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Methods - How we monitor

Where and how we collect the data

The soil quality monitoring programme is a screening tool designed to gather a large amount of information quickly and at a low cost to inform detailed environmental assessment of the regions soils. 

Monitoring sites

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

How this indicator is compiled

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).

Limitations

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.

Further indicator developments

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.

Quality control procedures

Soil samples were processed at the Landcare Research laboratory, which is Telarc registered and ISO 9000 accredited.