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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.

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)
  • Olsen phosphate (plant available phosphate)
  • pH (acidity or alkalinity).

Organic matter

The terms ‘soil organic matter’ and ‘soil organic carbon’ are used interchangeably. Total (organic) carbon is a measure of soil organic matter. When measured over time, the rate of soil organic matter turnover (total carbon content) provides information about soil health, for example:

  • the loss of soil biological activity (animal and plant communities in the soil such as worms and fungi)
  • soil nutrient depletion
  • changes in water infiltration and storage potential.

Soil nutrients

Soil nutrients are essential for plant growth and for maintaining soil health (see biological health above).

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 health

The physical health 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.

500 soils project

The 500 soils project monitored soil quality across New Zealand's 15 main soil orders using the six key measures discussed above:

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

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

  • arable cropping (for example, grains and fodder crops)
  • mixed cropping (for example, vegetables)
  • drystock pasture
  • dairy pasture
  • tussock grasslands
  • plantation (exotic) forestry
  • native forests.

A review of the 500 soils project was carried out in 2003.

This information has come from the latest national state of the environment report Environment New Zealand 2007.

 

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Last updated: December 2007