Making Sense of Chemical Indicators – Queensland
Most indicators of soil chemical quality measure dynamic soil properties i.e. properties that change over time and with management. These indicators are used to guide management decisions over the period of a rotation. It is important to monitor these indicators as they can act as constraints to yield, restricting crop growth and preventing the yield potential from being achieved.
- Indicators falling in the RED zone are high risk and need to be investigated urgently.
- Indicators falling in the AMBER zone are moderate risk and should be investigated further.
- Indicators falling in the GREEN zone are low risk, regular monitoring should be continued.
Soil pH (acidity and alkalinity)
pH is a measure of the concentration of hydrogen ions in the soil solution. The pH unit scale runs from 1 to 14, with 1 being most acid and 14 being most alkaline; soils normally fall in the range 3 – 8.
Acidic soils can restrict microbial activity, reduce the availability of essential nutrients and cause aluminium toxicity in the subsurface which retards root growth, restricting access to water and nutrients (figure 1). Application of agricultural lime is effective in treating soil acidity. (see Soil Acidity fact sheet).
Alkaline soils, particularly subsoils, often have high exchangeable sodium (sodicity) and toxic concentrations of Boron (see Boron factsheet).
Figure 1: Roots of barley grown in acidic subsurface soil (right) are shortened by aluminium toxicity.
Some crops show greater tolerance of acid or alkaline conditions and rotations can be optimised to reduce the impact of pH constraints.
Indicators of the nutrient content of soil can be used to guide paddock by paddock decisions about how much fertiliser to apply and when. Soil sampling to support fertiliser application must be carried out carefully to provide zoned paddock samples which are representative of the paddock.
Agronomic advice should be sought to guide the interpretation and use of indicators of soil nutrient status. Nutrient cycles in soil are complex, but are directly related to crop nutrient supply (figure 2).
A crop’s uptake of one nutrient is often linked to the uptake of other nutrients. Therefore, nutrient management needs to be considered in an integrated way. Seedling toxicity can also result when very high fertiliser additions increase electrical conductivity in the topsoil soon after application.
Figure 2: Crop showing symptoms of nitrogen deficiency (Photo: NJ Grundon)
Electrical conductivity in topsoil
The concentration of soluble salts in soil solution is measured by the electrical conductivity (EC) of soil and water mixed together at a ratio of 1:5. EC is expressed in units of deci Siemens per metre (dS/m).
EC is used to estimate the soluble salt concentration in soil, and is commonly used as a measure of salinity. The presence of high salt concentrations can stunt plant growth because water uptake by the roots is reduced by the increased osmotic potential of soil. Also, when salt concentration in the soil is high, there can be increased rates of leaf necrosis over the growing season.
EC is very variable over time and across a paddock, so further investigations of the site should be carried out by an expert.
Cation exchange capacity
Cation exchange capacity (CEC) is a measure of the soil’s ability to hold positively charged cations. The main cations present in soil are calcium, magnesium, sodium, potassium, and in acidic soils, aluminium and hydrogen. CEC is a very important soil property influencing soil structural stability, nutrient retention and availability, soil pH and how a soil reacts to fertilisers and other ameliorants.
Cation exchange capacity is an inherent characteristic of the soil, which means that it is largely outside a farmer’s control. It is strongly related to soil clay content and the type of clay in the soil and is mostly determined by the parent material of the soil. CEC is increased by clay and organic matter (see Cation Exchange Capacity – Queensland fact sheet).
CEC is reported as either milliequivalents per 100 grams of soil or centimoles(+) per kilogram of soil (these units are numerically equivalent). CEC can range from 1 meq/100 g in very sandy soils up to 50 meq/100 g in heavy clay soils.
Soils with higher CEC generally have a greater water holding capacity, greater ability to hold and retain cations against leaching and a greater ability to resist changes to soil pH. For these reasons, a soil with a high CEC will generally be more fertile than one with a low CEC. However, it should be noted that once the nutrient reserves or ability of a high CEC soil to resist acidification have been exhausted, high rates of fertiliser or lime will be required to restore soil fertility.
The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.
The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.