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 <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:
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).References
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.