What do we mean by soil quality
The physical, chemical and biological properties of different soils vary a great deal. Different soils are suited to different uses.
Soil quality defines the condition the soil is in for its current land use. Soil quality is often referred to as ‘soil health’.
The measure of quality relates to:
- amount of soil organic matter (humus)
- beneficial soil life, such as bacteria and earthworms
- physical condition (e.g. whether it has become compacted)
Soil quality characteristics
|Soil quality characteristic||Measure||What it tells us||Why it’s important|
|Fertility||Soil pH||Whether a soil is acid or alkaline.||Some plants and animals will only live in soils at a particular pH.|
|Olsen P (plant-available phosphate)||How much phosphate is available for plant growth.||Phosphate (P) is an essential nutrient for plants and animals. Plants get their P from phosphates in soil. Most New Zealand soils are low in phosphates and extra phosphate needs to be added for agricultural use.|
|Humus (organic matter)||Total C (carbon)||Organic matter content.||Organic matter helps the soil store water and release it in a steady way. Organic matter also helps a soil store nutrient and form a crumbly structure suitable for plant roots.|
|Total N (nitrogen)||Organic N reserves in the soil.||Nitrogen (N) is an essential nutrient for plants and animals.Nearly all the N in soil is in organic form (see anaerobic mineralisable N).|
|Biological activity||Anaerobic Mineralisable N (ANM)||How much of the total N is available to plants through microbial activity.||Mineralisable N is related to the amount and quality of organic matter and biological activity in soil. Soil life breaks down the total N in organic matter and converts it into forms plants can use.|
|Physical condition||Bulk density||Whether a soil is firm and compacted or loose and friable.||Plant roots cannot grow through compacted soil. Compacted soils also become waterlogged quickly, starving roots of oxygen. If a soil is too loose, it dries out very quickly and may be easily eroded.|
|Macropores||How many large pores there are in soil.||Large pores (0.03–0.3 mm diameter) are needed for air to reach roots and for water drainage. The larger pores are the first to be damaged by compaction.|
|Aggregate stability||How resistant soil aggregates are to breaking.||A stable ’crumbly’ texture lets water quickly soak into soil, doesn’t dry out too quickly, and allows roots to spread easily. This measure is most useful for soils used for horticulture and cropping.|
Measuring soil quality
Measuring soil quality shows how management practices for different land use affect our soils. There is no single test for soil quality because there are many things about soil that affect its quality. From the hundreds of possible measures, scientists have chosen eight key soil quality properties to measure in New Zealand soils.
Soil quality issues
Soil quality monitoring in two districts adjoining the Nelson City region (Marlborough District and Tasman District) show that cropping sites had slightly elevated Olsen P values, depleted total carbon and low macroporosity. On dairy sites, high Olsen P (higher than those required for optimal pasture production) and low macroporosity (indicating compaction) were of most concern. Some dry-stock pasture sites also showed evidence of soil compaction, and a larger number had low Olsen P values.
National soil quality issues
A national picture of soil quality issues for different land uses can be viewed in the Our Land 2018 report and at the StatsNZ Environmental Indicators website. The assessment used available regional monitoring data from 2014 to 2017 and found 83% or more of assessed sites were within the target range for five out of seven soil quality indicators (soil pH, total carbon, total nitrogen, anaerobically mineralisable nitrogen and bulk density). For the other two indicators (macroporosity and Olsen phosphorus), more than 48% of sites were outside target ranges (Figure 1).
In general, national soil quality issues are similar to the upper South Island with low macroporosity (i.e. high compaction) on pasture, high Olsen P on dairy, low Olsen P on dry stock, and high Olsen P and low total carbon on cropping and horticulture land.
Figure 1. Soil quality sites in the target range for given soil quality indicators, by land use (StatsNZ, 2018).
Looking after your soil
Maintaining soil quality is likely to improve plant growth and your production and is good for the environment. It can help make your business more financially and environmentally sustainable. There is a range of ways you can maintain soil quality for your current land use.
Most soil quality issues can be remedied through a change in soil management. However, it can take a very long time to move soil some properties to within target ranges. For example, soil Olsen P levels can be reduced at a rate of 1-2 units per year, so reducing Olsen P from 60 to 40 by reducing your fertiliser inputs could take 10-20 years.
Getting fertility right
- Maintain soil fertility levels appropriate to your production goals. These booklets provide fertility target ranges for different land uses:
- Carry out regular soil tests to check nutrient levels on the different soils of your property and areas where you have different management (e.g. hay paddocks, effluent paddocks, pasture versus cropping). Soil testing should be carried out every two to three years.
- Do a nutrient budget to make sure you’re putting on the appropriate amount of fertiliser for your soil and production needs. More information about nutrient management and a nutrient management plan template can be found on the Fertiliser Association of New Zealand For a comprehensive nutrient budget, you can use OVERSEER® or talk to your farm consultant.
Maintaining your soil organic matter
Soil organic matter (or humus) affects the chemical and physical properties of the soil helping maintain soil structure and porosity, water infiltration, moisture-holding capacity and plant nutrient availability. To maintain and improve soil organic matter you can:
- Grow annual cover crops between productive crops – these cover crops can then be mulched into the ground.
- Apply compost, organic manure or effluent to the soil. Adding organic matter helps the soil to store nutrients and water and improves soil structure.
- Reduce cultivation and soil disturbance to minimise the loss of organic matter. Try direct drilling seed when renewing pasture.
Keeping your soil alive
Fungi, bacteria and other micro-organisms in the soil make nutrients available for plant roots and recycle dead plant material. Earthworms distribute organic matter through the soil profile and improve soil structure, aeration and drainage. To help provide a healthy environment for soil life:
- Maintain a good cover of vegetation to protect the soil whenever possible.
- Minimise cultivation and soil compaction.
Looking after soil structure
Pugging, trampling and cultivating when wet will compact your soil and reduce grass and crop growth, as well as reducing soil drainage, resulting in more runoff to waterways. To protect your soil from compaction:
- Graze wetter paddocks before the wetter part of the season.
- Build pasture cover leading into the wet season, to protect against pugging.
- Graze land that is at risk of pugging with light stock.
- Use the farm bike rather than heavy tractors.
- Vary the depth of cultivation annually – to avoid forming a plough pan.
- Avoid working the soil when it is wet.
How can I check my soil quality?
You can assess your own soil quality using the Visual Soil Assessment (VSA) tool. This tool developed by Landcare Research is a simple field method to check your soil’s health. VSA booklets are available online free of charge. You will require some basic field assessment equipment. Nelson City Council staff may be available to assist you with developing an assessment programme.
SINDI is a freely available web-based soil quality interpretation tool developed by Manaaki Whenua-Landcare Research and regional councils. SINDI is designed to help you interpret the quality or health of a soil sample.
To use SINDI you will need to collect soil samples and have them analysed at a lab. The key soil properties that need to be measured are pH, anaerobic mineralizable N, Total carbon, Total nitrogen, Olsen phosphorus, bulk density and macroporosity. These are standard commercial tests available through soil laboratories. You do not need all 7 indicators to use SINDI but the more indicators you have, the better the soil quality interpretation.
OVERSEER® is a nutrient budgeting tool and can assist with managing your soil nutrients. The tool is designed for agriculture and horticulture properties and allows you create an annual nutrient budget to adjust the number of nutrients you are adding to your soil and estimate the nutrients leaving your property (nutrient outputs).
You will need to know the nutrient inputs for your property, such as:
- fertiliser inputs
- stock numbers
- supplement feed
- dairy shed effluent
Estimates for nutrient outputs from your property include:
- farm products – for example, milk, wool, meat
- transfer to non-productive areas - for example, races, stock camps, yards
- leaching and runoff losses to waterways
- gaseous losses to the air
Soil Trace elements
Trace elements can build up in the soil over time, which can lead to concentrations passing toxic thresholds. Issues include:
- Soil resource and agricultural sustainability issues including the poisoning of soil organisms (from microbes to invertebrates), in turn, reducing the function of the soil.
- Potential trade and human health issues including exceedance of food standards for crops and animal products and human health risk from increased dietary intake.
- Land use versatility issues including reduced ability to convert to more sensitive land use (e.g. conversion of pastoral land to cropping or farmland to housing).
Long term monitoring
Monitoring in the upper South Island districts has found that long term trends for trace elements are well within target ranges and similar to concentrations found by other regional monitoring of the same land uses.
The main trace elements we measure in New Zealand for soil quality monitoring include:
- Arsenic As
- Cadmium Cd
- Copper Cu
- Chromium Cr
- Lead Pb
- Nickel Ni
- Zinc Zn
- Mercury Hg
- Uranium U
- Flourine F
Trace elements (including heavy metals) in the soil are from either natural sources or anthropogenic (man-made) sources.
- Weathering of parent rocks during the process of soil formation.
- Biogenic release from living plants to the atmosphere.
- Release from decomposing vegetation and forest fires.
- Release from volcanic activity.
- Release from sea-salt spray.
- Application of fertilisers, manures or soil conditioners (present intentionally, or as an impurity).
- Application of pesticides that contain a trace element.
- Fossil fuel combustion followed by ashfall.
- Metal production/reprocessing and other manufacturing processes.
- Use, weathering and combustion or another disposal of industrial products containing metal.