Critical nutrient levels for canola in Western Australia

Other soil constraints affecting canola nutrition

Canola's ability to access nutrients can also be affected by the physical and chemical properties of the soil. Soil constraints which restrict root growth can limited the plants ability to access nutrients and soil chemical properties such as pH can affect the availability of some nutrients.

Critical levels for common soil constraints are presented here to provide some interpretation of soil tests specifically for canola crops.

As with macro and micro nutrients, a ‘traffic light’ format has been used with red indicating greater than 25% yield loss is likely, orange indicating between 15-25% yield loss, yellow between 5-15% yield loss and green less than 5% yield loss. Canola growers can use this information to easily determine the urgency of addressing a particular soil constraint.

These critical values can be used as a guide to the approximate levels of yield loss expected. They are necessarily simplified and generalised and are not intended or able to replace professional assessment and recommendations.

Soil pH and aluminium toxicity

Acidic soil types are widespread in Western Australia's grain growing region and soil acidity is a common constraint to canola yield which is more sensitive than wheat. Low soil pH can reduce the availability of certain nutrients such as phosphorus and can increase concentrations of toxic elements such as aluminium in the soil solution. Canola is sensitive to high aluminium levels in the subsoil and aluminium toxicity is the major problem associated with soil acidity in Western Australia.

Alkalinity can also reduce seed yield of canola crops on certain soil types.

For more information, and to view images of the symptoms of soil acidity in canola, see DPIRD’s guide to Diagnosing soil acidity in crops.

For general information about soil pH, it's impact on crop production and management options, refer to Soil acidity in Western Australia.

Interpreting acidic soil pH tests for canola

Interpreting alkaline soil pH tests for canola

Interpreting soil aluminium tests for canola

Soil sodicity

Dispersive or sodic soils are common in the south-west agricultural area of Western Australia where they occur mainly as duplex or gradational profiles on broad, flat landscapes with poor drainage. They are structurally unstable when wet and lack structure when dry. Soil aggregates collapse when the soil gets wet because the individual clay particles disperse into solution. This collapse of structure causes the soil to slump, lose porosity and become denser thus restricting crop root growth. This leads to problems including waterlogging, reduced plant growth, increased erosion risk, and poor trafficability when wet. These soils are difficult to manage, and can have multiple constraints to crop and pasture growth.

Soil testing that measures the exchangeable sodium percentage in soil solution is used to measure soil sodicity. Table 15 presents estimates of yield loss due to sodicity for 0-10cm soil tests. Sodicity below 10cm can also reduce yields, but impacts are likely to be less than for 0-10cm.

For more information, and to view images of the symptoms of poor seedbed soil structure, see DPIRD’s guide to Diagnosing poor seedbed soil structure.

For more information, and to view images of the symptoms of waterlogging in canola, see DPIRD’s guide to Diagnosing waterlogging in canola.

For more general information about dispersive and sodic soils in Western Australia including management options refer to Dispersive and sodic soils.

Interpreting soil sodicity tests for canola

Soil compaction

Livestock and light machinery can compact surface soil, and agricultural machinery or natural soil characteristics and conditions can lead to subsurface compaction. Some soils have greater capacity to resist compaction or to self-repair following compaction. Compacted sub-surface soil restricts crop root growth.

Soil compaction is not assessed as a part of standard soil testing, however a hand held penetrometer can be used to locate the depth and severity of a hard pan. Assessment must be done in moist soil conditions.

For more information, and to view images of the symptoms of soil compaction, see DPIRD’s guide to Diagnosing sandplain traffic pan

For more general information about soil compaction in Western Australia including management options, refer to Soil compaction.

Interpreting soil strength tests (compaction) for canola

Soil salinity

Dryland salinity is a major form of land degradation. More than one million hectares of broadacre farmland in Western Australia is estimated to be currently affected by dryland salinity. Through activities such as groundwater and soil analysis, landholders can confidently assess salinity risks and implement appropriate management responses.

Canola has a similar level of tolerance as wheat to soil salinity which is substantially less tolerant than barley. Crop growth is affected by the concentration of salt in the soil solution rather than just in the soil. When there is less water in the topsoil, the soil solution will be more concentrated, which may cause a higher yield loss even with a low salinity reading. As such, it is hard to assign salinity limits to yield loss categories. Instead, the traffic lights shown below correspond to severe, very high, high and low risk of yield decline due to salinity.

For more information, and to view images of the symptoms of soil acidity in canola, see DPIRD’s guide to Diagnosing soil salinity in canola.

For more general information about soil salinity in Western Australia, refer to Soil salinity.

Salinity is measured as electrical conductivity (EC) and is expressed in different units by different companies. This is based on a 1:5 soil:water solution. In Table 18, the data is expressed in millisiemens per metre (mS/m). If the units of your data are different to mS/m, you will need to convert it by multiplying with the conversion factors in Table 17:

Interpreting soil salinity tests for canola