Soil nutrient deficiencies and cation imbalances related to soil structure can have a compounding impact on overall soil health. These soils are likely to have poor plant and root growth, low ground cover and litter levels and, therefore, lower water holding capacity and soil organic matter levels. These factors combine to support fewer soil organisms, such as beneficial bacteria, fungi and earthworms.
Laboratory soil testing
Laboratory soil testing provides a range of information on nutrient levels, soil conditions (e.g. salinity or sodicity) and other measures, such as carbon status. Soil tests can identify if there is an aspect of the soil that is limiting production. They can also rate soil conditions against industry benchmarks. The results can be used to determine if inputs are required and what nutrient or product can be applied to address a deficiency. Understanding soil test results can provide confidence to invest in appropriate inputs.
Standard soil tests provide an analysis of:
- General fertility: nitrogen (N), phosphorus (P), sulphur (S) and potassium (K)
- Soil acidity: pH (usually in calcium chloride (CaCl2) and water) and exchangeable aluminium (Al)
- Salinity: electrical conductivity (EC)
- Soil structure: texture, colour, organic carbon, calcium:magnesium (Ca:Mg) ratio, exchangeable sodium percentage (ESP).
How the key components of soil health are measured
The three most commonly deficient nutrients are phosphorus, potassium and sulphur.
Phosphorous (P)
Phosphorous is measured as Olsen P or Colwell P. Olsen P measures the amount of nutrient immediately available to plants. Olsen P remains stable throughout the year. Colwell P measures the immediately available phosphorous (Olsen P) plus the phosphorous bound to the soil particles and released over time. This means the Colwell P is always higher than the Olsen P. The relationship between Olsen P and Colwell P varies with soil type.
Potassium (K)
Potassium levels are measured using the Colwell K test, which identifies the amount of nutrient immediately available to plants and remains relatively stable throughout the year.
Sulphur (S)
Sulphur is assessed using the KCL 40 test, which identifies the amount of sulphate sulphur immediately available to the plant. Other sulphur — elemental sulphur — is stored in organic matter and only becomes available as organic matter breaks down.
Trace elements
Trace elements such as copper (Cu), zinc (Zn), manganese (Mn), iron (Fe) and boron (B) are only required in small amounts. Soil testing only provides a guide to the levels in the soil and can help identify gross deficiencies or toxic levels. Tissue testing provides a more accurate measure of trace element imbalances.
Additional indicators of soil health
A number of other figures and calculations are provided in soil tests, which can used to calculate the rate and type of product to apply. These include the following:
- Phosphorous buffering index (PBI): a guide to the amount of phosphorous applied to a pasture or crop that is ‘locked up’ on soil’s clay particles and is not available for plant growth.
- Cation exchange capacity (CEC): cations are soil components that trap and release nutrients to the soil water.
- Soil texture: a measure of the proportion of sand, silt and clay in the soil. It influences the availability of some nutrients, a plant’s reaction to salinity and the requirement of soils for products such as lime and gypsum.
- Organic carbon: levels of organic carbon influence soil structure and CEC. The breakdown of organic carbon is critical to the release of other nutrients.
Taking soil samples for testing and interpret the results
The Taking a soil sample for laboratory testing tool provides the directions for collecting a soil sample for analysis at an accredited laboratory.
While soil tests can help identify the soil factor or factors limiting production, the target application of various products will depend on the soil type, rainfall, pasture species grown and stocking rates.
The Soil health benchmarks and guidelines for managing problem soils tool is designed to help interpret soil tests for any soil health problems (acidity, sodicity, salinity and trace elements).
Visual indicators of soil health
In addition to laboratory testing, visual indicators, the smell and feel of soil also are important indications of soil health. Healthy soils have a distinctive, ‘earthy ‘smell due to actinobacteria and other beneficial microbes. The actinobacteria present in a healthy soil produce geosmin, which in turn produces a distinctive slightly sweet smell. Unhealthy soils often have a sour or metallic smell.
Visual indicators of poor soil health include pasture discolouration and growth, large bare patches, areas with high weed growth, pooling water and low earthworm numbers. The best time to look for plant indicators of nutrient deficiencies are late winter and early spring.