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What is measured and why?

Soil health (or soil quality) is the biological, chemical, and physical condition of different soil types under specific land uses. Monitoring soil health identifies whether soils are degraded and the factors that contribute to degraded soils.

Degraded soil can result from:

  • soil compaction
  • reduced organic matter
  • an imbalance in soil nutrient status
  • a mismatch between soil pH (acidity/alkalinity of soil) in relation to land use
  • changes to the biological, chemical, and physical characteristics of a particular soil order (type).

Biological health

The biological health of soil is determined by measuring the level of potentially mineralisable nitrogen in the soil. Plants use this form of nitrogen for growth and it is an indicator of soil organism activity and health.

Soil nutrients are essential for plant growth and for maintaining soil health. Levels of nutrients may be affected by the excessive build-up of soil nutrients (from high stocking rates, imported feed high in nitrogen, or use of nitrogen fertiliser) or their removal. When excess nutrients are flushed through the soil by water flow, they can cause imbalances in the soil system or surrounding environment. For example, the excessive use of nitrogenous fertilisers may result in a build-up of nitrates in groundwater and rivers.

In the reverse situation, the mining of soil nutrients can also create a problem. Removing vegetation or animal products from the land can result in loss of soil nutrients. Cultivation and vegetation burn-off can also reduce soil fertility. Soil nutrients can be replaced by returning organic matter to the soil, using fertiliser, and using nitrogen-fixing pasture plants (such as clovers).

Chemical health

The chemical health of soil is determined by measuring the levels of the following chemical compounds or properties of the soil:

  • total carbon content (organic matter status)
  • total nitrogen content (organic nitrogen reserves)
  • mineralisable nitrogen (organic reserves)
  • Olsen phosphate (plant available phosphate)
  • pH (acidity or alkalinity).

Organic reserves

The terms ‘soil organic carbon’ and ‘soil organic matter’ are used interchangeably. Total carbon, total nitrogen and mineralisable nitrogen are measures of soil organic reserves. These measures provide information about soil health, for example:

  • the ability of the soil to maintain moisture and nutrients
  • the ability of the soil to support good  structure
  • reserves of nitrogen stored within organic matter in the soil
  • nitrogen available for plants to use for growth
  • the health of soil biological activity (animal and plant communities in the soil such as worms and fungi).

Soil fertility

Phosphorous is an essential nutrient for plant growth. Plants obtain phosphorous from phosphates in the soil, but many New Zealand soils have naturally low available levels. Phosphorous can be added for agricultural land uses, however, excessive levels can increase loss to waterways.

Soil acidity (pH)

Soil acidity is relevant to the nutrient management and biological function of soils. Soil acidification affects plant productivity. Acidification occurs naturally in soils but it can be accelerated by different land uses. For example, the application of fertilisers that contain ammonium or elemental sulphur can lead to acidification, as can the removal of soil nutrients (through cropping and nitrate leaching). Acidity can be manipulated with the addition of lime. In general, as soils become more acidic, their ability to support most kinds of vegetation is reduced.

Physical status

The physical status of the soil is determined by measuring macroporosity, which provides information on soil compaction, root environment and aeration (soil structure).

Soil compaction reduces pasture growth and, when coupled with high-surface soil fertility (that is, high availability of soil nutrients), increases the risk of phosphates and nitrogen reaching streams and lakes through greater surface run-off.

Soil health monitoring in New Zealand

Between 1999 and 2001, the 500 soils project monitored soil quality in 500 sites across New Zealand's 15 main soil orders using seven key measures and seven major land-use categories below. A review of the 500 soils project was carried out in 2003. Since then, regional councils have continued monitoring soil health, and by 2009 approximately 300 sites have been re-sampled. The key soil health measures are:

  • total carbon (organic carbon reserves)
  • total nitrogen (organic nitrogen reserves)
  • mineralisable nitrogen (organic reserves and biological health)
  • Olsen phosphate (plant available phosphate, fertility)
  • pH (acidity or alkalinity)
  • macroporosity (physical status)
  • bulk density (physical status).

These seven measures were monitored across seven major land-use categories:

  • arable cropping (eg, grains and fodder crops)
  • mixed cropping (eg, vegetables) (arable and mixed cropping were reported in the cropping category in 2009)
  • horticulture (eg, orchards and vineyards) (reported for the first time in 2009)
  • drystock pasture (ie, sheep and beef)
  • dairy pasture
  • tussock grasslands
  • plantation (exotic) forestry
  • native forests.
This information has come from the latest national state of the environment report Environment New Zealand 2007 and the Soil Health Environmental Snapshot Report Card.

Return to the soil health page.

Last updated: January 2011