Soil

Of the 15 soils represented in the NZ Soil Order Classification, nine soil orders are present in the Bay of Plenty. Of these, four soil orders are predominant in the Lake Rotorua catchment. They are Allophanic soils, Podzols, Pumice and Recent soils.

Most soils in the catchment are derived from pumice and have low bulk density, are well-drained with moderate to high water-holding capacity and low natural fertility.  Pumices that have been subject to high rainfall and historical native forest cover have developed into podzols. The allophanic soils to the south-west are formed on a mix of ash and Taupo pumice [1].

A brief outline of the four predominant soil orders is below.

Map: Distribution of Soil Orders in the Lake Rotorua Catchment

Podzols (purple area).

Podzols are strongly leached acid soils. The soils occur under high rainfall (generally exceeding 1800–2000 mm annually), mostly at higher elevations and have low natural nutrient levels. In the Bay of Plenty they are used for dry stock grazing and some dairying, forestry, and some cropping on the Mamaku Plateau. Podzols are predominant from lands south-west of Rotorua through to the Mamaku plateau in the northwest.

Pumice soils (green).

Pumice soils are soils that are dominated by pumice or pumice sand high in volcanic glass. Clay contents are generally less than 10 percent and soil strength is weak or very weak. The soils are resistant to pugging and, like the allophanic soils, have low to very low nutrient levels. The potential for erosion by water is high, especially when the surface vegetation and thin topsoil are removed. Summer droughts occur. These soils occur through Kaharoa to the north of Lake Rotorua to the lands around Okahu Bay at Lake Rotoiti and up around the hills to the east and northeast of Lake Rotorua.

Recent soils (yellow).

The main properties of recent soils include weak soil development, generally high base saturation, gravel or rock not strongly altered, high potential rooting depth, good drainage, low phosphate retention, high fertility, and susceptibility to erosion and/or sedimentation. Recent soils also occur on hilly or steep slopes where surfaces are renewed after erosion. Land use on such slopes is mostly dry stock or forestry, the latter restricted because of shallow profiles overlying parent rock. Many areas are in indigenous forest. These are among the most versatile soils in the Lake Rotorua catchment and are found primarily in the hills to the southeast of Lake Rotorua and around Lake Tarawera.

Allophanic soils (pink)

These soils have properties strongly influenced by clay minerals that are poorly crystallised or amorphous. They have weak soil strength and are sensitive with low bulk density. The soils are formed from layers of volcanic ash that are visible near the source (Rotorua area) and telescope together further away (East Coast area). They were called yellow-brown loams in previous soil classifications. The soils typically have dark yellowish-brown grading to yellowish-brown sandy loam to silt loam subsoils with high levels of phosphate-fixing allophane in the clay fraction.

Topsoils tend to be 18 cm or more deep with weakly developed structure and black to dark brown colours. The soils have a typically greasy feel when moistened and rubbed firmly between the fingers. Allophanic soils are generally moderately to strongly leached with low levels of exchangeable calcium, potassium, magnesium and sodium. Reserves of magnesium and potassium are low to very low. Available phosphorus is naturally low with high phosphate retention. Allophanic soils are friable to a great depth and do not have root-restricting layers.

Along coastal Bay of Plenty, these are ideal soils for deep-rooting subtropical plants such as kiwifruit, provided they are sheltered from salt-laden winds. Other uses are pasture (dairying, dry stock), or forestry on steeper slopes. Cropping, such as maize, needs careful management to preserve topsoil structure. These soils make up a large proportion of the soils around Rotorua city centre, from Western Heights through to Tihiotonga and south towards Horohoro and Tumunui.

Soil Texture

Texture is a basic property of the soil that is not easily changed. It affects other soil properties, such as water availability, permeability, drainage, and aeration. It also influences nutrient retention, the development of soil structure, and the ease of soil cultivation.

Texture is the relative proportions of the primary particles in the soil, namely sand (2.00–0.06 mm), silt (0.06–0.002 mm) and clay (<0.002 mm). Every soil contains a mixture of sand, silt and clay, and this is expressed as a textural class name such as sandy loam, silt loam, clay, etc. A soil that contains a balanced mixture of sand, silt and clay is called a loam. In general, soils of the Bay of Plenty do not contain appreciable amounts of clay.

Fig: Soil Texture triangle

The soil textural triangle shows all the textural class names that result in various combinations of sand, silt and clay. A more generalised, or simplified, textural triangle is used to group the textural classes into sandy, silty, loamy and clayey (see figure below).

Sandy soils have more large pores and fewer small pores. They have good aeration, but store much less water for plant use, and are considered droughty soils. In heavily-fertilised sandy soils, rapid water movement increases the risk of groundwater pollution through leaching of excess nutrients such as nitrate.

Leaching is the process of removal of soluble materials (nutrients, metals and pesticides) in solution by water draining through the soil. Strongly leached soils may occur anywhere, but are most common under high annual rainfall at higher elevations, as in the case of podzols.

Nutrient leaching is very relevant to the Bay of Plenty because many of the soils have sandy textures. Nutrients in sandy soils with low organic matter content are easily leached because the soils are freely draining and have low nutrient retention capacities. On the other hand, soils containing appreciable clay and organic matter do not leach as much because a greater proportion of inorganic nutrients is absorbed on the (mostly) negatively-charged exchange complex. Since texture is a basic soil property that is not easily changed, the addition of organic matter to sandy topsoils to increase their nutrient-holding capacity is very important.

To avoid under- and over-irrigation, it is important to properly monitor soil moisture in the farm. Tensiometers, gypsum blocks, neutron probes, time domain reflectometry (TDR) and frequency domain reflectometry (FDR) sensors are the main instruments that can be used for monitoring soil moisture.

More information on soil is available from Landcare Research, Bay of Plenty Regional Council and Rotorua Lakes Council.

References:

[1] Soils of the Bay of Plenty vol. 2 Central, Bay of Plenty Regional Council