Soil quality defines whether soils are in good condition for their current land use activity. Because soils have developed from different parent materials and have been influenced by a range of soil-forming factors, soils display a variety of physical, chemical and biological characteristics.
What this means is some soils types will be better suited to certain land uses activities than others and the same soil type will need to be managed differently under different land use activities. Where a soil’s characteristics match those needed for its current use, then we can say that soil is of good quality. This page explains what soil quality is like around the Marlborough region and what we can do to look after our soil.
Why do we Measure Soil Quality?
Measuring soil quality provides an early warning of the potential effects different primary land use activities may be having on long-term soil quality. It can help identify whether soil quality is degrading over time and what factors that may be contributing to soil degradation.
This information can then be used to help us to manage our soil resources in a sustainable manner into the future.
How do we measure soil quality?
There is no single measure for soil quality because there are many things about soil that affect its quality. From the array of possible measures, scientists have chosen eight key soil quality characteristics to measure in New Zealand soils. These have been grouped under biological, chemical and physical soil measurements.
Indicators used to assess soil quality
Soil quality characteristic
What is it telling us
Why is this important
Organic matter content
Total carbon is generally considered a good measure of organic matter in soil. Organic matter is important for soil quality because it helps soil retain moisture and nutrients, it contributes to a stable soil structure and it provides a source of food for soil microbes.
Total nitrogen (N)
Organic matter N content
Nitrogen is an essential nutrient for plants and animals. Typically in soils, organic matter N comprises more than 90% of the total N. However organic matter N needs to be mineralised to inorganic forms by soil microbes before it can be utilised by plants.
It is a measure of the acidity and alkalinity in soil
Most plants and soil organisms will have an optimum pH range for growth and the pH of the soil affects which species will grow best.
How much soluble phosphorus there is in soil
Phosphate (P) is an essential nutrient for plants and animals. Plants get their P from phosphates in soils. Most NZ soils are low in phosphate and extra is needed to be added to agricultural soils.
Are trace element accumulating in soils
Accumulation of some trace elements ie; Cd, F can result in plant and animal toxicity.
How much of the total N is available to plants through microbial activity
Mineralisable nitrogen is a useful measure of soil organic matter quality in terms of its ability to store nitrogen. It is also a useful indictor of soil microbial activity.
Dry bulk density
Whether a soil is compacted or loose
Compacted soils restrict air supply to plant roots, reduce root penetration, extension and germination and reduce capacity of the soil to store water. Loose soils may be subject to increased risk of erosion and dry out quickly.
The number of large pores in soil
Macropores are important for air penetration into soil, extension of roots into the soil and drainage of water. Typically macropores are the first to be lost when the soil is compacted.
How resistant soil aggregates are to breaking
A stable crumbly structure lets water quickly soak into the soil, doesn’t dry out too quickly and allows roots to spread out easily.