A Background to the Soil Map
This document contains important information for studying the soil map in more detail.
 
How to Read the Soil Map Before we start | Compilation of the Map | Soil Units | Legends | Colours | Symbols | Soil Names
Soil Boundaries | Reference Soil Profiles | Soil Areas | Cautionary Note | References

Before we start – an important note

This soil map has been quite a long time in the making. It was begun when the New Zealand Genetic Soil Classification (Taylor 1948; Taylor and Cox 1956; Taylor and Pohlen 1968) was operative. In this system, soil series and types were typically gathered together into genetic groups to which they belonged. Before the map was completed, the New Zealand Soil Classification (Hewitt 1992; 1998) was introduced. In this classification the higher order categories of Order, Group and Subgroup are more rigorously defined than were the old genetic groups. However, the new system has not as yet reached down to the more local categories of soil series and type. Nevertheless the new classification is acknowledged as having grown out of the old (Hewitt 1992; 1998 P5). In this circumstance it has been judged legitimate to gather the soil series and types identified in this map under the umbrella of both the new Soil Orders (distinguished by leading letter capitals) and the old genetic groups (all in lower case letters). This should not be interpreted as a perfect fit with the new system (sometimes the early series/type definitions were too broad for this to be possible) but rather as a meaningful, workable solution to bridge the gap between the two systems and enable the lower categories of series and type to find their proper place in the higher categories of Order and genetic group, which confers on them a wider meaning than that of mere local categories.

Compilation of the Map

In the first instance, the original hand-drawn soil map was digitized and compiled using Arc View software. The printed version of the map was derived from this source.

Subsequently, the map was recompiled using public domain software, for example Geotools, in order to make it accessible to a wider audience. At the same time special code was written to enable images of the soils corresponding to the soil units on the map to be portrayed on the screen. This interactive computer version of the map is available as a compact disc or alternatively may be accessed through the Massey University Image Web Server.

When using the electronic version of the map it is important to take full advantage of the “tool-bar” beneath the map, in order to enlarge the particular area of interest, be it farm, part of farm, river flat or terrace, and access the corresponding soil profile images and descriptions.

Soil Units

The soil map may be read at three different levels. The first level is that of the Soil Order (genetic group), the second that of the soil series and the third that of the soil type.

The Soil Order (genetic group) is the broadest level. Here we are recognising a major category of soil, the members of which share certain defining properties that are dependent ultimately on a common genesis. The map depicts four main Soil Orders or genetic groups, namely:

  • Recent Soils (recent soils)
  • Recent Gley Soils (gley recent soils)
  • Organic Soils (organic soils)
  • Pallic Soils (yellow-grey earths)

each of which is distinguished by its own colour on the map, for example, yellow for Recent Soils, green for Pallic Soils.

The intermediate level is that of the soil series. The soil series is defined as an assemblage of soils with similar soil profiles and similar soil temperature and soil moisture regimes, that are derived from the same or similar parent material. Soil series are given local geographic names, for example, Manawatu soil series, Kairanga soil series. Soil series are distinguished on the map by different shades of the colour of the Soil Order under whose banner they are gathered. Thus the Ohakea soil series carries a different shade of green from that of the Tokomaru soil series, with green, as we have already noted, being the chosen basic colour of the Soil Order of Pallic Soils.

The most detailed level is that of the soil type. Soil types are distinguished within soil series chiefly on the basis of soil texture (particle size), more particularly that of the topsoil, for example, Rangitikei loamy sand, Rangitikei fine sandy loam. The soil types are depicted on the map by symbols, thus Rkls for Rangitikei loamy sand and Rkfsl for Rangitikei fine sandy loam.

In a detailed soil map such as this, various other subtleties may be introduced at the soil type level and these are discussed under the section on symbols below.

The high and intermediate terraces on the Massey property are delineated by major terrace scarps. For convenience the soils on these are shown on the map as belonging to either Halcombe Hill Soils (HaH) or Halcombe Steepland Soils (HaS) depending on the steepness of slope, with 25o constituting the break point between them. These soils are highly variable but still fall under the umbrella of Pallic Soils.

Legends

Two legends accompany the soil map, a physiographic one and a pedological one.

The physiographic legend lists all the soils down to the soil type level that are to be found on the different landscape units of the area, for example, the soils that are located on the high terrace or those that are found on the river flats. It is the legend we can all feel familiar with. It helps us find our way around the terrain.

The pedological legend links the soil classes down to the soil series level with the wider Soil Orders, or genetic groups, so that they may be placed accurately within the overall context of New Zealand soils (the New Zealand Soil Classification) or indeed the soils of the world. For example, the Tokomaru soil series falls within the Order of Pallic Soils, while the Rangitikei soil series fits nicely within the Order of Recent Soils. The pedological legend widens our understanding of the soils.

Colours

Colours are used as an aid to distinguish the different classes of soils down to the series level. Thus:

  • different shades of yellow are used to differentiate those soil series that fall within the Order of Recent Soils
  • different shades of blue are used to distinguish soil series that fall within the Order of Recent Gley Soils
  • purple is used to designate the one soil series that falls within the Order of Organic Soils
  • different shades of green are used to distinguish those soil series and close associates that belong to the Order of Pallic Soils

Symbols

There are too many soil types to be differentiated on the map solely on the basis of colour. Therefore soil types are distinguished by symbols. It will be recalled that soil types within series are chiefly distinguished by textural differences, more particularly of the topsoil. The symbol for the soil type thus carries two components, the first two letters indicating the particular soil series followed by letters indicative of the textural class, thus:

  • Manawatu sandy loam: Mnsl
  • Kairanga silt loam: Kasil

In a detailed soil map such as this we sometimes find it useful to make further distinctions which are appended to the normal soil type symbol. Thus the common soil type on the level tops of the high terrace is the Tokomaru silt loam. We are aware, however, that the level tops are dissected by a number of gullies and the soils of these gullies are distinguished as a gully phase: Tksil(G). We are also aware that there are areas where the flat tops give way to gently sloping sides and the soils here are identified as belonging to an undulating phase: Tksil (U). These particular phase distinctions are essentially topographic.

A few soil types carry a single asterisk attached to them, thus Rangitikei fine sandy loam* (Rkfsl*). The asterisk signifies a fuzzy set. While in reality all soil classes are fuzzy, that is, not entirely pure, some are more fuzzy than others! Thus Rkfsl* is known to be highly variable in both texture and depth, but nevertheless, for the greater part, it is distinguishable as a fine sandy loam of reasonable depth.

A double asterisk indicates something different — a bisequal soil. A good example is the Rangitikei fine sandy loam**(Rkfsl**). In this soil only the upper part of the profile is a fine sandy loam. A former soil of silt loam texture lies buried beneath. The soil has experienced two separate stages of sediment deposition and soil development.

Yet another distinction is conveyed by the presence of the letter p prefixed to the soil type name. The letter p stands for para, meaning a soil similar to but not identical with the named soil type. A good example is the para Ohakea silt loam (pOHsil). By and large we encounter the characteristic Ohakea silt loam on reasonably extensive intermediate terraces. But there are some more confined locations where small streams have entrenched themselves in underlying terrace gravels producing local flats carrying soils of essentially similar properties to the normal Ohakea silt loam, but with subtle distinctions such as a thinner loess cover or greater accumulation of sesquioxide concretions (iron pan). The para prefix accommodates these broadly similar but not quite identical soils. Other para soils have been distinguished because they have been protected by stopbanks in relatively recent times and no longer receive the deposition of flood sediments characteristic of the normal soil.

These subtle distinctions are not meant to confuse! They simply acknowledge the reality of the complexity that is present in the soil distribution.

Soil Names

The name that we give to a soil is important because is establishes its identity and enables us to discuss it at will.

By picking up the symbol and colour for a particular soil depicted on the map we are then able to go to the legends to obtain its proper name. Thus Kasil translates readily to Kairanga silt loam, a Recent Gley Soil (gley recent soil) found on the river flats, while Tksil translates with equal facility to Tokomaru silt loam, a Pallic Soil (yellow-grey earth) found on the high terrace. The soil names are derived from two sources:

  1. the Soil Orders from Hewitt, A.E. 1998: New Zealand Soil Classification, 2nd Edition, Landcare Research Science Series No.1 (and the parallel soil genetic groups from Taylor, N.H. 1948: Soil Map of New Zealand, 1:2,027,520 scale. DSIR, Wellington).
  2. the soil series and types, with one exception, from Cowie, J.D. 1978: Soils and Agriculture of Kairanga County, North Island, New Zealand. Soil Bureau Bulletin 33. DSIR, Wellington.

The one exception is the new soil type name of Turitea silt loam (Tusil) used to replace the former name of Manawatu mottled silt loam. As an important imperfectly drained soil on the river flats, it was considered worthy of its own series/type name.

Soil Boundaries

This soil map has been digitized so that each particular soil class is contained within its own polygon or set of polygons. As a consequence the boundaries between the different soil units are depicted as solid lines. Sometimes the boundaries between soil classes are abrupt, as when, for example, the Tokomaru silt loam drops over the terrace edge onto the Halcombe Steepland Soil of the scarp. In this circumstance the solid line fits the field reality. In other circumstances, however, the boundaries between soil units may merge. This is most certainly the case between adjacent fuzzy sets and also, for example, between the Tokomaru silt loam proper and its undulating phase. In such circumstances the solid boundary line is rather misleading, as it tends to convey a precise separation of soil units when in fact they actually grade into one another. Only with experience in the field can one come to recognize soil boundaries that are sharp from those that are merging.

Reference Soil Profiles

In the course of the survey a great many soil profile pits were dug and the soils described in order to determine the soil pattern. Fifty of these have been selected as reference soil profiles to illustrate the properties of the different soil types. Thirty five are illustrated in colour, three in black and white, with the remaining twelve presented as straight text descriptions. In each case a simplified soil profile description is given in terms by the colour and texture of horizons, the soil is named and classified, and a brief comment made on its overall properties.

It is important to note that these reference soil profile descriptions relate to the soil as found at specific point locations on the ground. In the printed map these locations are shown as red dots, with the appropriate soil profile numbers alongside. However, in the electronic version of the map that is available in the public domain, whenever a particular polygon is flagged by clicking with the appropriate icon from the “tool-bar” the image of the soil profile corresponding to the soil type for that polygon appears on the screen. This image is derived from the reference profile for the soil type, although the position of the polygon pointed at may be a considerable distance away from the spot location (geodetic reference point) for the reference profile.

What we are relying on here is the consistency of the soil type mapping unit. It cannot be claimed that the soil of the polygon pointed at is identical with the reference profile portrayed. All soil mapping units, of necessity, carry an element of variation, some more so than others. None the less, with this understood, it can be claimed that the soil of the polygon pointed at will resemble in its essentials that of the image of the reference profile thrown on the screen.

Soil Areas

The areas for each of the soil units depicted on the map are shown in Table 1. We can see at a glance that the most extensive soil type is that of the Tokomaru silt loam and its associates occupying the high terrace (554 ha).

Next come the various soil types found on the river flats of the Manawatu River and the Turitea and Kahuterawa Streams (363 ha). But be warned, this set of soils is far from uniform, including as it does soils of many different textures and drainage classes.

The soils of the Ohakea Terrace, including those upon which the University campus is built, come next (159 ha). Here we have to deal with only a few soil types belonging to either the Ohakea or Ashhurst series. But note that these two series, while grading into one another, differ considerably in drainage and soil depth.

The soils of the major terrace scarps delineating the boundaries of the high and intermediate terraces (102 ha) are not to be ignored. Here slopes are hilly or steep and increasingly these have been given over to plantings of either native or exotic shrubs and trees.

Much further information may be gleaned from Table 1. For example, the individual soil types belonging to a particular soil series may be summed to give an overall total for the series. Thus we see, for example, that the well or moderately drained soils of the Rangitikei series account for 150 ha, compared with those of the Manawatu series, which occupy 113 ha.

With equal facility we may total up all the imperfectly or poorly drained soils on the property. With respect to soils belonging to these drainage classes, it is important to note that this is the drainage condition of the soil in its natural state. The Tokomaru silt loam, for example, carries the designation poorly drained. Only extensive tile and mole underground drainage has made it possible to conduct the modern dairy farming operation of Number Four Dairy Unit on this particular soil type.

Lastly, cross reference to the Pedological Legend enables us to calculate the areas of soils belonging to the different Soil Orders.

Final Cautionary Note

It is salutary to reflect that no soil map can be perfect. Consider for a moment the problem of scale. Whereas the electronic map may be zoomed into and out of at virtually any scale, the scale of the hard-copy map covering the whole Massey property is limited by the size of the paper it is printed on. For example, if AO (841 x 1189 mm) is the limiting paper size this restricts the scale at which the map can be printed to 1:6000. This means that, linearly, 6000 cm or 60 metres on the ground has to be squeezed into 1 cm on the map. Or, in terms of square measure, 60 m x 60 m = 3600 m2 = 0.36 ha on the ground has to be squeezed into 1 cm2 on the map (equivalent area). A square cm on the map is not a large area on which to depict the soil pattern over 0.36 ha (close to an acre) on the ground.

Scale is not the only thing. Other important factors are soil variability (always present), the state of knowledge at the time the map is being constructed, the skill of the pedologist and the tools at his or her disposal. With all that said we are lucky to have a soil map at all!

Finally, we should note that the development of the Massey farms, and the Turitea Campus in particular, has resulted in a number of man-made modifications to the natural landscape and soil pattern. Some examples are the construction of fresh water dams and effluent ponds, the diversion of minor streams underground, the removal of stones from the surface of playing fields, the development of the all-weather athletic track on what was once the para Ashhurst soil, the construction of landscaping mounds in the vicinity of buildings and sports areas, the creation of car parks, and the formation of spoil and waste-disposal dumps. Many, though not all, of these have been noted on the map.

References

Cowie, J.D. 1978: Soils and Agriculture of Kairanga County, North Island, New Zealand. Soil Bureau Bulletin 33. 92p., with 1:63360 scale soil map. DSIR, Wellington.

Hewitt, A.E. 1992: New Zealand Soil Classification. DSIR Land Resources Scientific Report No. 19. 133p.

Hewitt, A.E. 1998: New Zealand Soil Classification, 2nd edition. Landcare Research Science Series No.1. 133p.

Taylor, N.H. 1948: Soil Map of New Zealand, 1:2,027,520 scale. DSIR, Wellington.

Taylor, N.H.; Cox, J.E. 1956: The soil pattern of New Zealand. New Zealand Institute of Agricultural Science Proceedings. 17p.

Taylor, N.H.; Pohlen, I. 1968: Classification of New Zealand Soils. Pp. 15-33 in: Soils of New Zealand, Part 1. Soil Bureau Bulletin 26(1). 142p., with 1:1,000,000 scale soil map of New Zealand. DSIR, Wellington.

These pages and pictures are copyright Massey University unless otherwise stated!