Soil quality

Why we monitor soil quality

Waikato Regional Council monitors soil quality to provide information about how particular land uses are affecting soils in the long-term. Some land uses and land management practices may affect soil properties and reduce soil quality over time.

Some reductions in soil quality, such as lost organic matter, can take many decades to remedy. This can mean that future landowners are unable to grow crops or pasture of the same quality or need to use greater amounts of fertiliser to achieve the same result as previously.

The better the measured soil characteristic matches the guideline value for its current land use, the better the soil quality. Scores for individual soil quality characteristics are grouped together for each land use to give the percentage of sites in the Waikato region that meet (or fail to meet) soil quality guidelines. This is expressed as satisfactory soil quality and soil quality that is ‘of concern’.

We can get an idea of what the soil quality issues are for certain land uses by looking at which key soil quality characteristics don’t meet guideline values for that land use. For example, fertility imbalance and soil compaction are currently the main concerns identified for pastoral farming soils.

What's happening?

Good quality soils are those where key soil characteristics are in good condition for the current land use. We want soils to hold water and nutrients where they are readily available for plant roots to take them up, suppress pests and weeds, sequester carbon from the atmosphere, filter clean the water that flows through it into rivers, lakes, and aquifers, and prevent flooding by acting as a sponge during heavy rain. Good quality soils are full of beneficial organisms from microbes to earthworms that have important roles in natural recycling, inhibiting disease-causing organisms and soil formation (especially soil organic matter).

Waikato Regional Council measures soil quality for four main land use types in the Waikato region:

dairy farming
drystock farming (sheep, beef, deer etc.)
horticulture and cropping
plantation forestry.

About 15 percent of productive sites across the region had satisfactory soil quality for their current land use in 2019. This compares with 18 percent being satisfactory in 2010 and 43 per cent in 2005.

About 94 percent of sites under dairy farming had soil quality that is of concern, with excessively high fertility and/or soil compaction affecting about 85 percent of the sites.

Similarly, more than 88 percent of sites under drystock farming had soil quality that is of concern. About 70 percent of the sites are affected by soil compaction and/or excessive fertility, while about 24 percent of sites are affected by low fertility.

Nearly 90 percent of sites under cropping and horticulture has soil quality that is of concern. About 70 percent of the sites are affected by excessively high fertility while about 55 percent of the sites under cropping have low stability of aggregates. Nearly all cropping sites show loss of soil organic matter compared to other land uses.

About 58 percent of the land under forestry has soil quality measurements outside targets. Around 35 percent of sites are considered ‘loose’. ‘Loose’ is the opposite of compact and many of the region's soils are naturally loose. Loose soils are more prone to erosion. Trees reduce the risk of erosion. They protect the ground surface from the impact of rain, dry out wet soil by transpiring large amount s of water and bind earth with their roots. So the forestry land use allows production on what would otherwise be unproductive land. However, care is needed at harvest or conversion to another use where the land is made bare of vegetation. Conversely, sites where logging occurred showed evidence of disturbance and compaction from dragging logs and machinery.

How we monitor

Where and how we collect the data

Waikato Regional Council monitors soil quality at 150 sites across the Waikato region. Soil quality monitoring sites were selected to include the dominant soil types and main land use types in the region.

Monitoring frequency

Waikato Regional Council re-measures soil characteristics at a fifth of the sites (20 percent) each year. Each site is sampled approximately every 5 years.

Monitoring history

Measurements of soil quality began in 1995 and have continued through to the present. This programme is aligned with national soil quality monitoring as established and administered through the Land Monitoring Forum. Waikato Regional Council has now built up a baseline soil quality database to compare changes in soil quality against.

Measurement technique

Seven key soil quality characteristics are measured for each soil quality monitoring site. Soil samples are taken at each site and soil characteristics are analysed in the laboratory. Landcare Research carried out laboratory analyses of the baseline soil samples following standard laboratory procedures.

The table below lists the soil quality characteristics measured.

Table 1: Soil quality characteristics measured

Soil characteristic	What the measurement tells us	How it is measured
Chemical properties
Total carbon (C)	Organic matter status	Dry combustion, Leco CHN Analyser
Total nitrogen (N)	Level of organic nitrogen reserves	Dry combustion, Leco CHN Analyser
pH	Level of acidity or alkalinity	Glass electrode pH meter, 1:2.5 in water
Olsen phosphorus (P)	Level of plant available phosphate	Bicarbonate extraction, with phosphate in the extract measured using the molybdenum blue method and ascorbic acid reductant
Biological properties
Mineralisable nitrogen (N)	Level of readily mineralised nitrogen reserves	Accumulation of NH⁴⁺ following waterlogged incubation (5 g soil in 10 ml water) at 40°C for 7 days
Physical properties
Bulk density	Degree of soil compaction	Volume and mass of soil cores oven dried at 105°C
Macroporosity	Degree of soil compaction, quality of root environment, amount of aeration.	Drainage on pressure plates at a tension of –10 kPa

The soil quality indicator is expressed as the percentage of the region’s productive land area (divided into four key land use types) that meets (or fails) soil quality guidelines. The indicator is based seven key soil quality characteristics, which are analysed and aggregated.
The following steps explain the process used to compile the indicator:
1. Soil samples are sent to the laboratory for analysis of the key soil quality characteristics.
2. Soil quality results are analysed to identify all data points that fall outside the guideline values for each different soil and land use combination. These are termed exceedances. The percentage of exceedances for each soil and land use combination gives a measure of soil quality.
3. Soil sampling sites are biased towards certain land use and soil combinations. To account for bias when providing regional soil quality information, the exceedance data are weighted according to the total area of each land use found in the region. The weighting factors are shown in the table below. Note that the total for all land uses does not add up to 100 because of rounding errors and non-classified land.
Table 2: Weighting factors for each broad land use type and soil combination in Waikato region

Land use % of regional land area

Cropping 0.7

Horticulture 0.1

Forestry 11.6

Pasture - dairy 25.3

Pasture - other 32.1

Indigenous vegetation 29.0

Once the exceedance data have been corrected for bias, they are divided by the total regional land area to give the area in hectares for each land use type that meets (and fails) soil quality guidelines.This can then be expressed as the percentage of the region’s land area (by land use type) that has either satisfactory soil quality or soil quality ‘of concern’.
Guidelines and standards
Guidelines values for each soil quality characteristic were defined for key land use types and soil classes at a series of expert workshops in 1999. For more information about the guideline values used, check out the Landcare Research web-page explaining the approach used to develop a web-based soil indicators tool (SINDI).
Soil quality indicators and guideline ranges are still being developed and have not been validated for all soil and land use combinations. Therefore, guideline ranges may be refined over time.
Because soil quality indicators and guideline ranges are still being developed, there may be changes in the recommended ranges or more detailed land use types may be added. Current soil quality guidelines are taken from “Provisional targets for soil quality indicators in New Zealand", published by Landcare Research in 2003.
Soil samples were processed at the Landcare Research laboratory, which is Telarc registered and ISO 9000 accredited.

Land use	% of regional land area
Cropping	0.7
Horticulture	0.1
Forestry	11.6
Pasture - dairy	25.3
Pasture - other	32.1
Indigenous vegetation	29.0

Data and trends

The main soil quality issues in the Waikato region are soil compaction, excessive nutrients and loss of soil organic matter (SOM) with the associated decrease in biological activity of microorganisms.
More detailed information on trends for each of these issues can be found in Taylor, M., Cox, N., Littler, R, Drewry,J. (2017) 'Trends in soil quality monitoring data in the Waikato region 1995-2015', Waikato Regional Council Technical Report 2017/26. Hamilton, Waikato Regional Council. Click here to read the technical report.

Download the data file

The Excel spreadsheet below contains the source data for this indicator's graph and information.

The graph shows the percentage of sites meeting guidelines for satisfactory soil quality and soil quality ‘of concern’, by land use type.

The data were collected between 1996 and 2019.

Download the data file
13 KB

When this indicator is updated

The indicator is updated annually.