Programme overview

Greater Wellington Regional Council (GWRC) monitors soil quality as part of its State of the Environment programme, to meet the requirements of section 35 of the Resource Management Act (1991) and to provide information to measure Regional Plan policy effectiveness.

The soil quality monitoring programme consists of approximately 100 monitoring sites on a range of soils across the region under different land uses. The frequency of sampling is dependent on the intensity of the land use; dairying, cropping and market garden sites are sampled every 3-4 years, dry stock, horticulture and exotic forestry sites are sampled every 5-7 years, while indigenous vegetation sites are sampled every 10 years. This years’ report summarises monitoring results for cropping and market garden sites.

Monitoring objectives

  1. Provide information on the physical, chemical and biological properties of soils;
  2. Provide an early-warning system to identify the effects of primary land uses on long-term soil productivity and the environment;
  3. Track specific, identified issues relating to the effects of land use on long- term soil productivity;
  4. Assist in the detection of spatial and temporal changes in soil quality; and
  5. Provide information required to determine the effectiveness of regional policies and plans.

Monitoring indicators

Monitoring indicators are used to assess soil chemistry and fertility, and to understand soil physical condition. The indicators used are as follows:

Regional summary

Each monitoring site is benchmarked against relevant guidelines for the 2019/20 monitoring season, with the total indicators breached displayed below.

48% of sites had very depleted levels of organic carbon. Carbon is one of the basic building blocks of organic matter. Biologically, soil organic matter is the source of energy for soil micro-organisms (or microbes) and is a major reservoir of plant nutrients. Low levels of organic carbon signal concern about soil quality. Soil ecosystem functions may not be significantly affected, but there is either an increase in the likelihood of deleterious effects, the loss of intrinsic resources, or loss of a measure of resilience in the soil system. Very depleted levels indicate a large proportion of the soil organic resource has been lost and is likely to severely affect critical soil ecosystem functions, including those that affect productivity.

61% of sites had low or very low macroporosity due to compaction of soil. Macroporosity is a measure of the proportion of large pores in the soil that provide air supply to plant roots. When macroporosity is low, plant growth may be severely affected by reduced oxygen supply and soil drainage may be impeded.

High levels of Olsen phosphorus, above optimal ranges, were found at the majority of sites within the Kāpiti Coast Whaitua. Elevated levels of Olsen phosphorus can occur shortly after fertiliser application, but do not need to be consistently this high because there is little benefit to production, and it may risk increased phosphorus leaching into rivers and streams.

See the Resources page for links to more information on indicators of soil health.

Monitoring sites and indicator breaches

Map usage: Drag and scroll on the map to move and zoom in on areas of interest, hover on the site circles to see more information, hover over each outlined area to see Whaitua (main river catchments), and use the top right checkbox to show/hide “LUCAS 16 landcover” classifications.