The indicators selected to assess soil quality in SINDI reflect the idea that soil quality is not a single concept, but encompasses aspects of the soil physical structure, chemical fertility, nutrient storage, organic matter resources, and the biological life in the soil. There are potentially many indicators that can be used, but for any extensive national or regional monitoring scheme it is not practical to have more than a small core number. Note that other indicators excluded from SINDI still have value for specific questions. Additionally, these indicators were selected specifically for New Zealand soils and a different set of indicators may be more appropriate for soils from different areas.

The indicators themselves do not measure soil quality. Soil quality is a value judgement about how suitable a soil is for a particular use. The indicators measure attributes of a soil (e.g. pH, bulk density). Consequently different target values for indicators are needed for different land uses. For example, soils with pH less than 5 may be of suitable quality to grow radiata pine, but not for a good crop of white clover. Soils that are stony and excessively free-draining may be of poor quality for pasture production, but of excellent quality for vineyards.

Within the SINDI programme the quality criteria alter depending on the land use selected. We have not attempted to cover all possible land uses within the current SINDI package. The indicators are not intended as a basis for fertiliser requirements.

We propose a set of 7 indicators as shown in the Table below.

Indicator Soil Quality Information
1. Olsen phosphorus Plant-available phosphate
2. pH Acidity or alkalinity of soil
3. Anaerobically mineralisable Nitrogen Availability of nitrogen reserve, surrogate measure for soil microbial biomass
4. Total carbon Organic matter reserves, soil structure, ability to retain water
5. Total nitrogen Organic nitrogen reserves
6. Bulk density Soil compaction, physical environment for roots and soil organisms
7. Macroporosity Availability of water and air, retention of water, drainage properties

These 7 indicators describe the overall quality or condition of the soil. We used a multivariate statistical technique called Principal Component Analysis (PCA) to determine which of the soil indicators should be combined (Hair et al., 1995). From this analysis we identified four primary factors or groups that describe quality of a soil:

Group 1 - Olsen 2
This property makes up the first group representing the fertility status of the soil.

Olsen P is a measure of the plant-available phosphorus, which is greatly effected by fertiliser additions. In their natural state, most soils in New Zealand are of low nutrient status.

Group 2 — pH
One property makes up the second group representing the acidity status of the soil.

Soil pH is the acidity or alkalinity of soil, which controls the availability of many nutrients to plants. The acidity of soils of soils is greatly influenced by the applications of lime and fertilisers. In their natural state, most soils in New Zealand are acidic.
Group 3 — Anerobic Nitrogen, Total carbon, Total nitrogen
This group of soil properties represent the soil's organic resources .

This resource has an underlying supportive role for the other three groups. Total C and N provide a measure of the organic matter concentration and quality in a soil. Organic matter gives topsoil many of its unique characteristics, and provides a medium for water storage, a source of nutrients, habitat, and food supply for soil organisms, and retains soil chemicals within the root zone. These attributes characterise the intrinsic nature of a soil and are not readily modified.

Mineralisable nitrogen is a more dynamic measure of the organic N reserves of soil that are potentially mineralised by microorganisms into plant-available N. Being a measure of the mineralisable N reserves, and a surrogate for microbial biomass, the Mineralisable N measure provides an indicator of the biological status of soil. Mineralisable N can be markedly influenced by land use, particularly organic matter contents and N status of a soil.
Group 4 — Bulk density, Macroporosity
These properties formed the fourth group representing the physical status of soil.

Bulk density is a measure of soil compaction i.e., the mass of soil in a defined volume. Total porosity is a measure of the holes or voids in the soil mass. Voids are important to allow air to penetrate the soil, but also to give the soil an open structure to enable it to retain water. The larger pores or macropores are of particular importance for infiltration and drainage, but are easily lost when soil is compacted. The physical characteristics and the susceptibility to compaction are much influenced by soil mineralogy and amounts of sand, silt and clay.

Target ranges

A group of soil experts have developed soil response curves for each of the soil properties for different land use and soil order combinations. Each curve combines environmental and production criteria. It was intended that these values would be reevaluated from time to time so it is possible that the curves may change with changing knowledge.

Interpretation of site values for each of the 7 indicators is provided in the form of bar-graphs based on these response curves. The graphs are colour coded in a manner similar to a “stoplight”. The colours are designed to show significant impact (red), potential impact and therefore of concern (orange), and near optimal (green).


Hair, J.F.; Anderson, R.E.; Tatham, R.L.; Black, W.C. (1995). Multivariate Data Analysis. Prentice-Hall: London.

Sparling G., Lilburne, L., Vojvodić-Vuković M. 2008. Provisional targets for soil quality indicators in New Zealand. Landcare Research Science Series no. 34. Manaaki Whenua Press, Lincoln, New Zealand. First published in 2003 by Landcare Research New Zealand Ltd, reissued in 2008, with minor amendments.