greensands of marine o rigin, and early Quaternary gravels.

The soils of these deep loess-covered downlands are mostly dense grey soils, with a fragipan (or eqUivalent compact hOrizon) 50 -70 cm below the soil surface, and usually mottling and gam mate colour patterns (see Chapter 6). The Ngapara soils are found in the driest downlands around the lower Waitaki River (plate 12.8), Timaru soils (Plate 12 .9) on the downlands between the O pihi and Otaio Rivers, and the moister Claremont, Opuha and Kakahu soils closer to the frontal ranges behind Waimate, Fairlie and Geraldine. The fragipan , so dominant in the Ngapara and Timaru soils, is progressively modified as the soils become moister; at the same time the soils become more friable. with more developed structures, yellowish­brown subsoils and lower nutrient content because they are more leached (see Fig. 13.2).

Some of the downlands of North Otago and North Canterbury are also loess­covered Tertiary siltstones, sandsto nes and greensands of marine origin, but the presence of limestone and other calcareous rocks has given rise to an interesting range of landforms and soils. The most striking are the 'cuestas' of North Otago where the gently tilted limestone has been carved into broad tablelands separated

by escarpments and wide, shallow valley lIoors (Plate 12.8). In North Canterbury, the calcareous rock strata tend to be more steeply tilted and have been eroded into sharper hogsbacks and homoclinal ridges.

Where the mantle of quartzo-feldspathic loess over the calcareous rocks is deep. the soils are dense grey soils (Ngapara and Timaru soils); where the loess is only thin a range of calcareous soils occur (see p.190). On weakly weathered limestone these soils are shallow, with dark loamy topsoils - Oamaru soils (Plate 12.10) in North Otago and Waikari soils (Plate 1.5) in North Canterbury. Where the rocks are calcareous siltstones and sandstones (rather than limestone) the soils have the properties of both the dense grey soils and true calcareous soils. Examples are the Waikakahi soils of the Waihao Downs locality on the northern side of the lower Waitaki River and the Huihui soils of the Waikari district of North Canterbury. The Waikakahi soils have thick, friable, well-structured and fertile topsoils which make them ideal for cereal cropping (Plate 12.11), but the use of most of the calcareous soils of the two districts is restricted to semi-intensive sheep and cattle farming because of their shallowness and low available moisture - despite their higher fertility.

Stony Plains, Silty Downs 185

Plate 12.8 lIeft)

Looking northwards down the floor of the Awamoko Stream between Tokorahi and Ngapara, west of Oamaru. The lower Waitaki River near Duntroon is in the distance. The underlying limestone has been carved into a cuesta landscape, leaving broad tablelands separated by limestone escarpments above short talus slopes which spill out onto the shallow valley floors.

The tablelands are mantled with thick loess (Ngapara soils) of greywacke composition (from the bed of the Waitaki River) and, in this locality, the limestone contributes little to the soils (contrast with the Oamaru soi ls near Weston, Plate 12.10).

Plate 12.10

Oamaru soils are shallow calcareous soils which form where only thin layers of loess cover the underlying limestone. The deep (30 em) dark brown topsoils are friable and well­structured silt loams over brown, blocky structured B horizons (20 - 30 cm) resting on limestone (bedrock and colluvium).

They are slightly alkaline soils, with a high content of organic matter, and high capacity for the exchange of nutrients. Levels of available phosphorus, calcium, magnesium and potassium are high but sulphur is low Because of their high fertility, good drainage, and stable structure under cultivation, the deeper Oamaru soils are used for cereal cropping (Plate 12.11 ) and market gardening.

The downlands of North Otago, between Enfield and the Kakanui River, are a rolling landscape with a complex soil pattern of dense grey soils (Timaru soils, Plate 12.9), calcareous soils (Oamaru soils, Plate 12.10) and black swelling clays (Te Aneraki and Waiareka soils, Plate 12.12). Each grades into the others. Cereal cropping for barley and wheat is important in this soil landscape and market gardening is important on the Te Aneraki and Waiareka soils. In particular, the Kakanui district produces much of New Zealand's brussels sprout crop. The production of glasshouse tomatoes is another important horticultural activity.

Plate 12.12 (right)

The Waiareka soils are deep, weakly leached, black swelling clays which have developed in the basaltic tuffs and lavas in the downlands west and south of Oamaru. They are fertile, well­structured soils dominated by expanding smectite clays (such as montmori llonite) which account for the stickiness of the soils when wet. They are slower draining and more difficult to cultivate than the associated Oamaru soi ls (Plate 12.10), but their otherwise excellent chemica l and physical properties (and the gentle topography) have made this district one of the most productive market garden areas in the South Island.

Within the limestone cuestas of North Otago there are also pockets of basaltic tuffs and lavas. These are of similar early Tertiary age to the limestone and interbedded with it. The resulting soil landscape between Enfield and the Kakanui River is therefore a complex mosaic of calcareous soils (Oamaru soils), clay-rich soils of volcanic origin (Waiareka soils), and dense grey soils on the deep loess (Timaru soils). The Waiareka soils (Plate 12 .1 2), and the related Te Aneraki soils from mixed calcareous/volcanic parent materials, cover around 4000 ha and are important cropping and market gardening soils. They have well-developed topsoil structures but, because they are only weakly weathered and leached, they have higher topsoil pH, greater capacity for nutrient exchange, and a higher content of exchangeable nutrients than the volcanic clays of Northland (see Chapter 5). Whereas the strongly weathered volcanic clays of Northland (e.g., Waimatenui soils, Plate 5. 1 I) have kaolin and iron oxide clay minerals, the calcium and magnesium accumulating conditions in this low weatheringlleaching environment of North Otago have led to smectite clays dominating the properties of the soils. These swelling clays account for the sticky nature of the dark Waiareka soils, which are locally known as 'tarry soils' and are described as black swelling clays (p.191 and Fig. 1.6b).

Banks Peninsula Banks Peninsula consists of two deeply eroded Tertiary age volcanoes that are now joined to the mountainous hinterland by the alluvial Canterbury Plains (Plate 12. 13). The volcanic rocks are andesitic and basaltic lavas with some basaltic tuff and scoria. During the late Quaternary Ice Ages, loess swept up from the aggrading Canterbury Plains covered the dissected volcanic slopes . Subsequent erosion has removed most of this loess from the higher slopes, leaving a landscape which has developed in loess, volcanic materials, and mixed slope deposits.

The soil parent materials on Banks Peninsu la are subjected to a pronounced rainfall gradient, increaSing from 600 mm per annum in the rainshadow on the north-eastern slopes to over 1400 mm in the higher central regions which face the rain-bearing south-easterly winds (Fig. 12.2). The rainfall is less than 900 mm

Stony Plains, Sjlty Downs 187

Plate 12.13

Looking north-west from Dyers Pass Road on the Port Hills of Banks Peninsula, past the Cashmere and Hoon Hay suburbs of Christchurch, and across the Canterbury Plains to the Mt Hutt, Torlesse and PuketeraKi Ranges - frontal ranges of the Southern Alps.

The so ils of the plains in the foreground were poorly drained gley soils in their natural state (e .g. Temuka so ils, Plate 12.6J, but have since been improved (through drainage) and are capable of intensive pastoral and arable fa rming. The slopes of the Port Hills are a mixture of dense grey soils (Takahe soils) where loess has accumulated, and volcanic loamy clays (Cashmere soi ls, Plate 12. 14J where the underlying volcanic parent materials have been exposed (Fig. 12.2). These slopes are exposed to the desiccat ing north-westerly winds and usually suffer moisture deficiency between December and April. Many of the soils on the slopes are also susceptib le to tunnel ­gully erosion and rapid mass movement; residentia l development of the hill soi ls consequently requires care and an understanding of the properties of the soils involved (see Plate 14.14J.

188 The [ivins MamJe

Distinguishing features of stony te rrace soils PARENr MATERIALS AND LOCATION - stony terrace soils are common in alluvial landscapes throughout

New Zealand. Most are associated with the stony gravels laid down as fluvio-glaclal outwash surfaces or cold climate floodplains during the cold stages of the late Quaternary Period Ice Ages. Most of this stony alluvium is of greywacke composition, with local additions of schist. gran ite. gneiss and diori te They are particularly widespread on the Canterbury Plains. and on the Waimea (Nelson), Amuri. lower Waitaki and Waimea (Southland) Plai ns and inland basins of the South Island (Fig. 1.6b). Although many of the soils in the basins of Central Otago are shallow and stony. they have distinct properties which make it better to consider them as semi-arid soils (deSCribed in Chapter 10)

CLIMATE AN D VEGETATION - most stony terrace soils have developed in the typical dense grey soil climatic environment of the eastern lowlands - a pronounced dry season and a relatively low annual rainfall (550 - I 000 mm). In the ir natural state, the droughtier soils would have carried short tussock grassland. scattered woody shrubs and kanuka forest; as rainfall inc reased to 650-1000 mm. or as the soils increased in the depth of fine·textured material over gravels. there would have been a trend towards totara and matai fo rest In central New Zealand and close to the ranges of the South Island ther e are less extensive, but signi fi cant. areas of shal low and stony soils! n a brown earth climatic environment (no dry season and 1000 -1 400 mm of annual rainfall). These soils. such as Ashhurst. Kawhatau (plate 6.10). Heretaunga. Kopua, Ranzau (plate 8.14). Ahaura (Plate 11.8), Ruapu na. O reri and Monowai soils, are still free draining but in their more humid environment would have carried podocarplhardwood or beech forest. They are not shown as stony terrace soils on the soil maps: instead they are classed as shallow. stony variants of brown earths because:

o A horizons very dark greyish brown; o B horizons dark yellowish brown to light olive brown.

Gravels and stones dominate the morphology of profiles:

o soils with less than 45 cm of fine material over gravels are shallow soils: o soils with less than 20 cm of fine material (containing some gravels and Stones in the topsoil) over

gravels are stony soils.

TEXTURES AND STRUcrURES

o A horizons usually silt loam or stony silt loam to fi ne sandy loarn or stony fine sandy loam - depending on the extent to which the fine material has been e roded away; moderately developed c rumb strucrure;

o B horizons stony sil t loam to stony sandy loam; weakJy developed fi ne nut structure; o stones general ly make up 60 -85% by weight of subsoil, and clay only 15 -20% of the remaining fine

material.

FRIAB LE topsoils, loose subsoils unless compacted (or occasionally cemented, as in the Steward soils)

PLANT-AVAILABLE WATE R CAPACITY - medium in topsoils (I 5 - 20% of soil volume): very low in subsoils (6-12% of soil volume). The stoniness and lack of a significant depth of fine-textured topsoil severely reduces the ir water-holding capacity.

DRAINAGE - characteristics depend upon the depth of fine material over gravels. Most are well drained where 20-45 cm of fines over gravels; some are excessively drained (>20cm fines). Cemented subsoil pans can restrict drainage in some soils (such as Steward soils) which are Ii able to waterlog under irrigation.

CLAY MINERALS indicate the weakJy to moderately weathered narure of the soils:

o mica and chlorite predominate (50 --90% of day); some vermiculite: o allophane can make up 15 -45% of the day fractio n in subsoil horizons of some of these soils (e.g

Eyre, Lismore, Steward) in the Canterbury and lower Waitaki Plains - another example of allophane formi ng in acid ic conditions, in this case from the weathering of a1uminosilicate minerals like feldspars, rather than fro m volcanic glass (see deSCription of volcanic loarns in Chapter 2)

SOIL CHEMISTRY - the drier Stony terrace soils seem to be more leached than the fi ne-textured dense grey soils which occur in the same climatic environment; base satu ration is only low to medium (30 - 50%) and exchangeable magnesium Is lower. They are low in organic matter and plant-available phosphorus. Retention of phosphate Is general ly low, but higher than in dense grey soils

The stony terrace soils of the drier lowlands of eastern New Zealand are generally d roughty, rathe r infertile soils wh ich are difficult to farm intensively because the stones interfere with cultivation and the thin topsoil is very prone to wind e rosion. Without irrigatio n they are capable of producing only 4 - 7 tonnes/ha of pasture (or 6 -9 tonneslha of lucerne) per annum. With irrigation, this can be lifted to 7 - 12 tonneslha of pasture and 10 - 13 tonneslha of lucerne: a significant proportion of the plant-available water for this level of production is drawn from the underlyi ng gravels since pasture and lucerne roolS are capable of extending to a_ depth of more than I metre if moisture Is available for them to tap. They are responSive to sulphur, lime, phosphorus and molybdenum fertilisers. With irrigation they are attractive for dairying because of reduced puggi ng an d facial eczema problems.

Wind can be a significant consrraint to use of these soils. Periodicwindthrowofexo tic forest planrations on the Canterbury Plains has limited their use forforesay; with shelter, grapes, srrawberries and other ho rticultural c rops can thrive on such stony soils.

Atti tude 1m) 600

Pla ins

ANNUAL RAINFAll 750-900 mm

Takahesoils(fromloess) Cashmere soils (from basalt)

ANNUAL RAINFAll 900-1400 mm

Summllsoils(from/oess) Rapakisoils(from basalt)

NW 1 km

'---~------"

Fig. 12.2

Stylised cross·section of the Port Hills of Banks Peninsula, showing the relationship of soils to loess and basaltic parent materials, and climate. Below 300 m altitude, the annual rain fall is around 750 - 900 mm and the soils are only weakly leached; above 300 m, the rainfall is 900 - 1400 mm and the soils are moderately to strongly leached.

along the lower inner and outer flanks of both the Lyttelton and Akaroa calderas, and here the loess has developed into dense grey soils (fakahe soils). The topsoil of the Takahe soils has the typical weakly developed structure and the subsoil has a very firm fragipan. Where the rainfall exceeds 900 mm, and the period of summer soil·moisture deficit is only short, the soils (Pawson soils) develop brown earth characteristics (see Chapter 8). As the rainfall approaches 1400 mm the fragipan has almost disappeared and the subsoils have become more friable, better structured and more yellowish·brown in colour (Akaroa, Summit, Bossu soils).

Al l of these soils formed from loess and re lated slope deposits are susceptible to tunnel.gully or mass-movement erosion. Careful planning for residential use of such soils on the lower Port Hills adjacent to Christchu rch involves a numbe r of management safeguards, including avoidance of widespread removal of vegetation and cut-and-fi ll operations, and the efficient disposal of stormwater from roofs and paved surfaces.

Where the weathered volcanic rocks are the soil parent materials (usually on ridge crests and steep valley sides where the loess has been e roded away) volcanic loamy clays (see Chapter 3) occur. Although often shallow, these soils are generally well structured and well drained, with stony brown subsoils. Where the rainfall is below 900 mm they are mapped as Cashmere soils which are naturally high in available nutrie nts (Plate 12 .14); at higher altitudes and rainfall , Rapaki and Stewart soils are similar in morphology to the Cashmere soils but are more leached of nutrients and less fertile (Fig. 12.2).

Both the volcanic loamy clays and the loessial soils are used for sheep and cattle farm ing throughout the peninsula. Despite their lower natural fertiliry, the brown earths are more suitable for pasture production than either the dense grey soils or the volcanic loamy clays. Although the latter soils have a higher moisture­holding capacity per unit volume than the loessial soils, their shallowness to bedrock means that their tatal volume of plant-available water is limited. In the sheltered valleys of the Port Hills and the Akaroa Harbour there are virtually frost-free microclimates. Here the dense grey soils can be used for market gardening and flower growing provided irrigation water is available.

Banks Peninsula is as important a landmark to the city of Christchurch as are the more distant frontal ranges of the Southern Alps. Although the soil landscapes of the peninsula have a number of limitations for intensive urban or agricultural uses, the tradit ional extensive pastoralism of this hill country is very much in sympathy with the impo rtance that the local community places on this area for outdoor recreation and nature conservation.

Stony Plains. Silty Downs 189

Plate 12.14

Cashmere soils have developed through the in situ weathering of the basaltic rocks and tuff of Banks Peninsula under a rainfall of less than 900 mm per annum; they commonly contain a significant component of loess. They have friable, dark reddish-brown, stony silt loam topsoils with moderately developed nut structure, over friable dark brown silt loam to clay loam subsoils that have a strongly developed nut structure. Weathered basalt can be seen below 70 cm.

Cashmere soils are typical volcanic loamy clays with high contents of iron and aluminium oxides, which are largely responsible for their friable consistence and good structure . As a consequence, they are much less susceptible to erosion than the assoc iated dense grey soils (such as the Takahe soils).

190 The Ljy;nn Mamie

Distinguishing features of calcareous soils PARENT MATERIALS AND LOCATION - calcareous soils have developed on calcareou s parent

mate ria ls. usually rocks (limesto ne . marble. dolomite. ca lca reou s sandstone) but occasionally softer calcium-ric h deposits (such as cha lk and marl) of m arine or fresh wate r origin . Where the so ils are deep and almost w ho lly from calcareous m ate ria ls they are known Inte rna tionally as rendzinas. Most calcareous soils in New Zealand a re no t t rue rendzinas because they a.re too sh allow o r are diluted w ith no n -calca reou s loess or tephra Calcareous soils are widely distributed throughout New Zealand. They cover about 100000 ha. the m ost important areas being the downlands o f North and South Canterbury and North Otago (Fig. 1.6). Localised areas of hill a nd steep land soil s occur on calcareous rocks in the T e Kuitl. H awke 's Bay, Wairarapa. Nelso n, Buller. coastal Marlborough and 'Western Southland districts,

C LIMATE AND VEGETATIO N - calcareous soils have d eveloped under a 'Wide range of climatic conditio n s. In the typical d ry enviro nment of low land Cante rbu ry and Otago they are weakly leached a nd h ave a high conte nt o f excha ngeable nutrie nts; in a reas of highe r rainfall they have less free lime in their profiles and leaching has reduced their exchangeable nutrie nt content. Differences in indigenous vegetation are always a good indication of the occurre nce of calca reo us soils . As a general rule these soils favour the highe r fertility pre fe rring spec ies of tussock g rass or forest t ree . In d rier regio n s, totara. m atai and red beech are often found and , at highe r, 'Wetter altitudes. H all 's totara and silve r beech (In preference to mountain beech).

PRO FILE C H ARACTE RISTI CS

o profiles are often shallow (30 - 70 cm), contain fragments of limestone, and pass abruptly into the underlying whitish limestone;

o topsoils are usually deep (25 -30 cm) in relation to the rest of the profile. and black to dark grey in colou r;

o subsoils, where they occur, a re o live brown to brown

o A horizons I?amy. or clay loam, whh a strongly developed nut or granular struc ture; o B horizons vary from day loam to sandy loam depending on the mineralogy of the parent rock;

structures are usually moderately developed nut to blocky; o d ay content is very variable, depending on the degree of weathering

CONS ISTENCE - varies according to moisture content. Topsoils ofte n friable when dry but sticky when wet; subsoils (in deeper soils) usually sticky and firm

o medium in topsoils (20 - 25% of soil volume); o shallow profiles can be droughty because of free-draining nature and low total waler-storage capacity.

DRAI NAGE - can be a problem in deeper calcareous soils where shrinkage and swelling occu r with welti ng and drying; consequent changes in porosity can lead to water perching on the soil su rface during wet periods and deep c racking of the hard soil during su mmer.

BULK DENS ITY - low in topsoils (0.75 -0.9 TIm )).

CLAY MIN ERALS - predominantly smectites (including montmorillonite), clays wh ich are capable of varyi ng in volume by 50% or more between wet and dry conditions. The high content of smectites accou nts for theshrlnklswell properties of these soils, and contributes towards their high capacity for retai ning exchangeable nutrients. Someti mes minor amounts of zeolite (of pedogenic origin) occur.

o pH values are high (7 -8) because of the high content of free lime; o organiC matter contents are quite high, with particularly low ClN ratios (8 - 12); o capaCity for nutrient exchange is high and most of the weakly leached soils are saturated with

exchangeable nutrients; o levels of exchangeable calcium. magnesium and potassium and available phosphorus are usually high;

avail able sulphur is low; trace elements are usually not lacking; o retention of phosphate is low.

Calcareous soils are highly variable in the ir agronomic propert ies. large areas of shal low hill soils (usually in drie r eastern localities) are only su itable fo r extensive pastoral farming because of their droughtiness.

Where lhecalcareou s soils are deep and fertile they are well su ited to mixed cropping and intensive pastoralism. High production reqUires irrigation in the drier eastern regions oft.he South Island. In the more humid conditions of the King Count ry, Nelson and Southland , pastoral farming can lead to severe poaching of pastures In winte r, followed by hardening and cracking of the soil in su mmer. Calcareous soil s are not usually su bject to erosion because of thei r good structure, high con tent of organic maner, and high fe rtili ry which promotes rapid revegetation .

Distinguishing features of black swelling clays PARENT MATERIALS AND LOCATION - black swelling clays are a small group of soils in

New Zealand (less than 10 000 ha, Fig. 1.6) largely confined to the OamaTu locality in the South Island (Waiareka sOil s) and the Dargaville locality in Northland (Arapohue sO il s) . They are derived from rocks which have a low silica content (such as basaltic tuffs or lavas, and limestone); these rocks weather to soil parent materials which have a high content of reactive calcium and magnesium. In low leaching or slow drainage condit io ns the calcium and magnesium accum ulates and, instead of developing into volcanic clays or calcareous soil s, the parent materials give rise to black swell ing clay soil s whose properties are dominated by their high content of smectite clays.

PROFILE CHARACTERISTICS

o very dark greyish·brown topsoil s; o yellowish-brown B horizons.

TEXTURES AND STRUCTURES

o clayey topso il s. with strongly developed gra nular and nut structu re o clayey subso il s, with blocky structure; o clay content is usually SO - 70% of so il.

CONSISTE NC E - firm [Q very firm; sticky

PLANT-AVAILABLE WATER CAPACI1I'IDRAINAGE - med ium available moisture ( 14 -22% of soil volume) in subsoils. Total porosity is high but macroporosity is low; consequently, drainage is slow. The soils c rack when dry and rainfall tends to run down the cracks so that the soil is rewet at depth as well as from the soil surface.

BULK DENSI1I' - low in topsoil s (0.8 T/m\ higher in subsoils (1.0 T/m l).

CLAY MINERALS - virtually all smectites (gene rally montmorillonite) which are capable ofvarying in volume by 50% or more between wet and dry conditions. The high content of smectite clays accounts for the shrink/swell properties of these soils and their very high capacity for retaining nutrients in a form available to plants.

SOIL CHEMISTRY

o pH slightly acid (6.5); o moderately high leve ls of organic matter; o extremely high capacity for nutrient exchange; o high levels of calciu m, magneSium and potaSSium; o moderate retention of phosphate.

USES OF BLACK SWELLING CLAYS

Black swelling clays in North Otago are known locally as 'tarry so il s' and are highly valued for market gardening (especially the product ion of brussels sprouts but also potatoes, carrots. cau liflowers and lettuce) because of their high fert il ity and strongly developed structure.

There are very few limitatio ns to pastoral use of these soils - phosphate fe rt ilise rs are required in Northland and sulphur fertilise rs in North Otago. However, their specia l range of chem ical and physical properties suggests that cropping is a more sUitable use than pastoralism.

Because of the unique propert ies conferred by the smectite clays. black swelling clays are often incorporated into c ricket wickets. They not o nly prOVide a very hard playing surface when dry, but also manage to maintain a smooth surface if the pitch has to absorb moisture .

Scony Plains, Siley Downs 191