11 Soil Indicators
11.1 ISSUES
The effects of freshwater irrigation on soil are primarily physical, including alterations in water content, drainage and nutrient transport. Wastewater irrigation can have a more significant chemical and biological effect on soil properties. The primary sustainability goal for soil is the maintenance of productivity. Irrigation should be managed so that it has minimal adverse effects on the quality of the soil. This will ensure that the soil is healthy and remains productive in years to come. Proper management of irrigation water and wastewater as fertiliser can result in enhanced productivity. By using the correct indicators of soil productivity, the effects of irrigation can be gauged, and thus optimised. Soil productivity is affected directly and indirectly by the type of crop, management practices and soil quality, which in turn is affected by moisture, pH, organic matter, heavy metal content etc. Many of these soil characteristics are interdependent - changes in one characteristic result in changes in another. This means that monitoring a subset should highlight any changes in soil productivity.
11.2 POTENTIAL INDICATORS
- Depth of soil, topsoil
- Soil bulk density
- Soil organic matter
- Soil water holding capacity
- Soil moisture
- Soil porosity
- Soil Hydraulic conductivity
- Soil texture
- Soil surface breakdown of aggregates
- Soil infiltration rate/recession curves
- Soil cracking (wetting/drying cycles)
- Erosion
- Soil waterlogging
- Soil surface salinity
- Electrical conductivity
- Soil respiration
- Temperature
- Depth to groundwater
- Ponding
- Soil nutrients
- Organic Carbon
- Organic Nitrogen
- Leaching of chemicals
- pH
- Salinity
- Worm population
- Biodiversity (crops and animals)
There are many measures of soil health and productivity - chemical, physical and biological. Many farmers regularly check the health of their soil by sampling and analysis of nutrients such as nitrogen, phosphorus and potassium. A key set of indicators must be identified that are appropriate for monitoring soil productivity. Listed below are some examples of the type of indicators that may be used to assess the impact of irrigation on soil:
| Physical | Chemical | Biological |
11.3 RATIONALE FOR SELECTING INDICATORS
It is difficult to identify soil characteristics that are affected by irrigation alone. Soil properties can be altered by most farming activities - tillage practices and fertiliser application will change soil physical and chemical properties, as will the types of crops grown. It will be important when using these indicators to be aware of the management practices other than irrigation that will affect the soil quality and cause changes in indicator values or trends. The report Soil Quality Indicators for Sustainable Agriculture produced by the Lincoln Soil Quality Research Centre provides a complete set of indicators designed to provide a measure of the sustainability of the whole farm system. The four soil indicators recommended in this report are a subset of the complete set contained in the soil indicators report and have been selected as those soil characteristics which are most influenced by irrigation.
The recommended soil indicators are: soil water holding capacity, organic carbon content, condition of soil surface aggregates and pH. These four indicators give an overall assessment of soil health referred to as a soil health indicator,
The soil water holding capacity is a measure of soil health because an increase in the ability of a soil to hold water comes about as a result a higher organic content due to increased plant material, increased topsoil depth and improved soil structure. As irrigation promotes plant growth and health, it indirectly affects soil water holding capacity.
Monitoring the organic carbon content, and using the measure along with the knowledge of irrigation volumes, will determine the effect of irrigation on organic content of the soil, and assist the farmer in terms of optimising fertiliser usage, and determining the effects of wastewater irrigation. When assessing the effects of wastewater irrigation on soil, indicators such as organic carbon and nitrogen in the soil will be more significant than with freshwater irrigation, due to the nutrient content of the wastewater, and the general concern for nutrient levels in soils. There is often a limit to the amount of nitrogen that can be applied to a soil. This is based on concern for groundwater contaminants, especially in areas using groundwater as potable water source (regional council responsibility).
Surface sealing of a soil can occur when water has been applied with such intensity that it breaks down the soil aggregates at the surface. This can result in decreased infiltration and thus ponding of water. A measure of the soil aggregate breakdown at the surface will determine whether infiltration is being reduced due to irrigation, and enable the farmer to introduce methods to prevent or mitigate surface sealing- either by ploughing the field, or changing irrigation practices.
The chemical indicators listed in Soil Quality Indicators for Sustainable Agriculture are soil pH and available nutrients. pH is one of the most common indicators of soil quality used. The availability of many minerals and trace elements in the soil depends on the pH. The maintenance of a consistent pH will ensure that the availability of nutrients to the plants is optimised.
Increased salinity is a problem that has been encountered, to a greater or lesser extent, by irrigation schemes around the world. Salinity is not seen as a threat to irrigation schemes in New Zealand, and therefore has not been recommended as an indicator of soil health.
11.4 RECOMMENDED INDICATORS
| Soil Health Indicator (Soil Water Holding Capacity |
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